为什么没有带有“#if X”的警告当X未定义？ [英] Why no warning with "#if X" when X undefined?

问题描述

我偶尔写下这样的代码：

  // file1.cpp 
 #define DO_THIS 1 
 
 #if DO_THIS 
 // stuff 
 #endif 
  

在代码开发期间，我可以切换 DO_THIS 在0和1之间的定义。

最近我不得不重新排列我的源代码，并将一些代码从一个文件复制到另一个。但我发现我犯了一个错误，这两个部分已经分离如下：

  // file1.cpp 
 #define DO_THIS 1 
  

和

  // file2.cpp 
 #if DO_THIS 
 // stuff 
 #endif 
  

显然我修复了错误，但后来想到自己，为什么编译器不警告我？我的警告级别设置为4.为什么在X未定义时 #if X 可疑？

还有一个问题：有没有系统的方法，我可以找出是否我在其他地方犯了同样的错误？该项目是巨大的。

编辑：我可以理解没有警告，#ifdef，这是完全有道理。但是确实#if是不同的。

解决方案

gcc可以为此产生警告，但它可能不是标准要求： / p>

-Wundef

如果在#if指令中评估未定义的标识符，请警告。

I occasionally write code something like this:

// file1.cpp
#define DO_THIS 1

#if DO_THIS
    // stuff
#endif

During the code development I may switch the definition of DO_THIS between 0 and 1.

Recently I had to rearrange my source code and copy some code from one file to another. But I found that I had made a mistake and the two parts had become separated like so:

// file1.cpp
#define DO_THIS 1

and

// file2.cpp
#if DO_THIS
    // stuff
#endif

Obviously I fixed the error, but then thought to myself, why didn't the compiler warn me? I have the warning level set to 4. Why isn't #if X suspicious when X is not defined?

One more question: is there any systematic way I could find out if I've made the same mistake elsewhere? The project is huge.

EDIT: I can understand having no warning with #ifdef that makes perfect sense. But surely #if is different.

解决方案

gcc can generate a warning for this, but its probably not required by the standard:

-Wundef
Warn if an undefined identifier is evaluated in an `#if' directive.

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