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PONeed help understanding using C++ map as an associative array
 

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  1. PONeed help understanding using C++ map as an associative array
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    <p>I was going through Josuttis's "Using Map's as associative arrays" (from <a href="http://www.cppstdlib.com/" rel="nofollow noreferrer">The C++ Standard Library - A Tutorial and Reference, 2nd Edition</a>) and came across <a href="https://stackoverflow.com/questions/1540182/using-a-stdmap-as-an-associative-array">Using a std::map as an associative array</a> on Stack Overflow. Now I have more questions on the constructors that are called when inserting into a map.</p> <p>Here is my sample program (not using best coding practices; please excuse me for that):</p> <pre><code>class C { public: string s; C() { cout &lt;&lt; "default " &lt;&lt; endl;} C(const string&amp; p) : s(p) { cout &lt;&lt; "one param" &lt;&lt; endl;} C(const C&amp; obj) { if (this != &amp;obj) { s = obj.s; } cout &lt;&lt; "copy constr" &lt;&lt; endl; } C&amp; operator = (const C&amp; obj) { if (this != &amp;obj) { s = obj.s; } cout &lt;&lt; "copy initializer" &lt;&lt; endl; return *this; } }; int main() { map&lt;int,C&gt; map1; C obj("test"); cout &lt;&lt; "Inserting using index" &lt;&lt; endl; map1[1] = obj; cout &lt;&lt; "Inserting using insert / pair" &lt;&lt; endl; map1.insert(make_pair(2,obj)); } </code></pre> <p>The output for this program is:</p> <pre><code>one param Inserting using index default copy constr copy constr copy initializer Inserting using insert / pair copy constr copy constr copy constr copy constr </code></pre> <p>I was assuming that initializing the map by index should call the default constructor and followed by the assignment operator. </p> <p>But executing <code>map1[1] = obj</code> creates following output;</p> <pre><code>Inserting using index default copy constr copy constr copy initializer </code></pre> <p>Can someone help me to understand the initialization better?</p>
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    1. USmithuna
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    1. COYou dont need to test for copy construction to self.
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      1. PONeed help understanding using C++ map as an associative array
      1. USMartin York
    2. COAs a matter of fact, now i understand it is better to have pointers than objects in STL containers.
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      1. USmithuna
    3. CO@mithuna: Warning: Storing pointers instead of objects brings lifetime management issues, so I'd say that it is better to store objects. *If* this causes performance issues, you should see if the copy can be optimized. Then, *if* it can't, you could consider storing pointers instead of objects. In that case, you may be interested in the Boost.PointerContainer library (http://www.boost.org/doc/libs/1_40_0/libs/ptr_container/doc/ptr_container.html). ptr_map (http://www.boost.org/doc/libs/1_40_0/libs/ptr_container/doc/ptr_map.html), in the present case.
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      1. USÉric Malenfant
 

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