C预处理器可以执行整数运算吗？ [英] Can the C preprocessor perform integer arithmetic?

问题描述

正如问题所言， C 预处理器能够做到吗？

As the questions says, is the C preprocessor able to do it?

例如：

#define PI 3.1416
#define OP PI/100
#define OP2 PI%100

在预处理阶段是否可以计算OP和/或OP2？

Is there any way OP and/or OP2 get calculated in the preprocessing phase?

推荐答案

整数算术？运行以下程序以查找：

Integer arithmetic? Run the following program to find out:

#include "stdio.h"
int main() {
    #if 1 + 1 == 2
        printf("1+1==2\n");
    #endif
    #if 1 + 1 == 3
        printf("1+1==3\n");
    #endif
 }

答案是是，有一种方法使预处理器执行整数算术，即在预处理器条件下使用它。

Answer is "yes", there is a way to make the preprocessor perform integer arithmetic, which is to use it in a preprocessor condition.

但是请注意，您的示例不是整数算术。我刚刚检查了一下，如果您尝试让gcc进行浮点比较，它的预处理器就会失败。我还没有检查过该标准是否允许预处理器中进行浮点运算。

Note however that your examples are not integer arithmetic. I just checked, and gcc's preprocessor fails if you try to make it do float comparisons. I haven't checked whether the standard ever allows floating point arithmetic in the preprocessor.

常规宏扩展不评估整数表达式，它会留给编译器，可以通过预处理（在gcc中为-E）可以看到以下内容：

Regular macro expansion does not evaluate integer expressions, it leaves it to the compiler, as can be seen by preprocessing (-E in gcc) the following:

#define ONEPLUSONE (1 + 1)
#if ONEPLUSONE == 2
    int i = ONEPLUSONE;
#endif

结果为 int i =（1 + 1 ）； （可能还有一些东西来指示源文件名和行号等）。

Result is int i = (1 + 1); (plus probably some stuff to indicate source file names and line numbers and such).

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