what about std::make_pair?

This is a discussion on what about std::make_pair? within the C++ Programming forums, part of the General Programming Boards category; hi, what is it for? stl docu says template <class T1, class T2> pair<T1, T2> make_pair(const T1& x, const T2& ...

  1. #1
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    what about std::make_pair?

    hi,

    what is it for?

    stl docu says

    template <class T1, class T2>
    pair<T1, T2> make_pair(const T1& x, const T2& y)

    Equivalent to pair<T1, T2>(x, y). This is a global function, not a member function. It exists only for the sake of convenience.
    looking at the implementation of the VS2005 stl is seems to do something else:

    Code:
    template<class _Ty1,
    	class _Ty2> inline
    	pair<_Ty1, _Ty2> make_pair(_Ty1 _Val1, _Ty2 _Val2)
    	{	// return pair composed from arguments
    	return (pair<_Ty1, _Ty2>(_Val1, _Val2));
    	}
    it takes the pair members by value instead of const reference adding 2 copy constructor calls to one constructor call to pair(), a copy constructor call for the the return by value and another copy constructor call if the client code also catches by value instead of by const reference ... compared to a singe constructor call to pair().

    what kind of convenience is this? has std::make_pair any sense/advantage in any situation?

    thank you!

  2. #2
    C++ Witch laserlight's Avatar
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    Quote Originally Posted by pheres
    it takes the pair members by value instead of const reference adding 2 copy constructor calls to one constructor call to pair(), a copy constructor call for the the return by value and another copy constructor call if the client code also catches by value instead of by const reference ... compared to a singe constructor call to pair().
    Return value optimisation may make all these extra copies amount to nothing.

    Quote Originally Posted by pheres
    what kind of convenience is this? has std::make_pair any sense/advantage in any situation?
    The primary advantage is that it allows you to avoid having to repeat yourself by explicitly stating the template arguments.
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

  3. #3
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    That sounds reasonable. thanks!

  4. #4
    The larch
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    The standard library has a lot of "convenience" functions like that (in functional). I often add similar functions to classes where most/all arguments could be deduced from the constructor - particularly if the arguments have complex types.
    I might be wrong.

    Thank you, anon. You sure know how to recognize different types of trees from quite a long way away.
    Quoted more than 1000 times (I hope).

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