This is an example of the pimpl idiom using smart pointers.

CMakeLists.txt

cmake_minimum_required(VERSION 3.16)
project(pimpl_idiom)

set(CMAKE_CXX_STANDARD 17)

add_library(timer timer.cpp)

add_executable(pimpl_idiom main.cpp)
target_link_libraries(pimpl_idiom timer)

main.cpp

#include <iostream>
#include <chrono>
#include <thread>
#include "timer.h"

int main() {
    AutoTimer at("Angelos");
    at.UpdateName("Aris");
    std::this_thread::sleep_for(std::chrono::seconds (2));
    return 0;
}

timer.cpp

//
// Created by angelos on 30/05/2020.
//

#include "timer.h"
#include <iostream>
#if WIN32
#include <windows.h>
#else
#include <ctime>
#endif

class AutoTimer::TimerImpl {
public:
    [[nodiscard]] double GetElapsed() const {
#ifdef WIN32
        return (GetTickCounts() - mStartTime) / 1e3;
#else
        time_t end_time{};
        time(&end_time);
        return difftime(end_time, mStartTime);
#endif
    }

    std::string mName;
#ifdef WIN32
    DWORD mStartTime;
#else
    time_t mStartTime{};
#endif
};

AutoTimer::AutoTimer(std::string_view name) : mImpl{new AutoTimer::TimerImpl} {
    mImpl->mName = name;
#ifdef WIN32
    mImpl->mStartTime = GetTickCount();
#else
    time(&mImpl->mStartTime);
#endif
}

AutoTimer::~AutoTimer() {
    std::cout << mImpl->mName << ": took " << mImpl->GetElapsed() << " secs" << std::endl;
}

void AutoTimer::UpdateName(std::string_view new_name) const {
    mImpl->mName = new_name;
}

timer.h

//
// Created by angelos on 30/05/2020.
//

#ifndef PIMPL_IDIOM_TIMER_H
#define PIMPL_IDIOM_TIMER_H

#include <memory>
#include <string_view>

class AutoTimer {
public:
    explicit AutoTimer(std::string_view name);
    ~AutoTimer();

    void UpdateName(std::string_view new_name) const;

private:
    class TimerImpl;
    std::unique_ptr<TimerImpl> mImpl;
};


#endif//PIMPL_IDIOM_TIMER_H