c++: how does memberwise assignment work?

Possible Duplicate:
How are C array members handled in copy control functions?

If I don't override the operator = of a class, it will use default memberwise assignment.

But what does it mean?

struct A {
    int array[100];
};
A a;
A b=a;

No error. How does b copes a'sarray? Normally array_b = array_a is invalid.

Another exampe:

struct A {
    vector<int> vec;
};
A a;
A b=a;

How does b copes a'svec? Through assignment(vec_b = vec_a), constructor(vec_b = vector<int>(vec_a)) or other mystery way?