C预处理器 - 示例 [英] C Preprocessor - example

问题描述

#include<stdio.h>
#define SWAP(a, b, c)(c t; t=a, a=b, b=t)
int main()
{
    int x=10, y=20;
    SWAP(x, y, int);
    printf("%d %d\n", x, y);
    return 0;
}

为什么不允许int声明？我们可以在一个paranthesis中声明吗？是否允许？

why it is not allowing int to be declared?can we declare inside a paranthesis?whether it is allowed?

推荐答案

试试这个



#define SWAP（a，b，c）{ct; t = a，a = b，b = t;}



我插入了一个空格，并用圆括号括起来。

Try this instead

#define SWAP(a, b, c) {c t; t=a, a=b, b=t;}

I inserted a space, and exchanged parentheses for braces.

使用 std :: swap [ ^ ]

或者如果您使用预处理器进行此操作：

Use std::swap[^]
or if you''re set on doing this using the preprocessor:

#define SWAP(a,b,c) do {c t=a; a=b; b=t;}while(0)

执行构造确保您可以执行以下操作：

The do while construct ensures that you can do something like this:

if(x < y)
 SWAP(x,y,int);
else
 y++;
// rest of your code

没有不良副作用 - 我建议你使用std :: swap。

< br $>
最好的问候

Espen Harlinn

Without undesired side effects - I would advice you to use std::swap.

Best regards
Espen Harlinn

我不确定你会回答这些问题，但我强烈建议你坚持下去一本体面的C ++书并研究它。尝试使用预处理器宏解决编程问题是一个坏主意，并且可能导致许多难以诊断的问题。使用该语言使用适当的工具和结构编写代码。

I am not sure where you are going with these questions, but I would strongly recommend you get hold of a decent book on C++ and study it. Trying to solve programming problems with preprocessor macros is a bad idea and can lead to many difficult to diagnose problems. Use the language to write your code using the proper tools and constructs.

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