类成员函数当友元函数，编译错误?报错

1.环境：Win7 + codeBlock + gcc

2.问题：友元函数报错

--------------

G:\projects\friendFun2\A.h|11|error: 'B' has not been declared|
G:\projects\friendFun2\A.h||In member function 'void B::bPrint(A)':|
G:\projects\friendFun2\A.h|14|error: 'int A::m_A' is private|
G:\projects\friendFun2\B.cpp|17|error: within this context|
||=== Build finished: 3 errors, 0 warnings ===|

--------------

代码分为A.h,A.cpp,B.h,B.cpp,main.cpp

A.h

#ifndef A_H
#define A_H

#include "B.h"

class A
{
    public:
        A(int );
        virtual ~A();
        friend void B::bPrint(A aa);
    protected:
    private:
        int m_A;
};

#endif // A_H

B.h

#ifndef B_H
#define B_H


#include "A.h"


class B
{
    public:
        B();
        virtual ~B();
        void bPrint(A aa);
    protected:
    private:
};


#endif // B_H

B.cpp

#include "B.h"
#include <iostream>


using namespace std;
B::B()
{
    //ctor
}


B::~B()
{
    //dtor
}


void B::bPrint(A aa)
{
    cout << "in B :" << aa.m_A << endl;
}

A.cpp没什么好说的。

但是如果和在一个main.cpp的话，就没有上面的问题

#include <iostream>


using namespace std;
class A;


class B
{
    public:
        void bPrint(A aa);
};


class A
{
    public:
        friend void B::bPrint(A aa);
        A(int a):m_A(a){};
    private:
        int m_A;
};


void B::bPrint(A aa)
{
    cout << "in B :" << aa.m_A << endl;
}


int main()
{
    B b;
    A a(6);
    b.bPrint(a);
    return 0;
}