我们可以有递归宏吗？ [英] Can we have recursive macros?

问题描述

我想知道是否可以在 C / C ++中使用递归宏？如果是，请提供一个示例。

I want to know if we can have recursive macros in C/C++? If yes, please provide a sample example.

第二件事：为什么我无法执行以下代码？我在做什么错误？是因为递归宏？

Second thing: why am I not able to execute the below code? What is the mistake I am doing? Is it because of recursive macros?

# define pr(n) ((n==1)? 1 : pr(n-1))
void main ()
{
    int a=5;
    cout<<"result: "<< pr(5) <<endl;
    getch();
}

推荐答案

只有预处理，而不是实际编译。如果你想在宏中找到一个问题，这是有用的。例如使用 g ++ -E ：

Your compiler probably provides an option to only pre-process, not actually compile. This is useful if you are trying to find a problem in a macro. For example using g++ -E:

> g++ -E recursiveMacro.c

# 1 "recursiveMacro.c"
# 1 "<built-in>"
# 1 "<command line>"
# 1 "recursiveMacro.c"

void main ()
{
    int a=5;
    cout<<"result: "<< ((5==1)? 1 : pr(5 -1)) <<endl;
    getch();
}

正如你所看到的，它不是递归的。 pr（x）在预处理期间只更换一次。重要的是要记住的是，所有的预处理器是盲目地用另一个替换一个文本字符串，它实际上不评估表达式（x == 1）。

As you can see, it is not recursive. pr(x) is only replaced once during pre-processing. The important thing to remember is that all the pre-processor does is blindly replace one text string with another, it doesn't actually evaluate expressions like (x == 1).

您的代码无法编译的原因是 pr（5 -1）处理器，因此它在源代码中调用未定义的函数。

The reason your code will not compile is that pr(5 -1) was not replaced by the pre-processor, so it ends up in the source as a call to an undefined function.

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