为什么unordered_map的pieceplace_construct参数的位置需要默认构造函数？

Question

I have an unordered_map which stores <string, A> pairs. I wanted to emplace pairs with this snippet:

        map.emplace(std::piecewise_construct,
            std::forward_as_tuple(name),
            std::forward_as_tuple(constructorArg1, constructorArg1, constructorArg1));

But if my A class doesn't have a default constructor then it fails to compile with this error:

'A::A': no appropriate default constructor available

C:\\Program Files (x86)\\Microsoft Visual Studio 14.0\\VC\\include\\tuple 1180

Why does it need a default constructor and how can I avoid the use of it?

Answer 1

std::unordered_map needs default constructor is because of operator[] . map[key] will construct new element using default constructor if key is not exist in map .

You can totally use map without default constructor. Eg following program will compile with no error.

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

...

std::unordered_map<int, A> b;

b.emplace(std::piecewise_construct,
    std::forward_as_tuple(1),
    std::forward_as_tuple(2));
b.at(1).x = 2;