使用emplace_back向向量添加元素时，如何强制编译器使用默认构造函数？

Question

How can I force compiler to use default constructor when adding element to vector using emplace_back?

#include <vector>
#include <utility>

enum E {E1, E2};

struct A
{
  A(){}
  A(int i){}
};

int main()
{
  std::vector<std::pair<E, A>> testMap;

  testMap.emplace_back(E1, 10);
  // testMap.emplace_back(E2);
}

Commented out line gives following error: error C2664: 'std::pair<_Ty1,_Ty2>::pair(const std::pair<_Ty1,_Ty2> &)' : cannot convert parameter 1 from 'E' to 'const std::pair<_Ty1,_Ty2> &'

Answer 1

The accepted answer does call A 's default constructor. But it also calls A 's compiler-supplied move constructor. If you would like to call only A 's default constructor, you can do this:

testMap.emplace_back(std::piecewise_construct, std::forward_as_tuple(E1),
                                               std::forward_as_tuple());

Or here is a minor re-spelling:

testMap.emplace_back(std::piecewise_construct, std::forward_as_tuple(E1),
                                               std::tuple<>{});

These rewrites are admittedly much uglier than:

testMap.emplace_back(E1, A{});

But if saving a call to A(A&&) is something you want to do, this is how to do it.

Answer 2

Just like you can't construct a std::pair<E, A> from just E1 , you can't emplace it either. So just provide a default-constructed A :

testMap.emplace_back(E1, A{});