map::emplace() with a custom value type

Question

I\'m having trouble using map::emplace(). Can anyone help me figure out the right syntax to use? I am effectively trying to do the same thing as in this example. He

Answer 1

GCC 4.7 does not have full support of emplace functions.

You can see C++11 support in GCC 4.7.2 here.

Answer 2

A container's emplace member constructs an element using the supplied arguments.

The value_type of your map is std::pair<const int, Foo> and that type has no constructor taking the arguments { 5, 5, 'a', 'b' } i.e. this wouldn't work:

std::pair<const int, Foo> value{ 5, 5, 'a', 'b' };
map.emplace(value);

You need to call emplace with arguments that match one of pair's constructors.

With a conforming C++11 implementation you can use:

mymap.emplace(std::piecewise_construct, std::make_tuple(5), std::make_tuple(5, 'a', 'b'));

but GCC 4.7 doesn't support that syntax either (GCC 4.8 will when it's released.)