C ++宏乘法发生了什么

问题描述

#define MAX 265

std::cout << 0 * MAX << std::endl; //to my surprise, the output is 9 rather than 0

这个C ++宏乘法？

EDIT ：

以下是完整版本。

#include <stdio.h>
#include <string.h>
#include <iostream>

#define NAME_BYTES 256
#define VERSION_BYTES 256
#define SIZE_BYTES 32
#define USED_LOCK_COUNT_BYTES 32
#define LOCK_NAME_BYTES 256
#define LOCK_TYPE_BYTES 1
#define PID_BYTES 4
#define TID_BYTES 4
#define LOCK_BYTES LOCK_NAME_BYTES + LOCK_TYPE_BYTES + PID_BYTES + TID_BYTES
#define HEADER_BYTES NAME_BYTES + VERSION_BYTES + SIZE_BYTES + USED_LOCK_COUNT_BYTES

int main() {
  std::cout << "LOCK_BYTES: " << LOCK_BYTES << std::endl;
  std::cout << "HEADER_BYTES: " << HEADER_BYTES << std::endl;
  std::cout << "LOCK_BYTES * 0: " << 0 * LOCK_BYTES << std::endl;
}

这是我刚才得到的结果和编译器信息。

Here's the result I just got and the compiler info.

yifeng @ yifeng-Precision-WorkStation-T3400：〜/ Shared- / examples / IMPL $
./a.out LOCK_BYTES：265 HEADER_BYTES：576 LOCK_BYTES * 0：9

yifeng@yifeng-Precision-WorkStation-T3400:~/Shared-Memory-Solution/examples/IMPL$ ./a.out LOCK_BYTES: 265 HEADER_BYTES: 576 LOCK_BYTES * 0: 9

编辑：谢谢你们这么多人！我一直很高兴，我决定发布，虽然我得到这么多downvote。

EDIT: Thank you so much guys!! I've been most happy that I decided to post this, although I am getting so many downvotes. What a lesson to learn about MACRO!

推荐答案

您应该始终在宏定义的后面加上括号：

You should always put parentheses around macro definitions:

#define LOCK_BYTES (LOCK_NAME_BYTES + LOCK_TYPE_BYTES + PID_BYTES + TID_BYTES)

否则，代码会扩展为：

cout << 0 * LOCK_NAME_BYTES + LOCK_TYPE_BYTES + PID_BYTES + TID_BYTES

输出 LOCK_TYPE_BYTES + PID_BYTES + TID_BYTES 。

更好的是，不要使用宏，除非你真的需要。这些更好地表示为常量变量。

Better still, don't use macros unless you really have to. These are better represented as constant variables.

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