在头文件中使用声明 [英] using declaration in header files

问题描述

我一直在寻找头文件中使用 using-declarations 的一些说明（我正在四处搜索，但无法得到我正在寻找的答案）。我的研究结论迄今为止，在非全局范围内使用它们是可以的，而命名空间指令是不好的。我知道（至少我希望如此:)）。

所以在我的例子中我使用 shared_ptr s，但我需要支持在 std :: 命名空间中没有它们但在 std :: tr1 :: shared_ptr 的类都需要相同的 shared_ptr defing，所以我必须将正确的 #include 指令并在每个头文件中使用声明。所以我将这部分移到了一个单独的头文件中，所以我只有一个文件需要进行更改。通过预处理指令 HAS_SHAREDPOINTER 来决定使用哪个 shared_ptr ，如果用户有编译器支持 std :: shared_ptr 。

$ b SharedPtr.h ：

  #ifndef SHAREDPTR_H_ 
 #define SHAREDPTR_H_ 
 
 #ifdef HAS_SHAREDPOINTER 
 #include<存储器GT&; 
使用std :: shared_ptr; 
 #else 
 #include< tr1 / memory> 
使用std :: tr1 :: shared_ptr; 
 #endif 
 
 #endif / * SHAREDPTR_H_ * / 
  

现在在使用shared_ptr的每个头文件中，都包含这个头文件。例如在

ModelPar.h 中：

  #ifndef MODELPAR_H_ 
 #define MODELPAR_H_ 
 
 #include< string> 
 #include< set> 
 
 #includeSharedPtr.h
 $ b $ class ModelPar {
 private：
 std :: set< shared_ptr< ModelPar> >连接; 
 ... 
}; 
 
 #endif / * MODELPAR_H_ * / 
  

现在我想我这是错误的，因为包含我的任何头文件的用户（使用 shared_ptr s）在他的代码中也有对应的使用声明 。这是坏事，因为用户没有意识到这一点......所以我把我的使用声明放在全局范围内。要么？我有点困惑和困惑如何正确地做到这一点？提前致谢！

解决方案

好的，我自己找到了答案。我想我没有意识到命名空间中的using声明在名称空间相同的名称空间下仍然有效。现在Bjarne的话也更有意义，人们不应该污染全球命名空间:)。如果我仍然做错了，请纠正我。

SharedPtr.h：

  #ifndef SHAREDPTR_H_ 
 #define SHAREDPTR_H_ 
 
 #ifdef HAS_SHAREDPOINTER 
 #include< memory> 
 namespace blub {
使用std :: shared_ptr; 
} 
 #else 
 #include< tr1 / memory> 
 namespace blub {
使用std :: tr1 :: shared_ptr; 
} 
 #endif 
 
 #endif / * SHAREDPTR_H_ * / 
  

ModelPar.h：

  #ifndef MODELPAR_H_ 
 #define MODELPAR_H_ 
 
 #include< string> 
 #include< set> 
 
 #includeSharedPtr.h
 
 namespace blub {
 
 class ModelPar {
 private：
 std ::设置< shared_ptr的< ModelPar> >连接; 
 ... 
}; 
} 
 
 #endif / * MODELPAR_H_ * / 
  

I've been looking for some clarification for the usage of the using-declarations in header files (I was searching around but couldn't quite get the answer I'm looking for). What I my research concluded so far is, using them in the non-global scope is ok, while namespace directives are bad. That I understood (At least I hope so :)).

So in my example I use shared_ptrs, but I need to support older compilers that do not have them in the std:: namespace but in std::tr1:: for example. Since every class using the shared_ptr needs the same shared_ptr defition, I would have to put the correct #include directive and using declaration in each of these header files. So I moved this part into a seperate header file so I have only one single file where I need to make changes. The decision on which shared_ptr to use, is made via a preprocessor directive HAS_SHAREDPOINTER, which is set if the user has a compiler that supports std::shared_ptr.

SharedPtr.h:

#ifndef SHAREDPTR_H_
#define SHAREDPTR_H_

#ifdef HAS_SHAREDPOINTER
#include <memory>
using std::shared_ptr;
#else
#include <tr1/memory>
using std::tr1::shared_ptr;
#endif

#endif /* SHAREDPTR_H_ */

Now in every header file that uses the shared_ptr, I include this header file. For example in

ModelPar.h:

#ifndef MODELPAR_H_
#define MODELPAR_H_

#include <string>
#include <set>

#include "SharedPtr.h"

class ModelPar {
  private:
    std::set<shared_ptr<ModelPar> > connections;
  ...
};

#endif /* MODELPAR_H_ */

Now I think the way I did it is wrong, since the user that includes any of my header file (using the shared_ptrs) also has the corresponding using declaration in his code. And that's the bad thing, as the user is not aware of this... etc. So I put my using declarations in the global scope. Or? I'm kind of stuck and confused on how to do this properly? Thanks in advance!

解决方案

Ok I found "the" answer myself. I guess that I was not aware that the using declaration in a namespace is still valid in below namespace scopes with the same name. Now Bjarne's words also makes more sense, that one should not pollute the global namespace :). Please correct me though, if I'm still doing something wrong.

SharedPtr.h:

#ifndef SHAREDPTR_H_
#define SHAREDPTR_H_

#ifdef HAS_SHAREDPOINTER
#include <memory>
namespace blub {
using std::shared_ptr;
}
#else
#include <tr1/memory>
namespace blub {
using std::tr1::shared_ptr;
}
#endif

#endif /* SHAREDPTR_H_ */

ModelPar.h:

#ifndef MODELPAR_H_
#define MODELPAR_H_

#include <string>
#include <set>

#include "SharedPtr.h"

namespace blub {

  class ModelPar {
  private:
    std::set<shared_ptr<ModelPar> > connections;
    ...
  };
}

#endif /* MODELPAR_H_ */

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