"Exchange" of includes - c++

Question

As you can see, the title is a little nonsense, it's because I really don't know how to call that, but know that I tried hard to find a better title.
I hope you understand what I'm trying to say seeing the rest of the question.
Let's suppose that I have 3 classes, "A", "B", and "C".
The class "A" includes "B" and "C".
The class "B" includes "A"
The class "C" includes "A"
Let's go the code...
#include <iostream>
#include <b.hpp>
#include <c.hpp>
class A
{
public:
A()
{
std::cout << "Hello from A" << std::endl;
A a;
B b;
}
};
#include <iostream>
#include <a.hpp>
class B
{
public:
B()
{
std::cout << "Hello from B" << std::endl;
}
};
#include <iostream>
#include <a.hpp>
class C
{
public:
C()
{
std::cout << "Hello from C" << std::endl;
}
};
This way, everything works fine, the output:
Hello from A
Hello from B
Hello from C
BUT, if I do that:
#include <iostream>
#include <a.hpp>
#include <vector>
class B
{
public:
B()
{
std::cout << "Hello from B" << std::endl;
}
private:
std::vector<A> garage;
};
I got a cascade of errors, including this one (actually, the main one, because of that one, there are others):
error: 'A' was not declared in this scope
std::vector garage;
This is exactly what I want, do you have an idea what I can do? Thank you.
#edit - As answer to #Kerrek SB
I tried to create separated files to each one, header and sources. (.hpp and .cpp) and the error persist.
A.hpp
#ifndef A_HPP
#define A_HPP
#include <iostream>
#include <B.hpp>
#include <C.hpp>
class A
{
public:
A();
};
#endif
A.cpp
#include <A.hpp>
A::A()
{
std::cout << "Hello from A" << std::endl;
B b;
C c;
}
B.hpp
#ifndef B_HPP
#define B_HPP
#include <iostream>
#include <A.hpp>
#include <vector>
class B
{
public:
B();
private:
std::vector<A> garage;
};
#endif
B.cpp
#include <B.hpp>
B::B()
{
std::cout << "Hello from B" << std::endl;
}
C.hpp
#ifndef C_HPP
#define C_HPP
#include <iostream>
#include <A.hpp>
class C
{
public:
C();
};
#endif
C.cpp
#include <C.hpp>
C::C()
{
std::cout << "Hello from C" << std::endl;
}
main.cpp
#include <iostream>
#include <A.hpp>
int main(int argc, char ** argv)
{
A a;
return 0;
}
gcc
g++ -O2 -std=c++11 -I"." -o exchange A.cpp B.cpp C.cpp main.cpp
output
In file included from ./A.hpp:5:0,
from ./C.hpp:5,
from C.cpp:1:
./B.hpp:13:17: error: 'A' was not declared in this scope
std::vector<A> garage;

Answer 1

Separate the class definitions from the class member definitions:
#include <iostream>
#include <vector>
class A { public: A(); };
class B { public: B(); private: std::vector<A> garage; };
class C { public: C(); };
A::A()
{
std::cout << "Hello from A" << std::endl;
// A a; // no, because that's idiotic
B b;
}
B::B()
{
std::cout << "Hello from B" << std::endl;
}
C::C()
{
std::cout << "Hello from C" << std::endl;
}
In a larger project, you would probably have separate header and source files for each of those.