Re: map inserts by reference or via copy ctor

Victor Bazarov <>
Thu, 25 Sep 2008 13:17:26 -0400
puzzlecracker wrote:

Does map copy the object, both key and value during the insert?


is this example correct:

std::map<std::string T> my_map;

I think you meant

   std::map<std::string, T> my_map;

template<typename T>
void foo(const std::string str, const T & t){

Passing the string by value? You ought to do

    void foo(const std::string & str, const T & t){

(although it doesn't make it incorrect, per se).

    my_map.insert(make_pair(str, t));

'make_pair' will either create a wrong pair (the second template
argument would be a reference to const, or it will be the right pair,
but then creation of it will make the first copy. The insertion itself
will [also] make a copy since the temporary created by 'make_pair' will
be destroyed at the end of the full expression.


template<typename T>
void bar()
    foo(std::string("Key1"),T(param1, param2));

Unless you have some other overload of 'foo', there is no need to spell
out the type of the first argument, there exists an implicit conversion
from a character pointer to 'std::string'. You could just write

    foo("Key1", T(param1, param2));


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