In fact your example would build if A’s constructor were implemented in a compile unit that knows the type of B.
An std::vector instance has a fixed size, no matter what T is, since it contains, as others said before, only a pointer to T. But the vector’s constructor depends on the concrete type. Your example doesn’t compile because A() tries to call the vector’s ctor, which can’t be generated without knowing B. Here’s what would work:
A’s declaration:
// A.h
#include <vector>
class B; // Forward declaration.
class A
{
public:
A(); // only declare, don't implement here
private:
std::vector<B> v;
};
A’s implementation:
// A.cpp
#include "A.h"
#include "B.h"
A::A() // this implicitly calls vector<B>'s constructor
{
std::cout << v.size() << std::endl;
}
Now a user of A needs to know only A, not B:
// main.cpp
#include "A.h"
int main()
{
A a; // compiles OK
}