C++ Circular dependency due to enum class nested

Question

I have a circular dependency created by a nested enum.

class A
{
    enum class A_enum {};

    void method_which_uses_class_B() {}
}

class B
{
    void method_which_uses_A_enum() {}
}

That gives you an idea of the overall structure.

Specifically, I have the enum class defined in this way:

... file containing class A

class A
{
    enum class A_enum
    {
        ITEM_A, ITEM_B
    };

    // then the rest of A goes here
}

// in my main code file

#include "class B file"
#include "class A file"

... code

The solution therefore would be to move the enum class out of class A and put code/files/include's in the correct order to prevent the problem.

But it's rather nice to have the enum class nested inside class A for my purposes. Is there a way of pre-defining that there will be a class A and what the enum inside that class will be? I tried the following, but it didn't work because I was defining the class twice:

class A;

enum class A::A_enum
{
    contenta, contentb
};

or

class A
{
    enum class A_enum {...};
}

then

class A
{
    // the rest of it
    // obviously doesn't work because already defined class A
}

Answer 1

The only way to handle this situation is to implement A::method_which_uses_class_B() after the definition of class B.

class A
{
    enum class A_enum {};

    void method_which_uses_class_B();
};

class B
{
    void method_which_uses_A_enum() {}
};

void A::method_which_uses_class_B()
{
}

Answer 2

Forward Declarations can solve this problem:

class A;
class B;

class A {
    public:
        enum class A_enum {};
        void method_which_uses_class_B();
};

class B {
    public:
        void method_which_uses_A_enum();
};

void A::method_which_uses_class_B() {
    A::A_enum instance_of_a_enum;
    A instance_of_a;
    B instance_of_b;
}

void B::method_which_uses_A_enum() {
    A::A_enum instance_of_a_enum;
    A instance_of_a;
    B instance_of_b;
}