整个类范围内的C ++名称空间别名 [英] C++ namespace alias in entire class scope

问题描述

我希望能够在类声明中使用名称空间别名，但会遇到编译器语法错误.

I expected to be able to use a namespace alias in a class declaration but get a compiler syntax error.

struct MyClass
{
    namespace abc = a_big_namespace;
    void fn() {
        abc::test();
    }
};

使它起作用的唯一方法是在每个函数中都使用别名.

The only way I can get it to work is to put the alias in every function.

void fn() {
    namespace abc = a_big_namespace;
    abc::test();
}

此外，我希望能够对功能参数使用别名.我还没有找到解决方法.

Additionally I would like to be able to use the alias for function parameters. I haven't found a work-around for that.

void fn(abc::testType tt) {
    abc::test(tt);
}

有没有办法做我想做的事?

Is there a way to do what I want?

我的解决方案

我发现我不需要为特定的问题使用未命名的命名空间，并且可以简单地做到这一点:

I found that I didn't need unnamed namespace for my particular problem and can simply do this:

namespace myspace
{
    namespace abc = a_big_namespace;

    struct MyClass
    {
       void fn(abc::testType tt) {
          abc::test(tt);
       }
    };
}

要切换到另一个库，这就是我的别名名称空间所指的，我只是更改别名.这种方法甚至允许我在一个文件中两次使用相同的类，每次引用一个不同的库. 感谢您的所有帮助.

To switch to the other library, which is what my alias namespace refers to I just change the alias. This method even allows me to have the same class in a single file twice, each time refering to a different library. Thanks for all your help.

推荐答案

命名空间别名为非法.

仅在名称空间范围或功能范围内的 中允许允许.

Its allowed in only in namespace scope or function scope.

您可以在命名空间范围内创建别名.但这将成为永久别名，也可以从其他文件中使用该别名.但是解决方案很简单:您可以使用 unnamed 命名空间来防止其他文件中看到别名(以及大命名空间中的所有符号).这是可以做到的:

You can make alias at namespace scope. But that will be make permanent alias which can be used from other files as well. But the solution is simple : you can use unnamed namespace to prevent alias (and therefore all symbols from the big namespace) from being visible from other files. This is how it can be done:

//MyFile.cpp
namespace myspace
{ 
    namespace   //this is unnamed namespace
    {
       namespace abc = a_big_namespace;     
    }
    struct MyClass 
    {
      void fn() 
      { 
         abc::test();  //don't worry, this will work!
      } 
    };
}

//OtherFile.cpp

myspace::abc::test(); //error - that means, prevention worked.

从其他文件看不到别名.编译OtherFile.cpp时，GCC(4.5.0)说，

The alias is not visible from other files. When compiling OtherFile.cpp, GCC (4.5.0) says,

'myspace :: abc'尚未声明

'myspace::abc' has not been declared

这证明别名abc在MyFile.cpp中仅仅可见.感谢 unnamed 命名空间.

That proves the alias abc is visible only in MyFile.cpp. Thanks to unnamed namespace.

演示: http://www.ideone.com/2zyNI (尽管它没有演示OtherFile概念.我无法ideone.com上有多个文件)

Demo : http://www.ideone.com/2zyNI (though it doesn't demonstrate OtherFile concept. I cannot have more than one file at ideone.com)

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