Import nested classes into namespace - C++

Question

Say I have a class like this:

class A {
public:
    class B {
        // ...
    };
    static void f();
    // ...
};

I can refer to B as A::B and to f() as A::f(), but can I import B and f() into the global/current namespace? I tried

using A::B;

but that gave me a compilation error.

Was the compilation error, "error: ‘A’ is not a namespace"? — johnsyweb, Jul 25 '12 at 10:10
@iammilind but you can achieve that behavior with a workaround. — Eitan T, Jul 25 '12 at 10:31

Sergey K. · Answer 1 · 2012-07-25T10:54:20.940

33

Here are two workarounds for your problem:

1) Class B:

typedef A::B B;

2) Function f():

inline void f()
{
    A::f();
}

But think twice before using them.

Edit: In C++11 you can do auto f = A::f; but this actually creates a pointer to a function and functions pointers cannot be inlined.

edited Jul 25 '12 at 10:54

answered Jul 25 '12 at 10:35

Sergey K.

24,894
13
106
174

For the `typedef`or `using` solutions, any idea to make it work even with private nested class? – Dreamer Feb 11 '17 at 01:05

score 19 · Accepted Answer · edited May 23 '17 at 11:46

19

You should be able to use namespace aliases for the class:

using B = A::B;

However you can't do that with the member function, not even with static member functions.

Edit: According to this SO answer (What is the difference between 'typedef' and 'using' in C++11) this should be valid, and actually creates a type alias in the same way that typedef does. However, it's C++11 only.

There is a workaround for static member functions in C++11, by declaring a variable pointing to the static function:

struct Foo
{
    static void bar()
        { }
};

auto bar = Foo::bar;

Edit: Of course, having a global variable pointing to a static member function is possible in the older C++ standard as well, but it's more messy than using the auto keyword of C++11. In the example above it would be:

void (*bar)() = Foo::bar;

edited May 23 '17 at 11:46

Community

1
1

answered Jul 25 '12 at 10:41

Some programmer dude

400,186
35
402
621

2

``using B = A::B;`` this will not work. Another point is that function pointers cannot be inlined. – Sergey K. Jul 25 '12 at 10:48
5

@SergeyK. Works fine for me, with GCC 4.7.1. But you're right about the inlining. – Some programmer dude Jul 25 '12 at 10:51
3

1.cpp:9:7: error: expected nested-name-specifier before 'B' using B = A::B; ``(gcc.EXE (GCC) 4.8.0)`` – Sergey K. Jul 25 '12 at 10:52
3

@SergeyK. Actually, the `using` keyword syntax is legal in C++11. When compiling with GCC did you use the `-std=c++11` or `-std=c++0x` flags? – Some programmer dude Aug 07 '12 at 06:26

score 11 · Answer 3 · answered Jul 25 '12 at 10:09

11

You can typedef

typedef A::B B;

answered Jul 25 '12 at 10:09

Andrew

24,218
13
61
90

5

You could also create `f()` in the global namespace that calls `A::f()`. – Eitan T Jul 25 '12 at 10:30

Import nested classes into namespace - C++

3 Answers3

Linked

Related