A simple example of the Pimpl pattern using unique_ptr to store the object. This example is intended to show that although I want to hide the implementation, `unique_ptr` wants to call `A_impl::~A_impl` which isn't available from the header and that this issue can be ameliorated by defining `A::~A() = default;` in the implementation

main.cpp

#include "Pimpl.h"
int main(int argc, char * argv[]) {

    A a;
    a.f();
}

Pimpl.cpp

#include "A.h"
#include <iostream>
class A_impl {
    public:
    void f() const {
        std::cout << "F!" << std::endl;
    }
};

A::A(): _obj{std::make_unique<A_impl>()} {}
//Can still use the implicitly-declared destructor with = default!
A::~A() = default;

void A::f() const {
    assert(_obj);
    _obj->f();
}

Pimpl.h

#pragma once
#include <memory>

// I don't want to expose this class to the public
class A_impl;

class A {
	public:
	A();
  // This requires that Imported be fully defined in the header
	//~A() = default;
  // So instead we have to implement it
	~A();
  void f() const;

	private:
	std::unique_ptr<A_impl> _obj;
};