Внутренний приватный класс

aclass.cpp

#include <aclass.hpp>

A::A() {
    x = 30;
}

int A::f() {
    x += 10;
    b.f();
    return x + b.getX();
}

A::B::B() {
    x = 138;
}

void A::B::f() {
    x++;
}

int A::B::getX() {
    return x;
}

aclass.hpp

#pragma once 

class A {
  private:
    class B {
      public:
        B();
        void f();
        int getX();

      private:
        int x;
    };

  public:
    A();
    int f();

  private:
    B b;
    int x;
};

main.cpp

#include <aclass.hpp>
#include <iostream>

int main() {
    A a;
    std::cout << a.f() << std::endl;
}

Makefile

all: main

bin:
	mkdir bin

bin/aclass.o: src/aclass.cpp | bin
	g++ -c -g -Iinclude -std=c++11 src/aclass.cpp -o bin/aclass.o

bin/main.o: src/main.cpp | bin
	g++ -c -g -Iinclude -std=c++11 src/main.cpp -o bin/main.o

main: bin/aclass.o bin/main.o
	g++ bin/aclass.o bin/main.o -o main

clean:
	rm -Rf bin main