预处理在编译器中到底意味着什么 [英] What does preprocessing exactly mean in compiler

问题描述

我试图理解typedef和define之间的区别.在关于SO的先前问题中，有很多不错的帖子a>，但是我听不懂

I am trying to understand the difference between a typedef and define. There are a lot of good posts specially at this previous question on SO, however I can't understand the post that states

#define 是预处理器令牌:编译器本身将永远看不到它.
typedef 是一个编译器令牌:预处理器并不关心它.

#define is a preprocessor token: the compiler itself will never see it.
typedef is a compiler token: the preprocessor does not care about it.

任何人都可以再详细解释一下.我对这里的预处理器一词感到困惑.

Could anyone please explain this with a little more detail. I am confused with the term preprocessor here.

推荐答案

预处理器是在之前运行的程序，本质上执行文本替换.当您写时:

The pre-processor is a program that runs before the compiler and essentially performs text substitution. When you write:

#define X 10

int main() 
{
    int x = X;
}

预处理器将该文件作为输入，执行该操作并输出:

The pre-processor take that file as input, does it's thing, and outputs:

int main() 
{
    int x = 10;
}

然后，编译器使用预处理后的输出执行操作.

And then the compiler does its thing with the pre-processed output.

typedef 是编译器可以理解的结构.当您写时:

typedef on the other hand is a construct that the compiler understands. When you write:

typedef unsigned int uint_32;

编译器知道 uint32 实际上是 unsigned int 的别名.该别名由编译器本身处理，比简单的文本替换包含更多的逻辑.举一个简单的例子，这很明显:

The compiler knows that uint32 is actually an alias for unsigned int. This alias is handled by the compiler itself and involves a bit more logic than simple text substitution. This becomes obvious with a simple example:

typedef int my_int;

int main()
{
    unsigned my_int x;  // oops, error
}

如果 typedef 是一种简单的文本替换机制(如预处理器一样)，则该方法会起作用，但不允许这样做，并且将无法编译.

If a typedef were a simple text substitution mechanism (like the pre-processor is) then that would work, but it is not allowed and will fail to compile.

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