std::make_pair, c++11 and explicit template parameters [duplicate]

Question

Reedited: first of all, this is just a matter of curiosity, I do know, that std::pair or lots of other solutions could eradicate this issue.

Can you tell me, what is exactly behind of this following problem? This code is a simple example working on c++03 and fails on c++11.

    std::pair<int*,int**> getsth(int* param)
    {
        return std::make_pair<int*,int**>(param, 0);
    }

    int main(int argc, char* argv[])
    {
        int* a = new int(1);
        std::pair<int*,int**> par = getsth(a);
        std::cout << *par.first;
        return 0;
    }

I do know how to fix it to be compatible with both standards here, but it is annyoing me, that I don't know, what is exactly behind make_pair in this case.

Thanks!

edited: a compile error message from Coliru:

main.cpp: In function 'std::pair<int*, int**> getsth(int*)':
main.cpp:8:47: error: no matching function for call to 'make_pair(int*&, int)'
     return std::make_pair<int*,int**>(param, 0);
                                               ^
main.cpp:8:47: note: candidate is:
In file included from /usr/local/include/c++/4.9.2/bits/stl_algobase.h:64:0,
                 from /usr/local/include/c++/4.9.2/bits/char_traits.h:39,
                 from /usr/local/include/c++/4.9.2/ios:40,
                 from /usr/local/include/c++/4.9.2/ostream:38,
                 from /usr/local/include/c++/4.9.2/iostream:39,
                 from main.cpp:1:
/usr/local/include/c++/4.9.2/bits/stl_pair.h:276:5: note: template<class _T1, class _T2> constexpr std::pair<typename std::__decay_and_strip<_Tp>::__type, typename std::__decay_and_strip<_T2>::__type> std::make_pair(_T1&&, _T2&&)
     make_pair(_T1&& __x, _T2&& __y)
     ^
/usr/local/include/c++/4.9.2/bits/stl_pair.h:276:5: note:   template argument deduction/substitution failed:
main.cpp:8:47: note:   cannot convert 'param' (type 'int*') to type 'int*&&'
     return std::make_pair<int*,int**>(param, 0);
                                               ^
main.cpp:9:1: warning: control reaches end of non-void function [-Wreturn-type]
 }

Answer 1

Rvalue references happened. Where std::make_pair in C++03 has the signature

template< class T1, class T2 >
std::pair<T1,T2> make_pair( T1 t, T2 u );

In C++11, it has

template< class T1, class T2 >
std::pair<V1,V2> make_pair( T1&& t, T2&& u );

V1 and V2 are (usually) std::decay<T1|T2>::type. C++14 adds constexpr, but that does concern us here.

This means that the explicit function template specialization std::make_pair<int*, int**> accepted, in C++03, parameters of type int* and int**, while the new one in C++11 accepts int*&& and int**&&.

Binding 0 to int**&& is not a problem, but param is an lvalue and cannot be bound to an rvalue reference to int*. And that is why your code explodes in C++11.

For this reason,

return std::make_pair<int*&, int**>(param, 0);

works with both C++03 and C++11 -- param can be bound to int*&, and the resulting std::pair<int*&, int**> can be converted to the std::pair<int*, int**> that the function wants to return in both revisions.

That is rather ugly, though, and std::make_pair is not really meant to be used this way. As @T.C. points out in the comments, if you know the types the std::pair should have, just use

return std::pair<int*, int**>(param, 0);

Answer 2

You need to remove the template parameters of make_pair. Here is more info on why.

Also, you should pass nullptr to make_pair, not 0.