C ++预处理器如何工作? [英] How does C++ Preprocessors work?

问题描述

我目前正在研究开源C ++项目，以了解有关C ++的更多信息.我正在查看的项目之一位于:

I am currently looking at open source C++ projects to learn more about C++. One of the project I am look at is located at:

https://github.com/Myzilla-Web-Resources/OpenBlox/

我了解大多数来源，但我不了解的是他如何使用预处理器声明C ++类.

I understand most of the source, but something I don't understand is how he is using the Preprocessors to declare a C++ class.

例如，

static_init.h

static_init.h

#define DECLARE_STATIC_INIT(ClassName) \
static void static_init_func(); \
static OpenBlox::static_init_helper ClassName##_static_init_helper

namespace OpenBlox{
class static_init_helper{
    public:
        static_init_helper(init_func_type f){
            static_init::instance().add_init_func(f);
        }
};

Instance.h

Instance.h

#define DECLARE_CLASS(Class_Name) \
    virtual Instance* cloneImpl(); \
    virtual QString getClassName(); \
    virtual int wrap_lua(lua_State* L); \
    DECLARE_STATIC_INIT(Class_Name); \
    protected: \
        static QString ClassName; \
        static QString LuaClassName

Frame.h

namespace ob_instance{
class Frame: public GuiObject{
    public:
        Frame();
        virtual ~Frame();

        virtual void render();

        DECLARE_CLASS(Frame);
};
}
#endif

请注意，Frame.h继承了所有Instance.h函数/处理器.有人可以向我解释这是如何工作的吗?

Please note that Frame.h inherited all of Instance.h functions/processors. Could someone explain to me how this work?

推荐答案

预处理器只是查找和替换文本，因此在显示的 Frame 定义中，预处理器首先看到 DECLARE_CLASS(框架)，并将其替换为 DECLARE_CLASS 宏的内容，成为

The preprocessor is just a text find and replace, so in the Frame definition you showed, the preprocessor first sees DECLARE_CLASS(Frame) and replaces it with the content of the DECLARE_CLASS macro, becoming

namespace ob_instance{
class Frame: public GuiObject{
    public:
        Frame();
        virtual ~Frame();

        virtual void render();

        virtual Instance* cloneImpl();
        virtual QString getClassName();
        virtual int wrap_lua(lua_State* L);
        DECLARE_STATIC_INIT(Frame);
    protected:
        static QString ClassName;
        static QString LuaClassName;
};
}

(我整理了格式，实际上整个替换文本都在一行上.)

(I cleaned up the formatting, in reality the entire replacement text is on one line).

然后备份到插入文本之前，再次开始通读，并看到 DECLARE_STATIC_INIT(Frame)并替换为:

It then backs up to just before the text it inserted, starts reading through again, and sees DECLARE_STATIC_INIT(Frame) and replaces that:

namespace ob_instance{
class Frame: public GuiObject{
    public:
        Frame();
        virtual ~Frame();

        virtual void render();

        virtual Instance* cloneImpl();
        virtual QString getClassName();
        virtual int wrap_lua(lua_State* L);
        static void static_init_func();
        static OpenBlox::static_init_helper Frame_static_init_helper;
    protected:
        static QString ClassName;
        static QString LuaClassName;
};
}

( ## 是令牌连接运算符)

为您提供最终的 Frame 类定义.

Giving you the final Frame class definition.

正如Chris Beck在评论中提到的那样，您可以使用 -E 标志对gcc或clang进行编译，使编译器输出预处理的文件，而不是对其进行编译.

As mentioned by Chris Beck in the comments, you can use the -E flag to gcc or clang to have the compiler output the preprocessed file instead of compiling it.

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