How can we move implementation to multiple header file without circularity?

[alwaystudent]

for single .cpp file- and .h file- we can move implementation from .cpp to .h file, but for multiple files, i can not do this for my case and occurred circularity :
my case is something like this (inclusion guard ignored):

//a.h

#include"stdio.h"
#include"b.h"

class A
{
public:
void showfromA(const B& b);
int index;
};

//a.cpp

#include"a.h"
void A::showfromA(const B& b)
{
printf("b.index=%i",b.index);
}

//b.h

class B
{
public:
void showfromB();
int index;
};

//b.cpp

#include"a.h"
void B::showfromB()
{
A a;
a.index=1;
printf("a.index=%i",a.index);
}

//main.cpp

#include"b.h"
main()
{
B b;
b.showfromB();
}
circularity occurred because a.h include b.h but b.cpp include a.h.
**when .h files and .cpp files are separated ,the code is OK** and we do not have circularity but when we try to merge .h file and .cpp file we encounter to circularity between class A and B and compile error. note that i want to move method implantation from .cpp file into * the class definition* in .h file.