Declaring template types to call to make_pair

Question

Maybe I'm being stupid, but I don't see what the problem with this code is. I'm starting to resort to doubting my tools. I'm using Clang (C++11), and its libc++. If it's my fault, I apologise and thank in advance whoever points out my stupidity, but if it's not, maybe this is a deeper issue..

std::map<int, bool> map_;

map_.insert(std::make_pair<int, bool>(5, true)); //Use a literal, compiles OK

int x = 5;
map_.insert(std::make_pair<int, bool>(x, true)); //Use a local of same type, compile error: 'no known conversion from 'int' to 'int &&' for 1st argument'

You should not provide template arguments, but let deduction do its job: `std::make_pair(x, true)` — juanchopanza, Jan 19 '14 at 23:54
`std::make_pair` would normally take that and give it a type of `int & &&`, or `int &`. Now you've gone and made it do `int &&`, which can't be used with an lvalue. — chris, Jan 19 '14 at 23:55
Yeah, that fixes it. But I wonder why.. enforcing the template version of the function that matches the arguments should yield the same implementation, surely? — Coran, Jan 19 '14 at 23:56
@ Chris. I see, so the definition adds the & on to the passed type, adding a further level of indirection.. that makes sense now! Thanks — Coran, Jan 19 '14 at 23:58
@Coran, See the term "universal reference" and you'll find all sorts of good resources. — chris, Jan 19 '14 at 23:58
Yes, it appears to be a duplicate, and the other has a more expanded explanation. Though the answer from 0x499602D2 is very good! Thank you — Coran, Jan 20 '14 at 00:05

David G · Accepted Answer · 2014-01-20T14:53:17.307

std::make_pair takes a "universal reference" to its arguments. A universal reference can bind to pretty much anything. This is the actual signature:

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

When you explicitly supply the type (unnecessarily I might add), it turns the function call into this:

... ( int&& t, bool&& u );

Meaning the arguments are no longer universal references but rvalue-references. Rvalue-references can only bind to rvalues, not lvalues, hence the error for the first argument.

The solution to this issue is to allow template argument deduction to do its job:

map_.insert(std::make_pair(x, true));

Declaring template types to call to make_pair

1 Answers1