何时使用包括防护装置? [英] When to use include guards?

问题描述

我知道在头文件中使用包含保护是为了防止某些东西被定义两次.不过，使用此代码示例是完全可以的:

I know that the use of include guards in header files is to prevent something from being defined twice. Using this code sample though, was completely fine:

foo.c

#include <stdio.h>
#include <string.h>
#include "bar.h"

int main() {
    printf("%d", strlen("Test String"));
    somefunc("Some test string...");
    return 0;
}

bar.h

#ifndef BAR_H_INCLUDED
#define BAR_H_INCLUDED
void somefunc(char str[]);
#endif

bar.c

#include <stdio.h>
#include <string.h>
#include "bar.h"

void somefunc(char str[]) {
    printf("Some string length function: %d", strlen(str));
}

上面的代码片段是用gcc -Wall foo.c bar.c -o foo编译的，没有错误.但是，<stdio.h>和<string.h>都被包含而没有包含保护.当我将bar.h剥离到单个语句void somefunc(char str[]);时，仍然没有错误.为什么没有错误?

The above snippets are compiled with gcc -Wall foo.c bar.c -o foo and there is no error. However, both <stdio.h> and <string.h> were included without an include guard. There is still no error when I strip bar.h down to the single statement void somefunc(char str[]);. Why is there no error?

推荐答案

首先，包含保护的主要目的是防止某些内容在同一翻译单元中被两次声明. 在同一个翻译单元中"部分是这里的关键.您使用两个不同翻译单元进行的实验与包含卫士的目的无关.它没有显示任何东西.它甚至没有远程关系.

Firstly, the primary purpose of include guards is to prevent something from being declared twice in the same translation unit. The "in the same translation unit" part is the key here. Your experiment with two different translation units has nothing to do with the purpose of include guards. It does not demonstrate anything. It is not even remotely related.

为了利用包含保护，您必须将相同的头文件(显式或隐式)两次包含在一个实现文件中.

In order to take advantage of include guards you have to include (explicitly or implicitly) the same header file twice into one implementation file.

第二，仅因为某些头文件没有包含保护措施，并且仅因为您将该头文件两次包含在同一个转换单元中并不意味着它必然会触发错误.为了导致错误，标头必须包含特定的不可重复"类型的声明.没有每个标头都包含此类令人讨厌的声明.并非每个声明都在这种意义上令人反感.

Secondly, just because some header file has no include guards and just because you included that header file twice into the same translation unit does not mean that it will necessarily trigger errors. In order to lead to errors the header must contain declarations of specific "non-repeatable" kind. No every header contains such offending declarations. Not every declaration is offending in this sense.

您的bar.h(已发布)实际上是无害的.正式地，您不需要在bar.h中包含防护.它只有一个函数声明，可以在一个翻译单元中重复多次.因此，多次包含此标头不会导致错误.

Your bar.h (as posted) is actually harmless. Formally, you don't need include guards in your bar.h. It has a single function declaration, which can be repeated many times in one translation units. So, including this header multiple times will not lead to errors.

但是在您的bar.h

struct SomeStruct
{
  int i;
};

，然后将其两次包含在同一实现文件中，最终将出现错误.此错误是旨在防止包括防护措施的错误.该语言禁止在同一翻译单元中重复相同结构类型的完整声明.

and then just include it twice in the same implementation file, and you will end up with an error. This error is what include guards are intended to prevent. The language prohibits repeating full declarations of the same struct type in the same translation unit.

包含保护通常会无条件地放置在头文件中.我很确定，它们也在<stdio.h>和<string.h>内部.目前尚不清楚为什么您声称这些标头是在没有包含保护的情况下被包含的".您是否检查了这些文件?无论如何，无论如何，您使用两个不同翻译单元进行的实验无论如何都不会显示任何相关信息.

Include guards are typically placed in header files unconditionally. They are, I'm quite sure, present inside <stdio.h> and <string.h> as well. It is unclear why you claim that these headers "were included without an include guard". Did you check inside these files? In any case, again, your experiment with two different translation units does not demonstrate anything relevant anyway.

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