Pimpl.cc

//
#ifndef Engine_h
#define Engine_h

// #include <memory>

namespace {
    class Engine {
    public:
        Engine();
        ~Engine();
    private:
        struct Impl;
        //std::unique_ptr<Impl> pImpl;
        Impl *pImpl;
    };
}

#endif /* Engine_h */

// .cc
#include "Engine.h"

namespace {
    
    struct Engine::Impl {
        int test();
        char arr[40];
    };
    
    Engine::Engine() {
        pImpl = new Impl;
    }
    
    Engine::~Engine() {
        delete pImpl;
    }
}