强制一个包含文件包含在另一个之前 [英] Force one include file to be included before another

问题描述

想象一下我有两个 .hpp 文件：

  #ifndef _DEF_FILE_1_ 
 #define _DEF_FILE_1_ 
 inline void some_function_1（）{
 / * do stuff * / 
} 
 #endif 
  

和

  #ifndef _DEF_FILE_2_ 
 #define _DEF_FILE_2_ 
 #ifdef _DEF_FILE_1_ 
 inline void some_function_2（）{
 / * do stuff using some_function_1（）* / 
} 
 #else 
 inline void some_function_2（）{
 / *做相同的东西，不使用some_function_1（）* / 
} 
 #endif 
 #endif 
  

当我不知道文件是以什么顺序包含时，我的问题出现，例如：
在 main .cpp 我可以有像：

  #includefile1.hpp
 #includefile2.hpp
 int main（）{
 some_function_2（）; 
 / *将调用使用some_function_1（）的函数* / 
} 
  

或

  #includefile2.hpp
 #includefile1.hpp
 int main （）{
 some_function_2（）; 
 / *将调用不使用some_function_1（）的函数* / 
} 
  

$ b b

有一种方法可以确保一旦 file1.hpp 和 file2.hpp
，则 some_function_2（）将调用 some_function_1（）？

PS：一个解决方案是在 file2.hpp 中加入 file1.hpp 不能做
，因为我开发一个代码，可能或可能不依赖于最终用户可能有或可能没有的一些库
。

PPS：我可以想到的唯一的其他解决方案（即使我不知道如何
实现这一点）是删除的定义 some_method_2（）包含
file1.hpp ，然后重新包含 file2.hpp p>

解决方案

我相信正确的解决方案是使用SFINAE机制重写 some_function_2（）和模板而不是预处理技巧。这样，实例化将发生在cpp文件中，如果 some_function_1（）存在，并且include的顺序不重要。

Imagine I have two .hpp files:

#ifndef _DEF_FILE_1_
#define _DEF_FILE_1_
inline void some_function_1(){
    /*do stuff*/
}
#endif

and

#ifndef _DEF_FILE_2_
#define _DEF_FILE_2_
#ifdef _DEF_FILE_1_
inline void some_function_2(){
    /*do stuff using some_function_1()*/
}
#else
inline void some_function_2(){
    /*do the same stuff without using some_function_1()*/
}
#endif
#endif

My problem arises when I don't know in which order the files are included, e.g: in the main.cpp i can have something like :

#include "file1.hpp"
#include "file2.hpp"
int main(){
    some_function_2();
    /*will call the function that uses some_function_1()*/
}

or

#include "file2.hpp"
#include "file1.hpp"
int main(){
    some_function_2();
    /*will call the function that doesn't use some_function_1()*/
}

Is there a way to make sure that as soon as both file1.hpp and file2.hpp are included, then some_function_2() will call some_function_1()?

PS: One solution would be to include file1.hpp in file2.hpp but I can't do that because I developp a code that may or may not depend on some library that the end-user may or may not have.

PPS: The only other solution I can think of (even if I don't know how to achieve this) would be to "delete" the definition of some_method_2() when file1.hpp is included and then reinclude file2.hpp.

解决方案

I believe proper solution would be to rewrite some_function_2() using SFINAE mechanism and template instead of preprocessor tricks. That way instantiation will happen in cpp file where it would be known if some_function_1() exists and order of include will not matter.

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