std::map emplace without copying value

The arguments you pass to map::emplace get forwarded to the constructor of map::value_type, which is pair<const Key, Value>. So you can use the piecewise construction constructor of std::pair to avoid intermediate copies and moves.

std::map<int, Foo> m;

m.emplace(std::piecewise_construct,
          std::forward_as_tuple(1),
          std::forward_as_tuple(2.3, "hello"));

Live demo

In C++17 this can more easily be achieved with the try_emplace method.

map<int,Foo> m;
m.try_emplace(1, 2.3, "hello");

This addition to the standard library was covered in paper N4279 and should already be supported in Visual Studio 2015, GCC 6.1 and LLVM 3.7 (the libc++ library).