为什么std :: map需要一对？

Question

That syntax of:

std::map<int, int> m;
m.insert(std::make_pair(1, 42));

seems a bit crazy.

Why is there no alternative insert(K k, V v) method which would provide a much saner:

std::map<int, int> m;
m.insert(1, 42);

Yes, I'm aware of m[1] = 42 , but it has its own problems (creating an extra copy of the value object).

Answer 1

I can't tell you why that construct isn't allowed. Perhaps to keep insert similar to other containers' insert method. However, since c++11, there is map::emplace that does what you want.

std::map<int, int> m;
m.emplace(1, 42);

Answer 2

m.insert(x, y); forces a copy of the two parameters; whereas m.insert(std::make_pair(1, 42)); allows the call to get a const& pair which avoids all copying. That way, map can contain uncopyable objects (or heavy-duty copy objects)