C++ pimpl example

// PIMPL
#include <iostream>
#include <string>

template <typename T>
class Pimpl {
protected:
    T *const impl;
public:
    Pimpl();
    ~Pimpl();
};

template <typename T>
Pimpl<T>::Pimpl(): impl(new T()) {}

template <typename T>
Pimpl<T>::~Pimpl() {
    delete impl;
}

class Aimpl;

class A: public Pimpl<Aimpl> {
public:
    void inc();
    void dec();
    friend std::ostream& operator<<(std::ostream& os, const A& a);
};

class Aimpl {
public:
    int i = 0;
};

void A::inc() {
    ++impl->i;
}

void A::dec() {
    --impl->i;
}

std::ostream& operator<<(std::ostream& os, const A& a) {
    os << a.impl->i << "\n";
    return os;
}

int main()
{
  A a;
  a.inc();
  std::cout << a;
  a.dec();
  std::cout << a;
}