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<div class="identifier-type">function template</div><div class="header"><string></div><h1 itemprop="name"><span class="namespace" title="namespace std">std::</span><span class="token">operator+</span></h1>
<div itemprop="articleBody"><p><div class="codehilite"><pre><span></span><code><span class="k">namespace</span><span class="w"> </span><span class="nn">std</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="k">class</span><span class="w"> </span><span class="nc">charT</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">traits</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">Allocator</span><span class="o">></span>
<span class="w"> </span><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">></span>
<span class="w"> </span><span class="k">operator</span><span class="o">+</span><span class="p">(</span><span class="k">const</span><span class="w"> </span><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">>&</span><span class="w"> </span><span class="n">lhs</span><span class="p">,</span>
<span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">>&</span><span class="w"> </span><span class="n">rhs</span><span class="p">);</span><span class="w"> </span><span class="c1">// (1) C++03</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="k">class</span><span class="w"> </span><span class="nc">charT</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">traits</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">Allocator</span><span class="o">></span>
<span class="w"> </span><span class="k">constexpr</span><span class="w"> </span><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">></span>
<span class="w"> </span><span class="k">operator</span><span class="o">+</span><span class="p">(</span><span class="k">const</span><span class="w"> </span><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">>&</span><span class="w"> </span><span class="n">lhs</span><span class="p">,</span>
<span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">>&</span><span class="w"> </span><span class="n">rhs</span><span class="p">);</span><span class="w"> </span><span class="c1">// (1) C++20</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="k">class</span><span class="w"> </span><span class="nc">charT</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">traits</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">Allocator</span><span class="o">></span>
<span class="w"> </span><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">></span>
<span class="w"> </span><span class="k">operator</span><span class="o">+</span><span class="p">(</span><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">>&&</span><span class="w"> </span><span class="n">lhs</span><span class="p">,</span>
<span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">>&</span><span class="w"> </span><span class="n">rhs</span><span class="p">);</span><span class="w"> </span><span class="c1">// (2) C++11</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="k">class</span><span class="w"> </span><span class="nc">charT</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">traits</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">Allocator</span><span class="o">></span>
<span class="w"> </span><span class="k">constexpr</span><span class="w"> </span><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">></span>
<span class="w"> </span><span class="k">operator</span><span class="o">+</span><span class="p">(</span><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">>&&</span><span class="w"> </span><span class="n">lhs</span><span class="p">,</span>
<span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">>&</span><span class="w"> </span><span class="n">rhs</span><span class="p">);</span><span class="w"> </span><span class="c1">// (2) C++20</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="k">class</span><span class="w"> </span><span class="nc">charT</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">traits</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">Allocator</span><span class="o">></span>
<span class="w"> </span><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">></span>
<span class="w"> </span><span class="k">operator</span><span class="o">+</span><span class="p">(</span><span class="k">const</span><span class="w"> </span><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">>&</span><span class="w"> </span><span class="n">lhs</span><span class="p">,</span>
<span class="w"> </span><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">>&&</span><span class="w"> </span><span class="n">rhs</span><span class="p">);</span><span class="w"> </span><span class="c1">// (3) C++11</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="k">class</span><span class="w"> </span><span class="nc">charT</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">traits</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">Allocator</span><span class="o">></span>
<span class="w"> </span><span class="k">constexpr</span><span class="w"> </span><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">></span>
<span class="w"> </span><span class="k">operator</span><span class="o">+</span><span class="p">(</span><span class="k">const</span><span class="w"> </span><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">>&</span><span class="w"> </span><span class="n">lhs</span><span class="p">,</span>
<span class="w"> </span><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">>&&</span><span class="w"> </span><span class="n">rhs</span><span class="p">);</span><span class="w"> </span><span class="c1">// (3) C++20</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="k">class</span><span class="w"> </span><span class="nc">charT</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">traits</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">Allocator</span><span class="o">></span>
<span class="w"> </span><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">></span>
<span class="w"> </span><span class="k">operator</span><span class="o">+</span><span class="p">(</span><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">>&&</span><span class="w"> </span><span class="n">lhs</span><span class="p">,</span>
<span class="w"> </span><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">>&&</span><span class="w"> </span><span class="n">rhs</span><span class="p">);</span><span class="w"> </span><span class="c1">// (4) C++11</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="k">class</span><span class="w"> </span><span class="nc">charT</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">traits</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">Allocator</span><span class="o">></span>
<span class="w"> </span><span class="k">constexpr</span><span class="w"> </span><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">></span>
<span class="w"> </span><span class="k">operator</span><span class="o">+</span><span class="p">(</span><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">>&&</span><span class="w"> </span><span class="n">lhs</span><span class="p">,</span>
<span class="w"> </span><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">>&&</span><span class="w"> </span><span class="n">rhs</span><span class="p">);</span><span class="w"> </span><span class="c1">// (4) C++20</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="k">class</span><span class="w"> </span><span class="nc">charT</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">traits</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">Allocator</span><span class="o">></span>
<span class="w"> </span><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">></span>
<span class="w"> </span><span class="k">operator</span><span class="o">+</span><span class="p">(</span><span class="k">const</span><span class="w"> </span><span class="n">charT</span><span class="o">*</span><span class="w"> </span><span class="n">lhs</span><span class="p">,</span>
<span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">>&</span><span class="w"> </span><span class="n">rhs</span><span class="p">);</span><span class="w"> </span><span class="c1">// (5) C++03</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="k">class</span><span class="w"> </span><span class="nc">charT</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">traits</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">Allocator</span><span class="o">></span>
<span class="w"> </span><span class="k">constexpr</span><span class="w"> </span><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">></span>
<span class="w"> </span><span class="k">operator</span><span class="o">+</span><span class="p">(</span><span class="k">const</span><span class="w"> </span><span class="n">charT</span><span class="o">*</span><span class="w"> </span><span class="n">lhs</span><span class="p">,</span>
<span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">>&</span><span class="w"> </span><span class="n">rhs</span><span class="p">);</span><span class="w"> </span><span class="c1">// (5) C++20</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="k">class</span><span class="w"> </span><span class="nc">charT</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">traits</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">Allocator</span><span class="o">></span>
<span class="w"> </span><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">></span>
<span class="w"> </span><span class="k">operator</span><span class="o">+</span><span class="p">(</span><span class="k">const</span><span class="w"> </span><span class="n">charT</span><span class="o">*</span><span class="w"> </span><span class="n">lhs</span><span class="p">,</span>
<span class="w"> </span><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">>&&</span><span class="w"> </span><span class="n">rhs</span><span class="p">);</span><span class="w"> </span><span class="c1">// (6) C++11</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="k">class</span><span class="w"> </span><span class="nc">charT</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">traits</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">Allocator</span><span class="o">></span>
<span class="w"> </span><span class="k">constexpr</span><span class="w"> </span><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">></span>
<span class="w"> </span><span class="k">operator</span><span class="o">+</span><span class="p">(</span><span class="k">const</span><span class="w"> </span><span class="n">charT</span><span class="o">*</span><span class="w"> </span><span class="n">lhs</span><span class="p">,</span>
<span class="w"> </span><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">>&&</span><span class="w"> </span><span class="n">rhs</span><span class="p">);</span><span class="w"> </span><span class="c1">// (6) C++20</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="k">class</span><span class="w"> </span><span class="nc">charT</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">traits</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">Allocator</span><span class="o">></span>
<span class="w"> </span><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">></span>
<span class="w"> </span><span class="k">operator</span><span class="o">+</span><span class="p">(</span><span class="n">charT</span><span class="w"> </span><span class="n">lhs</span><span class="p">,</span>
<span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">>&</span><span class="w"> </span><span class="n">rhs</span><span class="p">);</span><span class="w"> </span><span class="c1">// (7) C++03</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="k">class</span><span class="w"> </span><span class="nc">charT</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">traits</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">Allocator</span><span class="o">></span>
<span class="w"> </span><span class="k">constexpr</span><span class="w"> </span><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">></span>
<span class="w"> </span><span class="k">operator</span><span class="o">+</span><span class="p">(</span><span class="n">charT</span><span class="w"> </span><span class="n">lhs</span><span class="p">,</span>
<span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">>&</span><span class="w"> </span><span class="n">rhs</span><span class="p">);</span><span class="w"> </span><span class="c1">// (7) C++20</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="k">class</span><span class="w"> </span><span class="nc">charT</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">traits</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">Allocator</span><span class="o">></span>
<span class="w"> </span><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">></span>
<span class="w"> </span><span class="k">operator</span><span class="o">+</span><span class="p">(</span><span class="n">charT</span><span class="w"> </span><span class="n">lhs</span><span class="p">,</span>
<span class="w"> </span><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">>&&</span><span class="w"> </span><span class="n">rhs</span><span class="p">);</span><span class="w"> </span><span class="c1">// (8) C++11</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="k">class</span><span class="w"> </span><span class="nc">charT</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">traits</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">Allocator</span><span class="o">></span>
<span class="w"> </span><span class="k">constexpr</span><span class="w"> </span><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">></span>
<span class="w"> </span><span class="k">operator</span><span class="o">+</span><span class="p">(</span><span class="n">charT</span><span class="w"> </span><span class="n">lhs</span><span class="p">,</span>
<span class="w"> </span><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">>&&</span><span class="w"> </span><span class="n">rhs</span><span class="p">);</span><span class="w"> </span><span class="c1">// (8) C++20</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="k">class</span><span class="w"> </span><span class="nc">charT</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">traits</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">Allocator</span><span class="o">></span>
<span class="w"> </span><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">></span>
<span class="w"> </span><span class="k">operator</span><span class="o">+</span><span class="p">(</span><span class="k">const</span><span class="w"> </span><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">>&</span><span class="w"> </span><span class="n">lhs</span><span class="p">,</span>
<span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">charT</span><span class="o">*</span><span class="w"> </span><span class="n">rhs</span><span class="p">);</span><span class="w"> </span><span class="c1">// (9) C++03</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="k">class</span><span class="w"> </span><span class="nc">charT</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">traits</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">Allocator</span><span class="o">></span>
<span class="w"> </span><span class="k">constexpr</span><span class="w"> </span><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">></span>
<span class="w"> </span><span class="k">operator</span><span class="o">+</span><span class="p">(</span><span class="k">const</span><span class="w"> </span><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">>&</span><span class="w"> </span><span class="n">lhs</span><span class="p">,</span>
<span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">charT</span><span class="o">*</span><span class="w"> </span><span class="n">rhs</span><span class="p">);</span><span class="w"> </span><span class="c1">// (9) C++20</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="k">class</span><span class="w"> </span><span class="nc">charT</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">traits</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">Allocator</span><span class="o">></span>
<span class="w"> </span><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">></span>
<span class="w"> </span><span class="k">operator</span><span class="o">+</span><span class="p">(</span><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">>&&</span><span class="w"> </span><span class="n">lhs</span><span class="p">,</span>
<span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">charT</span><span class="o">*</span><span class="w"> </span><span class="n">rhs</span><span class="p">);</span><span class="w"> </span><span class="c1">// (10) C++11</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="k">class</span><span class="w"> </span><span class="nc">charT</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">traits</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">Allocator</span><span class="o">></span>
<span class="w"> </span><span class="k">constexpr</span><span class="w"> </span><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">></span>
<span class="w"> </span><span class="k">operator</span><span class="o">+</span><span class="p">(</span><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">>&&</span><span class="w"> </span><span class="n">lhs</span><span class="p">,</span>
<span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">charT</span><span class="o">*</span><span class="w"> </span><span class="n">rhs</span><span class="p">);</span><span class="w"> </span><span class="c1">// (10) C++20</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="k">class</span><span class="w"> </span><span class="nc">charT</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">traits</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">Allocator</span><span class="o">></span>
<span class="w"> </span><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">></span>
<span class="w"> </span><span class="k">operator</span><span class="o">+</span><span class="p">(</span><span class="k">const</span><span class="w"> </span><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">>&</span><span class="w"> </span><span class="n">lhs</span><span class="p">,</span>
<span class="w"> </span><span class="n">charT</span><span class="w"> </span><span class="n">rhs</span><span class="p">);</span><span class="w"> </span><span class="c1">// (11) C++03</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="k">class</span><span class="w"> </span><span class="nc">charT</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">traits</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">Allocator</span><span class="o">></span>
<span class="w"> </span><span class="k">constexpr</span><span class="w"> </span><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">></span>
<span class="w"> </span><span class="k">operator</span><span class="o">+</span><span class="p">(</span><span class="k">const</span><span class="w"> </span><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">>&</span><span class="w"> </span><span class="n">lhs</span><span class="p">,</span>
<span class="w"> </span><span class="n">charT</span><span class="w"> </span><span class="n">rhs</span><span class="p">);</span><span class="w"> </span><span class="c1">// (11) C++20</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="k">class</span><span class="w"> </span><span class="nc">charT</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">traits</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">Allocator</span><span class="o">></span>
<span class="w"> </span><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">></span>
<span class="w"> </span><span class="k">operator</span><span class="o">+</span><span class="p">(</span><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">>&&</span><span class="w"> </span><span class="n">lhs</span><span class="p">,</span>
<span class="w"> </span><span class="n">charT</span><span class="w"> </span><span class="n">rhs</span><span class="p">);</span><span class="w"> </span><span class="c1">// (12) C++11</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="k">class</span><span class="w"> </span><span class="nc">charT</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">traits</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">Allocator</span><span class="o">></span>
<span class="w"> </span><span class="k">constexpr</span><span class="w"> </span><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">></span>
<span class="w"> </span><span class="k">operator</span><span class="o">+</span><span class="p">(</span><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">>&&</span><span class="w"> </span><span class="n">lhs</span><span class="p">,</span>
<span class="w"> </span><span class="n">charT</span><span class="w"> </span><span class="n">rhs</span><span class="p">);</span><span class="w"> </span><span class="c1">// (12) C++20</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="k">class</span><span class="w"> </span><span class="nc">charT</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">traits</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">Allocator</span><span class="o">></span>
<span class="w"> </span><span class="k">constexpr</span><span class="w"> </span><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">></span>
<span class="w"> </span><span class="k">operator</span><span class="o">+</span><span class="p">(</span><span class="k">const</span><span class="w"> </span><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">>&</span><span class="w"> </span><span class="n">lhs</span><span class="p">,</span>
<span class="w"> </span><span class="n">type_identity_t</span><span class="o"><</span><span class="n"><a href="../../string_view/basic_string_view.html">basic_string_view</a></span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="o">>></span><span class="w"> </span><span class="n">rhs</span><span class="p">);</span><span class="w"> </span><span class="c1">// (13) C++26</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="k">class</span><span class="w"> </span><span class="nc">charT</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">traits</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">Allocator</span><span class="o">></span>
<span class="w"> </span><span class="k">constexpr</span><span class="w"> </span><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">></span>
<span class="w"> </span><span class="k">operator</span><span class="o">+</span><span class="p">(</span><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">>&&</span><span class="w"> </span><span class="n">lhs</span><span class="p">,</span>
<span class="w"> </span><span class="n">type_identity_t</span><span class="o"><</span><span class="n"><a href="../../string_view/basic_string_view.html">basic_string_view</a></span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="o">>></span><span class="w"> </span><span class="n">rhs</span><span class="p">);</span><span class="w"> </span><span class="c1">// (14) C++26</span>
<span class="p">}</span>
</code></pre></div>
</p>
<ul>
<li>type_identity_t[link /reference/type_traits/type_identity.md]</li>
<li>basic_string_view[link /reference/string_view/basic_string_view.md]</li>
</ul>
<h2>概要</h2>
<p><code>basic_string</code> オブジェクトの連結を行う。</p>
<ul>
<li>(1) : <code>basic_string</code>オブジェクト同士を連結する</li>
<li>(2) : 右辺値の<code>basic_string</code>オブジェクトと<code>basic_string</code>オブジェクトを連結する</li>
<li>(3) : <code>basic_string</code>オブジェクトと右辺値の<code>basic_string</code>オブジェクトを連結する</li>
<li>(4) : 右辺値の<code>basic_string</code>オブジェクト同士を連結する</li>
<li>(5) : 文字配列と<code>basic_string</code>オブジェクトを連結する</li>
<li>(6) : 文字配列と右辺値の<code>basic_string</code>オブジェクトを連結する</li>
<li>(7) : 文字と<code>basic_string</code>オブジェクトを連結する</li>
<li>(8) : 文字と右辺値の<code>basic_string</code>オブジェクトを連結する</li>
<li>(9) : <code>basic_string</code>オブジェクトと文字配列を連結する</li>
<li>(10) : 右辺値の<code>basic_string</code>オブジェクトと文字配列を連結する</li>
<li>(11) : <code>basic_string</code>オブジェクトと文字を連結する</li>
<li>(12) : 右辺値の<code>basic_string</code>オブジェクトと文字を連結する</li>
<li>(13) : <code>basic_string</code>オブジェクトと<code><a href="../../string_view/basic_string_view.html">basic_string_view</a></code>オブジェクトを連結する</li>
<li>(14) : 右辺値の<code>basic_string</code>オブジェクトと<code><a href="../../string_view/basic_string_view.html">basic_string_view</a></code>オブジェクトを連結する</li>
</ul>
<h2><a class="cpprefjp-defined-word" data-desc="関数呼び出し式の評価結果となるオブジェクト・値">戻り値</a></h2>
<h3>C++17まで</h3>
<ul>
<li>
<p>(1) <code><a href="op_constructor.html">basic_string</a><charT, traits, Allocator>(lhs).<a href="append.html">append</a>(rhs)</code></p>
</li>
<li>
<p>(2) <code>std::<a href="../../utility/move.html">move</a>(lhs.<a href="append.html">append</a>(rhs))</code></p>
</li>
<li>
<p>(3) <code>std::<a href="../../utility/move.html">move</a>(rhs.<a href="insert.html">insert</a>(0, lhs))</code></p>
</li>
<li>
<p>(4) <code>std::<a href="../../utility/move.html">move</a>(lhs.<a href="append.html">append</a>(rhs))</code><br />
(<code>std::<a href="../../utility/move.html">move</a>(rhs.<a href="insert.html">insert</a>(0, lhs))</code> とも等価)</p>
</li>
<li>
<p>(5) <code><a href="op_constructor.html">basic_string</a><charT, traits, Allocator>(lhs) + rhs</code> </p>
</li>
<li>
<p>(6) <code>std::<a href="../../utility/move.html">move</a>(rhs.<a href="insert.html">insert</a>(0, lhs))</code> </p>
</li>
<li>
<p>(7) <code><a href="op_constructor.html">basic_string</a><charT, traits, Allocator>(1, lhs) + rhs</code></p>
</li>
<li>
<p>(8) <code>std::<a href="../../utility/move.html">move</a>(rhs.<a href="insert.html">insert</a>(0, 1, lhs))</code></p>
</li>
<li>
<p>(9) <code>lhs + <a href="op_constructor.html">basic_string</a><charT, traits, Allocator>(rhs)</code> </p>
</li>
<li>
<p>(10) <code>std::<a href="../../utility/move.html">move</a>(lhs.<a href="append.html">append</a>(rhs))</code> </p>
</li>
<li>
<p>(11) <code>lhs + <a href="op_constructor.html">basic_string</a><charT, traits, Allocator>(1, rhs)</code></p>
</li>
<li>
<p>(12) <code>std::<a href="../../utility/move.html">move</a>(lhs.<a href="append.html">append</a>(1, rhs))</code></p>
</li>
</ul>
<h3>C++20から</h3>
<ul>
<li>
<p>(1), (9) : 以下と等価
<br />
<div class="codehilite"><pre><span></span><code><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">></span><span class="w"> </span><span class="n">r</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">lhs</span><span class="p">;</span>
<span class="n">r</span><span class="p">.</span><span class="n"><a href="append.html">append</a></span><span class="p">(</span><span class="n">rhs</span><span class="p">);</span>
<span class="k">return</span><span class="w"> </span><span class="n">r</span><span class="p">;</span>
</code></pre></div>
</p>
</li>
<li>
<p>(2), (10) : 以下と等価
<br />
<div class="codehilite"><pre><span></span><code><span class="n">lhs</span><span class="p">.</span><span class="n"><a href="append.html">append</a></span><span class="p">(</span><span class="n">rhs</span><span class="p">);</span>
<span class="k">return</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">move</span><span class="p">(</span><span class="n">lhs</span><span class="p">);</span>
</code></pre></div>
</p>
</li>
<li>
<p>(3), (6) : 以下と等価
<br />
<div class="codehilite"><pre><span></span><code><span class="n">rhs</span><span class="p">.</span><span class="n"><a href="insert.html">insert</a></span><span class="p">(</span><span class="mi">0</span><span class="p">,</span><span class="w"> </span><span class="n">lhs</span><span class="p">);</span>
<span class="k">return</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">move</span><span class="p">(</span><span class="n">rhs</span><span class="p">);</span>
</code></pre></div>
</p>
</li>
<li>
<p>(4) : 呼び出しの後でも<code>lhs</code>と<code>rhs</code>は有効だが<a class="cpprefjp-defined-word" data-desc="未規定の動作。事前の断りなく処理系によって異なる動作をする" href="../../../implementation-compliance.html#dfn-unspecified-behavior">未規定</a>な状態のままであることを除いて、以下と等価
<br />
<div class="codehilite"><pre><span></span><code><span class="n">lhs</span><span class="p">.</span><span class="n"><a href="append.html">append</a></span><span class="p">(</span><span class="n">rhs</span><span class="p">);</span>
<span class="k">return</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">move</span><span class="p">(</span><span class="n">lhs</span><span class="p">);</span>
</code></pre></div>
</p>
</li>
<li>
<p>(5) : 以下と等価
<br />
<div class="codehilite"><pre><span></span><code><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">></span><span class="w"> </span><span class="n">r</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">rhs</span><span class="p">;</span>
<span class="n">r</span><span class="p">.</span><span class="n"><a href="insert.html">insert</a></span><span class="p">(</span><span class="mi">0</span><span class="p">,</span><span class="w"> </span><span class="n">lhs</span><span class="p">);</span>
<span class="k">return</span><span class="w"> </span><span class="n">r</span><span class="p">;</span>
</code></pre></div>
</p>
</li>
<li>
<p>(7) : 以下と等価
<br />
<div class="codehilite"><pre><span></span><code><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">></span><span class="w"> </span><span class="n">r</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">rhs</span><span class="p">;</span>
<span class="n">r</span><span class="p">.</span><span class="n"><a href="insert.html">insert</a></span><span class="p">(</span><span class="n">r</span><span class="p">.</span><span class="n"><a href="begin.html">begin</a></span><span class="p">(),</span><span class="w"> </span><span class="n">lhs</span><span class="p">);</span>
<span class="k">return</span><span class="w"> </span><span class="n">r</span><span class="p">;</span>
</code></pre></div>
</p>
</li>
<li>
<p>(8) : 以下と等価
<br />
<div class="codehilite"><pre><span></span><code><span class="n">rhs</span><span class="p">.</span><span class="n"><a href="insert.html">insert</a></span><span class="p">(</span><span class="n">rhs</span><span class="p">.</span><span class="n"><a href="begin.html">begin</a></span><span class="p">(),</span><span class="w"> </span><span class="n">lhs</span><span class="p">);</span>
<span class="k">return</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">move</span><span class="p">(</span><span class="n">rhs</span><span class="p">);</span>
</code></pre></div>
</p>
</li>
<li>
<p>(11) : 以下と等価
<br />
<div class="codehilite"><pre><span></span><code><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">></span><span class="w"> </span><span class="n">r</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">lhs</span><span class="p">;</span>
<span class="n">r</span><span class="p">.</span><span class="n"><a href="push_back.html">push_back</a></span><span class="p">(</span><span class="n">rhs</span><span class="p">);</span>
<span class="k">return</span><span class="w"> </span><span class="n">r</span><span class="p">;</span>
</code></pre></div>
</p>
</li>
<li>
<p>(12) : 以下と等価
<br />
<div class="codehilite"><pre><span></span><code><span class="n">lhs</span><span class="p">.</span><span class="n"><a href="push_back.html">push_back</a></span><span class="p">(</span><span class="n">rhs</span><span class="p">);</span>
<span class="k">return</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">move</span><span class="p">(</span><span class="n">lhs</span><span class="p">);</span>
</code></pre></div>
</p>
</li>
<li>
<p>(13) : 以下と等価
<br />
<div class="codehilite"><pre><span></span><code><span class="n">basic_string</span><span class="o"><</span><span class="n">charT</span><span class="p">,</span><span class="w"> </span><span class="n">traits</span><span class="p">,</span><span class="w"> </span><span class="n">Allocator</span><span class="o">></span><span class="w"> </span><span class="n">r</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">lhs</span><span class="p">;</span>
<span class="n">r</span><span class="p">.</span><span class="n"><a href="append.html">append</a></span><span class="p">(</span><span class="n">rhs</span><span class="p">);</span>
<span class="k">return</span><span class="w"> </span><span class="n">r</span><span class="p">;</span>
</code></pre></div>
</p>
</li>
<li>
<p>(14) : 以下と等価
<br />
<div class="codehilite"><pre><span></span><code><span class="n">lhs</span><span class="p">.</span><span class="n"><a href="append.html">append</a></span><span class="p">(</span><span class="n">rhs</span><span class="p">);</span>
<span class="k">return</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">move</span><span class="p">(</span><span class="n">lhs</span><span class="p">);</span>
</code></pre></div>
</p>
</li>
</ul>
<h2>備考</h2>
<p>(5), (6) の形式の <code>lhs</code>、および、(9), (10) の形式の <code>rhs</code> の文字列長算出のために <code>traits::length()</code> が使用される</p>
<h2>アロケータの伝播</h2>
<p>C++20からこの演算子による文字列連結時にアロケータがどのように伝播するかが変更された。C++17までの仕様及び各実装とC++20からの各<a class="cpprefjp-defined-word" data-desc="同名の関数を異なる引数・テンプレート・制約などで複数定義すること。または同名の関数の集合">オーバーロード</a>利用時の結果オブジェクトへのアロケータ供給元は次のようになる。なお、SOCCCは<code><a href="../../memory/allocator_traits/select_on_container_copy_construction.html">select_on_container_copy_construction</a></code>の略。</p>
<table border="1" bordercolor="#888" style="border-collapse:collapse">
<thead>
<tr>
<th><a class="cpprefjp-defined-word" data-desc="同名の関数を異なる引数・テンプレート・制約などで複数定義すること。または同名の関数の集合">オーバーロード</a></th>
<th>C++17まで</th>
<th>GCC</th>
<th>clang</th>
<th>MSVC</th>
<th>C++20から</th>
</tr>
</thead>
<tbody>
<tr>
<td>(1) : <code>lhs + rhs</code></td>
<td><code>lhs</code>からのSOCCC</td>
<td><code>lhs</code>からのSOCCC</td>
<td><code>lhs</code></td>
<td>新規に<a class="cpprefjp-defined-word" data-desc="デフォルト初期化。`T x;`構文での初期化。トリビアルにデフォルト構築可能な型の場合は初期化されず（オブジェクトのlifetimeは開始される）、そうではないクラス型はデフォルトコンストラクタで初期化する。配列型の場合は各要素が再帰的にデフォルト初期化される">デフォルト構築</a></td>
<td><code>lhs</code>からのSOCCC</td>
</tr>
<tr>
<td>(2) : <code>std::move(lhs) + rhs</code></td>
<td><code>lhs</code></td>
<td><code>lhs</code></td>
<td><code>lhs</code></td>
<td><code>lhs</code></td>
<td><code>lhs</code></td>
</tr>
<tr>
<td>(3) : <code>lhs + std::move(rhs)</code></td>
<td><code>rhs</code></td>
<td><code>rhs</code></td>
<td><code>rhs</code></td>
<td><code>rhs</code></td>
<td><code>rhs</code></td>
</tr>
<tr>
<td>(4) : <code>std::move(lhs) + std::move(rhs)</code></td>
<td><code>lhs</code>または<code>rhs</code></td>
<td><code>lhs</code></td>
<td><code>lhs</code></td>
<td><code>lhs</code></td>
<td><code>lhs</code></td>
</tr>
<tr>
<td>(5) : <code>"lhs" + rhs</code></td>
<td>新規に<a class="cpprefjp-defined-word" data-desc="デフォルト初期化。`T x;`構文での初期化。トリビアルにデフォルト構築可能な型の場合は初期化されず（オブジェクトのlifetimeは開始される）、そうではないクラス型はデフォルトコンストラクタで初期化する。配列型の場合は各要素が再帰的にデフォルト初期化される">デフォルト構築</a></td>
<td>新規に<a class="cpprefjp-defined-word" data-desc="デフォルト初期化。`T x;`構文での初期化。トリビアルにデフォルト構築可能な型の場合は初期化されず（オブジェクトのlifetimeは開始される）、そうではないクラス型はデフォルトコンストラクタで初期化する。配列型の場合は各要素が再帰的にデフォルト初期化される">デフォルト構築</a></td>
<td><code>rhs</code></td>
<td>新規に<a class="cpprefjp-defined-word" data-desc="デフォルト初期化。`T x;`構文での初期化。トリビアルにデフォルト構築可能な型の場合は初期化されず（オブジェクトのlifetimeは開始される）、そうではないクラス型はデフォルトコンストラクタで初期化する。配列型の場合は各要素が再帰的にデフォルト初期化される">デフォルト構築</a></td>
<td><code>lhs</code>からのSOCCC</td>
</tr>
<tr>
<td>(6) : <code>"lhs" + std::move(rhs)</code></td>
<td><code>rhs</code></td>
<td><code>rhs</code></td>
<td><code>rhs</code></td>
<td><code>rhs</code></td>
<td><code>rhs</code></td>
</tr>
<tr>
<td>(7) : <code>'l' + rhs</code></td>
<td>新規に<a class="cpprefjp-defined-word" data-desc="デフォルト初期化。`T x;`構文での初期化。トリビアルにデフォルト構築可能な型の場合は初期化されず（オブジェクトのlifetimeは開始される）、そうではないクラス型はデフォルトコンストラクタで初期化する。配列型の場合は各要素が再帰的にデフォルト初期化される">デフォルト構築</a></td>
<td>新規に<a class="cpprefjp-defined-word" data-desc="デフォルト初期化。`T x;`構文での初期化。トリビアルにデフォルト構築可能な型の場合は初期化されず（オブジェクトのlifetimeは開始される）、そうではないクラス型はデフォルトコンストラクタで初期化する。配列型の場合は各要素が再帰的にデフォルト初期化される">デフォルト構築</a></td>
<td><code>rhs</code></td>
<td>新規に<a class="cpprefjp-defined-word" data-desc="デフォルト初期化。`T x;`構文での初期化。トリビアルにデフォルト構築可能な型の場合は初期化されず（オブジェクトのlifetimeは開始される）、そうではないクラス型はデフォルトコンストラクタで初期化する。配列型の場合は各要素が再帰的にデフォルト初期化される">デフォルト構築</a></td>
<td><code>lhs</code>からのSOCCC</td>
</tr>
<tr>
<td>(8) : <code>'l' + std::move(rhs)</code></td>
<td><code>rhs</code></td>
<td><code>rhs</code></td>
<td><code>rhs</code></td>
<td><code>rhs</code></td>
<td><code>rhs</code></td>
</tr>
<tr>
<td>(9) : <code>lhs + "rhs"</code></td>
<td>新規に<a class="cpprefjp-defined-word" data-desc="デフォルト初期化。`T x;`構文での初期化。トリビアルにデフォルト構築可能な型の場合は初期化されず（オブジェクトのlifetimeは開始される）、そうではないクラス型はデフォルトコンストラクタで初期化する。配列型の場合は各要素が再帰的にデフォルト初期化される">デフォルト構築</a></td>
<td><code>lhs</code>からのSOCCC</td>
<td><code>lhs</code></td>
<td>新規に<a class="cpprefjp-defined-word" data-desc="デフォルト初期化。`T x;`構文での初期化。トリビアルにデフォルト構築可能な型の場合は初期化されず（オブジェクトのlifetimeは開始される）、そうではないクラス型はデフォルトコンストラクタで初期化する。配列型の場合は各要素が再帰的にデフォルト初期化される">デフォルト構築</a></td>
<td><code>lhs</code>からのSOCCC</td>
</tr>
<tr>
<td>(10) : <code>std::move(lhs) + "rhs"</code></td>
<td><code>lhs</code></td>
<td><code>lhs</code></td>
<td><code>lhs</code></td>
<td><code>lhs</code></td>
<td><code>lhs</code></td>
</tr>
<tr>
<td>(11) : <code>lhs + 'r'</code></td>
<td>新規に<a class="cpprefjp-defined-word" data-desc="デフォルト初期化。`T x;`構文での初期化。トリビアルにデフォルト構築可能な型の場合は初期化されず（オブジェクトのlifetimeは開始される）、そうではないクラス型はデフォルトコンストラクタで初期化する。配列型の場合は各要素が再帰的にデフォルト初期化される">デフォルト構築</a></td>
<td><code>lhs</code>からのSOCCC</td>
<td><code>lhs</code></td>
<td>新規に<a class="cpprefjp-defined-word" data-desc="デフォルト初期化。`T x;`構文での初期化。トリビアルにデフォルト構築可能な型の場合は初期化されず（オブジェクトのlifetimeは開始される）、そうではないクラス型はデフォルトコンストラクタで初期化する。配列型の場合は各要素が再帰的にデフォルト初期化される">デフォルト構築</a></td>
<td><code>lhs</code>からのSOCCC</td>
</tr>
<tr>
<td>(12) : <code>std::move(lhs) + 'r'</code></td>
<td><code>lhs</code></td>
<td><code>lhs</code></td>
<td><code>lhs</code></td>
<td><code>lhs</code></td>
<td><code>lhs</code></td>
</tr>
<tr>
<td>(13) : <code>lhs + sv</code></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td><code>lhs</code>からのSOCCC</td>
</tr>
<tr>
<td>(14) : <code>std::move(lhs) + sv</code></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td><code>lhs</code></td>
</tr>
</tbody>
</table>
<p>表にあるように、C++17までの仕様に完全に準拠している実装は無かった上に各実装によって伝播仕様がバラバラだったため、この変更によって影響を受けるコードはほぼ無いと思われる。</p>
<h2>例</h2>
<h3>基本的な使い方</h3>
<p><div class="yata" id="5a5b944643b7eb709ec1dc76bdaf86231a131e33"><div class="codehilite"><pre><span></span><code><span class="cp">#include <a href="../../iostream.html"><iostream></a></span>
<span class="cp">#include <a href="../../string.html"><string></a></span>
<span class="kt">int</span><span class="w"> </span><span class="nf">main</span><span class="p">()</span>
<span class="p">{</span>
<span class="w"> </span><span class="n"><a href="../basic_string.html">std::string</a></span><span class="w"> </span><span class="n">s1</span><span class="p">(</span><span class="s">"Hell"</span><span class="p">);</span>
<span class="w"> </span><span class="n"><a href="../basic_string.html">std::string</a></span><span class="w"> </span><span class="n">s2</span><span class="p">(</span><span class="s">"world"</span><span class="p">);</span>
<span class="w"> </span><span class="n"><a href="../basic_string.html">std::string</a></span><span class="w"> </span><span class="n">s3</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">s1</span><span class="w"> </span><span class="n"><span style="color:#ff0000">+</span></span><span class="w"> </span><span class="sc">'o'</span><span class="w"> </span><span class="n"><span style="color:#ff0000">+</span></span><span class="w"> </span><span class="s">", "</span><span class="w"> </span><span class="n"><span style="color:#ff0000">+</span></span><span class="w"> </span><span class="n">s2</span><span class="w"> </span><span class="n"><span style="color:#ff0000">+</span></span><span class="w"> </span><span class="sc">'!'</span><span class="p">;</span>
<span class="w"> </span><span class="n"><a href="../../iostream/cout.html">std::cout</a></span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n">s3</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="sc">'\n'</span><span class="p">;</span>
<span class="p">}</span>
</code></pre></div>
</div></p>
<h4>出力</h4>
<p><pre><code>Hello, world!
</code></pre></p>
<h3>string_viewとの連結 (C++26)</h3>
<p><div class="yata" id="140d8b7a3041e4dc85e9f9f24b88f3060d371279"><div class="codehilite"><pre><span></span><code><span class="cp">#include <a href="../../iostream.html"><iostream></a></span>
<span class="cp">#include <a href="../../string.html"><string></a></span>
<span class="cp">#include <a href="../../string_view.html"><string_view></a></span>
<span class="kt">int</span><span class="w"> </span><span class="nf">main</span><span class="p">()</span>
<span class="p">{</span>
<span class="w"> </span><span class="n"><a href="../basic_string.html">std::string</a></span><span class="w"> </span><span class="n">s</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="s">"Hello"</span><span class="p">;</span>
<span class="w"> </span><span class="n"><a href="../../string_view/basic_string_view.html">std::string_view</a></span><span class="w"> </span><span class="n">sv</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="s">", World!"</span><span class="p">;</span>
<span class="w"> </span><span class="c1">// string <span style="color:#ff0000">+</span> <a href="../../string_view/basic_string_view.html">string_view</a> (13)</span>
<span class="w"> </span><span class="n"><a href="../basic_string.html">std::string</a></span><span class="w"> </span><span class="n">result1</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">s</span><span class="w"> </span><span class="n"><span style="color:#ff0000">+</span></span><span class="w"> </span><span class="n">sv</span><span class="p">;</span>
<span class="w"> </span><span class="n"><a href="../../iostream/cout.html">std::cout</a></span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n">result1</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="sc">'\n'</span><span class="p">;</span>
<span class="w"> </span><span class="c1">// string&& <span style="color:#ff0000">+</span> <a href="../../string_view/basic_string_view.html">string_view</a> (14)</span>
<span class="w"> </span><span class="n"><a href="../basic_string.html">std::string</a></span><span class="w"> </span><span class="n">result2</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n"><a href="../basic_string.html">std::string</a></span><span class="p">(</span><span class="s">"Hello"</span><span class="p">)</span><span class="w"> </span><span class="n"><span style="color:#ff0000">+</span></span><span class="w"> </span><span class="n">sv</span><span class="p">;</span>
<span class="w"> </span><span class="n"><a href="../../iostream/cout.html">std::cout</a></span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n">result2</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="sc">'\n'</span><span class="p">;</span>
<span class="p">}</span>
</code></pre></div>
</div></p>
<h4>出力</h4>
<p><pre><code>Hello, World!
Hello, World!
</code></pre></p>
<h2>関連項目</h2>
<table border="1" bordercolor="#888" style="border-collapse:collapse">
<thead>
<tr>
<th>名前</th>
<th>説明</th>
</tr>
</thead>
<tbody>
<tr>
<td><code><a href="append.html">append</a></code></td>
<td>文字／文字列を追加する</td>
</tr>
<tr>
<td><code><a href="push_back.html">push_back</a></code></td>
<td>文字を追加する</td>
</tr>
<tr>
<td><code><a href="insert.html">insert</a></code></td>
<td>文字／文字列を挿入する</td>
</tr>
</tbody>
</table>
<h2>バージョン</h2>
<h3>言語</h3>
<ul>
<li>C++03</li>
<li>C++26 : (13), (14)</li>
</ul>
<h3>処理系</h3>
<ul>
<li><a href="../../../implementation.html#clang">Clang</a>: ??</li>
<li><a href="../../../implementation.html#gcc">GCC</a>: ??</li>
<li><a href="../../../implementation.html#icc">ICC</a>: ??</li>
<li><a href="../../../implementation.html#visual_cpp">Visual C++</a>: ??</li>
</ul>
<h2>参照</h2>
<ul>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p1165r1.html" target="_blank">P1165R1 Make stateful allocator propagation more consistent for operator+(basic_string)</a></li>
<li><a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p0980r1.pdf" target="_blank">P0980R1 Making <code>std::string</code> constexpr</a></li>
<li><a href="https://open-std.org/jtc1/sc22/wg21/docs/papers/2024/p2591r5.html" target="_blank">P2591R5 Concatenation of strings and string views</a><ul>
<li>C++26で<code>string_view</code>との連結が追加された</li>
</ul>
</li>
</ul></div>
</div>
</div>
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