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POWhat happened when call std::map's operator[] or insert
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  1. POWhat happened when call std::map's operator[] or insert
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    copied!<p>I have following code:</p> <pre><code>#include &lt;functional&gt; // std::less #include &lt;map&gt; #include &lt;iostream&gt; using namespace std; class Key { public: Key() {cout &lt;&lt; "Key Constructor" &lt;&lt; endl;} ~Key() {cout &lt;&lt; "Key Destructor" &lt;&lt; endl;} Key(const Key&amp; key) {cout &lt;&lt; "Key Copy Constructor" &lt;&lt; endl;} bool operator &lt; (const Key&amp; k1) {return true;} }; int main() { map&lt;Key, int&gt; mymap; Key k; cout &lt;&lt; "operator[]"&lt;&lt;endl; mymap[k] = 1; map&lt;Key, int&gt; mymap2; cout &lt;&lt; "insert"&lt;&lt;endl; mymap2.insert(std::make_pair(k, 1)); cout &lt;&lt; "=========" &lt;&lt; endl; } </code></pre> <p>And the output is:</p> <pre><code>$ g++ test.cpp -fpermissive $ ./a.out Key Constructor operator[] Key Copy Constructor Key Copy Constructor Key Destructor insert Key Copy Constructor Key Copy Constructor Key Copy Constructor Key Copy Constructor Key Destructor Key Destructor Key Destructor ========= Key Destructor Key Destructor Key Destructor </code></pre> <p>Could anyone please explain why mymap[k] = 1; invoke 2 copy constructor and mymap2.insert(std::make_pair(k, 1)); invokes 4 copy constructor? and does that mean operator[] is much more efficient than insert?</p> <p>Thanks.</p> <p><strong>Summary:</strong></p> <p>Thanks user 6502 and petersohn for your insight, I now guess the reason for the 2 extra copy constructor for insert is as below:</p> <ul> <li><a href="http://www.cplusplus.com/reference/utility/make_pair/?kw=make_pair" rel="nofollow">make_pair</a> is a function, it make a copy inside function first, and then return the copy - that is one extra copy</li> <li>make_pair(k, 1) will create a <code>pair&lt;Key, int&gt;</code>, but the required <a href="http://www.cplusplus.com/reference/map/map/" rel="nofollow">value_type</a> is <code>pair&lt;const&amp; Key, int&gt;</code>, the type conversion will cause another extra copy</li> </ul> <p>So in case 2, if I use:</p> <pre><code>mymap2.insert(std::pair&lt;const Key, int&gt;(k, 1)); </code></pre> <p>The number of copy constructors get called will be the same with operator[]</p> <p>As 6502 noted, following <a href="http://www.cplusplus.com/reference/map/map/operator%5B%5D/" rel="nofollow">claim</a> has been changed hence not true anymore:</p> <blockquote> <p>A call to this function(operator[]) is equivalent to: (*((this->insert(make_pair(x,mapped_type()))).first)).second</p> </blockquote> <p>operator[] is implemented different to avoid extra copy introduced by make_pair()</p>
 

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