C++ type is incomplete, can't access methods and fields, can't include

Question

I have a problem with C++ project. I'm using resharper c++ for visual studio 2017 if thats relevant.

When I have 2 classes (A, B) and they include each other, sometimes resharper blacks out #include "A.h" in B as it would be unused, even if I have some use of A in class B, and I can't compile, because it doesn't know what "A" is.

Resharper even offers to add #include "A.h" for me, but if i click to accept it, it says:

cannot perform this action most likely because of errors in the source code

I can "fix" it by adding "class A;" just before declaring class B, but then it sees A as it would not have any methods or fields.

Here is minimal complete example of code that generates the error.

A.h file

#ifndef A_H
#define A_H
#include "B.h"

class A
{
public:
    B** bs;

    A() : bs(new B*[5])
    {
        for (int i = 0; i < 5; i++)
        {
            bs[i] = nullptr;
        }
    };

    void do_something_in_A()
    {
        //something
    }
};

#endif

B.h file

#ifndef B_H
#define B_H

#include "A.h"

class A; //if I don't add it I can't compile, if I add it I cannot access methods and fields in A as shown below

class B
{
public:
    A* a;
    B(A* a) : a(a) {};

    void do_something_in_B()
    {
        //a->do_something_in_A(); doesn't work
    }
};

#endif

main.cpp file

#pragma once

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

int main()
{
    A a;
    system("pause");
    return 0;
}

I think that error could be created by circular dependency, but I couldn't find any working solution after searching for it quite long.

If possible, I would like to get not only the answer how to fix it, but some kind of explanation how it works and why is it wrong, so I can avoid similar errors in the future.

I hope I the error well enough. Thank you in advance for any answers.