Externing a C++ class to avoid including its header file

I have created a static library for the following class, libtgbotengine.a and externed the class to be used in a different project.

tgbotengine.h

#include <tgbot/tgbot.h>
// Other headers
// ...
class TgBotEngine{
public:
    // Class constructors and functions
    // ...
    void start();
    
private:
    TgBot::Bot m_bot;
    std::vector<TgBot::BotCommand::Ptr> m_commands;
    // Other members
    // ...
}

extern TgBotEngine myTgBotEngine;

In a different project, I want to link libtgbotengine.a with the following cpp file. My goal is to not to include tgbotengine.h. Is externing myTgBotEngine help me achieve this?

project2.cpp

int main(){
    
    myTgBotEngine.start();
    
    return 0;
}

Answer

The upcoming modules will probably allow what you want to do. Unfortunately, they are still experimental…

The only portable way I know to fully hide the implementation details is to split your class between a public interface and a private implementation. This is common to Java developpers.

Here it would look like:

1. The interface part:

   The header

    // Other headers
    // ...
    class TgBotEngine {
    public:
        // public functions
        // ...
        virtual void start() = 0;
        static std::unique_ptr<TgBotEngine> build(/*ctor parameters*/);
    }
   

   Source:

    #include <tgbot/tgbotimpl.h>
    #include <tgbot/tgbot.h>
   
    std::unique_ptr<TgBotEngine> TgBotEngine::build(/*ctor parameters*/) {
        return std::make_unique<TgBotEngineImpl>(/*ctor parameters*/);
    }
   

2. The implementation part

    #include <tgbot/tgbot.h>
    // Other headers
    // ...
    class TgBotEngineImpl: public TgBotEngine {
    public:
        // Class constructors and functions
        // ...
        void start();
   
    private:
        TgBot::Bot m_bot;
        std::vector<TgBot::BotCommand::Ptr> m_commands;
        // Other members
        // ...
    }
   

You can then use it that way:

#include "tgb.h"
#include <memory>

int main() {
    std::unique_ptr<TgBotEngine> engine = TgBotEngine::build(/* ctor parameters*/);
    engine->start();
    ...
}