为什么std :: vector不能使用前向声明？

如果我创建一个这样的类:

// B.h
#ifndef _B_H_
#define _B_H_

class B
{
private:
    int x;
    int y;
};

#endif // _B_H_

并像这样使用它:

// main.cpp
#include 
#include 

class B; // Forward declaration.

class A
{
public:
    A() {
        std::cout << v.size() << std::endl;
    }

private:
    std::vector v;
};

int main()
{
    A a;
}

// A.h
#include 

class B; // Forward declaration.

class A
{
public:
    A(); // only declare, don't implement here

private:
    std::vector v;
};

// A.cpp
#include "A.h"
#include "B.h"

A::A() // this implicitly calls vector's constructor
{
    std::cout << v.size() << std::endl;
}

// main.cpp
#include "A.h"

int main()
{
    A a; // compiles OK
}