C ++如何在不调用构造函数的情况下分配对象

Question

I have a class:

class C {
public:
    C(): _a(a++){}
private:
    static int a;
    int _a;
};

int C::a = 0;
C c = C(); // c._a is 0
C d = C(); // d._a is 1
C e = C(); // e._a is 2

This works out as I'd expect: a and obj._a increment by 1. But if I assign c, d, and e to an unordered_map, they increment by 2:

unordered_map<int, C> map;
C c = C(); // c._a is 0
map[0] = c;
C d = C(); // d._a is 2
map[1] = d;
C e = C(); // e._a is 4
map[2] = e;

How can I assign c, d, and e to map and still make the static int a increment by 1?

Answer 1

The problem comes from your map. When you access map[0], the map will look for the entry 0 and if it does not exists, will instantiate it and returns to you a reference to the new instance. Try using map.emplace(0, c) instead to add an item in your map.

Answer 2

Your problem is that the expression map[0] is creating a default constructed 'C' (and incrementing the counter), and then that object is assigned to.

The solution is to directly insert the object in the map with:

    map.insert({0,c});