Question

For example:

std::vector<std::pair<std::string, bool > > v;
std::pair<std::string, bool> pr;
v.push_back( pr );

Assuming std::pair has defined a move assignment operator. Will the call to v.push_back automatically use the
move assignment or I need to specifically ask for it like so?

v.push_back( std::move(pr) );

Answer 1

You will only get a move if the argument of the function (in this case, the argument of push_back) is an rvalue, as well as in certain situations when you return objects from a function.

In your example, pr is not an rvalue, so you won’t get it moved.

However, if you – for example – pass a temporary object to the vector, like this:

v.push_back(std::pair<std::string,bool>());

This will be an rvalue and trigger a move.

You can also trigger the move by explicitly casting the argument to an rvalue in the way you suggested:

v.push_back(std::move(pr));

Note, however, that in this case you won’t be able to use pr after the call in a meaningful way any more as its contents have been moved away.

(Of course, another precondition for a move is that the function you call actually accepts rvalue references. For vector push_back this is indeed the case.)