CODEWITHSUNDEEP
c++scopenamespacesaliasnamespace-alias
Ant
Ant

Reputation: 1708

How to make a namespace alias available only to a class

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();
    }
};

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();
}

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);
}

Is there a way to do what I want?

EDIT: my solution

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);
       }
    };
}

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 referring to a different library.

Upvotes: 52

Views: 22599

Answers (4)

Sarfaraz Nawaz
Sarfaraz Nawaz

Reputation: 361442

Namespace alias in the class definition is illegal, as specified by the language specification.

Its allowed in only in namespace scope or function scope.

You can make an alias at the namespace scope. But this will create a permanent alias which can be used from other files as well. However, the solution is simple: you can use an unnamed namespace to prevent the 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.

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

'myspace::abc' has not been declared

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

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

Upvotes: 50

Yochai Timmer
Yochai Timmer

Reputation: 49251

The scope of a namespace alias is a code block.

So you can put it in any code block.

BUT, you can't put it inside a class, because that will mean it's a member of the class.
A namespace alias can't be a member.

More about namespace aliases:

What is the scope of a namespace alias in C++?
Namespaces

Upvotes: 4

Bo Persson
Bo Persson

Reputation: 92261

You can of course also put the alias outside the class:

namespace abc = a_big_namespace;     

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

Upvotes: -2

Björn Pollex
Björn Pollex

Reputation: 76788

It works if you declare the alias outside of the struct.

Upvotes: -2

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