如何有效地使用emplace_back（对）？

Question

I have

using namespace std;   // for convenience in SO question only
vector<pair<array<int,3>, int>> foo;

and want to emplace_back an element with pair::first holding {i,j,k} and pair::second holding q . The only way I could get this compiling was with the rather clumsy

foo.emplace_back(piecewise_construct,
                 forward_as_tuple(i,j,k),
                 forward_as_tuple(q));

Is this efficient (ie is it guaranteed that the tuple s will be optimised away)? Or is there another way that is guaranteed efficient?

(I tried

foo.emplace_back(std::initializer_list<int>{i,j,k}, q);

but to no avail with gcc 4.8.1). I know that I can avoid this problem by defining

struct element : std::pair<std::array<int,3>,int>
{
  element(int i, int j, int k, int q)
  { first={i,j,k}; second=q; }
};

vector<element> foo;
foo.emplace_back(i,j,k,q);

but I'd prefer to do without such extra code.

Answer 1

std::array<T, N> doesn't have any constructor taking a std::initializer_list<U> for any U , not even for U = T . Don't mistake the fact that it can be initialised from a braced-init-list for the presence of such a constructor.

Instead, actually create an array.

foo.emplace_back(std::array<int,3>{i,j,k}, q);

There are hardly any guarantees about efficiency, and this is not guaranteed to be more efficient than your tuple approach. In practice, it shouldn't make much of a difference, I expect implementations to optimise both forms of emplace_back to the same machine code. If you really care, and you doubt that, measure it.