是否有一种解决方法可以从另一个名称空间内部定义成员？

Question

This may seem a bit esoteric, but I have a class structure like this, where a singleton template class is defined in a different namespace than the class actually using it.

namespace F{
    template<typename>
    struct Foo{
        static Foo instance;
    };
}

namespace B{
    struct Bar{};
    F::Foo<Bar> F::Foo<Bar>::instance; //error C2888
}

Which yields:

error C2888: 'Foo<void> Foo<void>::instance' : symbol cannot be defined within namespace 'B'

I know that this is how it's supposed to be, but in my instance Foo is part of my library and Bar is defined by the client, so they won't necessarily be part of the same namespace. The part where Foo<void>::instance is defined is part of a macro, so I can hide complexity from the user.

Is there any way that enables me to define the member of a class from inside another namespace?

Answer 1

A simple solution for your particular need (assuming that the example code provided is relevant) is to use a Meyers' singleton , eg

namespace F{
    template<class>
    struct Foo{
        static auto instance() -> Foo& {
            static Foo the_instance;
            return the_instance;
        }
    };
}

Alternatively you can provide a general definition of the static member in the header file, which works fine due to a special exemption for templates in the ODR:

namespace F{
    template<class>
    struct Foo{
        static Foo instance;
    };

    template<class Type>
    Foo<Type> Foo<Type>::instance;
}

---

Amendment :

Here's a concrete example of the last method. It works nicely with g++ 4.8.2 and Visual C++ 12.0 (2013). I can not remember any problem with it with earlier compiler versions.

main.cpp

 auto main() -> int {} 

xh

#include "x.h"

#include <iostream>

struct A {};
static bool u = !(std::cout << "A " << &F::Foo<A>::instance << std::endl);

#include "x.h"

a.cpp

 #include "xh" #include <iostream> struct A {}; static bool u = !(std::cout << "A " << &F::Foo<A>::instance << std::endl); #include "xh" 

a2.cpp

H:\dev\test\so\0169>
main.cpp
a.cpp
a2.cpp
b.cpp
Generating Code...

H:\dev\test\so\0169>
A 00988A50
A 00988A50
B 00988A6E

H:\dev\test\so\0169>_

b.cpp

 #include "xh" #include <iostream> struct B {}; static bool u = !(std::cout << "B " << &F::Foo<B>::instance << std::endl); 

Building and running with Visual C++:

\nH:\\dev\\test\\so\\0169>  \nmain.cpp \na.cpp \na2.cpp \nb.cpp \nGenerating Code... \n\nH:\\dev\\test\\so\\0169>  \nA 00988A50 \nA 00988A50 \nB 00988A6E \n\nH:\\dev\\test\\so\\0169>_ \n