Copy elision when creating object inside emplace()

Question

I see a lot of code at work where people use emplace and emplace_back with a temporary object, like this:

struct A {
    A::A(int, int);
};

vector v;
v         


        

Answer 1

The easy answer is no; elision doesn't work with perfect forwarding. But this is c++ so the answer is actually yes.

It requires a touch of boilerplate:

struct A {
  A(int, int){std::cout << "A(int,int)\n"; }
  A(A&&){std::cout<<"A(A&&)\n";}
};

template
struct maker_t {
  F f;
  template
  operator T()&&{ return f(); }
};

template
maker_t> maker( F&& f ) { return {std::forward(f)}; }

vector v;
v.emplace_back(maker([]{ return A(1,2); }));

live example.

Output is one call to A(int,int). No move occurs. In c++17 the making doesn't even require that a move constructor exist (but the vector does, as it thinks it may have to move the elements in an already allocated buffer). In c++14 the moves are simply elided.