loop quoting , why error happens

[misserwell]

I have two class as below: I want to test how class to work:
#include <iostream>

using namespace std;

class A
{
private:
B bInstance;
public:
A()
{
cout<<"A class is created! B class is invoked"<<endl;
}

};

class B
{
private:
A aInstance;
public:
B()
{
cout<<"B class is created! A class is invoked"<<endl;
}

};

int main()
{
A a;
B b;
}

but when I compile them in a file , I always get an error :(ClassThink.cpp is the name of saved file)
ClassThink.cpp:8: error: 'B' does not name a type
BTW, I use g++ compiler , thanks in advance

[ultifinitus]

You haven't declared the 'B' class before your declaration of your 'A' class. So when you try to make an instance of the aforementioned class, it throws an error. The compiler doesn't know what you're talking about.

Just throw a prototype at the top.

class B;

should do it.

[alex58765]

It is because class B is created after class A, so you can only use class A in class B.

[firedraco]

Actually the main problem here is that A contains a B. However, B also contains an A. And it goes in circles forever...You'll need to change them to pointers or something probably (or at least one to a pointer).

Then you can forward declare in .hpp and #include in the .cpp.

Argh, Articles section isn't up yet...

[misserwell]

Thank you all, especially firedraco , I got