C++:使类及其某些数据成员仅在 namespace 中可用

Question

是否可以让一个类只在命名空间内可用？还是有另一种方式，不使用命名空间？我正在努力创建一个框架，但不希望该框架的用户可以访问所有类，而只能访问特定类。

但是:无论如何，用户应该能够访问所有定义以创建指向这些类的指针变量。此外，他不应该能够访问这些类的所有数据成员，但我希望我的框架能够访问所有数据成员。

这可能吗？

示例(只是对我的请求的解释):

/* T2DApp.h */
namespace T2D {
    // I don't want the user to be able to create an instance of this class (only pointer vars), but the framework should be able to.
    class T2DApp {
    public:
        // constructor, destructor... //

        SDL_Window*  Window;
        SDL_Surface* Surface;

        bool Running = false;
    }
}

/* T2D.h */
#include "T2DApp.h"

void init();

/* T2D.cpp */
#include "T2D.h"

void init() {
    T2D::T2DApp app;       // function in framework is able to create new instance of T2DApp.
    app.Window.Whatever(); // every data member should be available to framework directly without getter methods.
    app.Window.Whatever(); // dito
    app.Running = true;    // dito
}

/* [cpp of user] */
#include "T2D.h"

void main(etc.) {
    ...
    T2D::T2DApp app;    // User shouldn't be able to create an instance of T2DApp
    T2D::T2DApp* p_app; // but he should still be able to "see" the class definition for creating pointers
    ...
    p_app.Running = true;     // User shouldn't be able to access this data member
    p_app.Window.Whatever();  // But he should be able to access the other data members
    p_app.Surface.Whatever(); // dito
    ...
}

非常感谢您:)

Answer 1

可以用 Pimpl idiom :

"Pointer to implementation" or "pImpl" is a C++ programming technique that removes implementation details of a class from its object representation by placing them in a separate class, accessed through an opaque pointer.