模板继承的嵌套类正向声明

Question

What's the proper way to inherit from a template class with the template argument being a nested class within the inheriting class?

class SomeClass : public TemplateClass<NestedClass>
{
     class NestedClass {};
};

Answer 1

There's no way to do specifically that. If you really have to inherit from TemplateClass<NestedClass> , you'll have to move the NestedClass declaration out of SomeClass .

Answer 2

I can't see how you could do this properly. There is this:

template<class T> class TemplateClass {};

class NestedClass {};

class SomeClass : public TemplateClass<NestedClass>
{
    typedef NestedClass NestedClass;
};

but that's just faking it...

Answer 3

You must atleast forward declare the NestedClass:

class NestedClass;
template<class T> class TemplateClass{};
class SomeClass : public TemplateClass<NestedClass>
{
     class NestedClass{};
};

This works. Tested on MinGW c++ on windows.

Update: @jon I tried the following on with gcc version 3.4.5 on Windows XP:

#include <iostream>

class NestedClass;
template<class T> class TemplateClass{};
class SomeClass : public TemplateClass<NestedClass>
{
public:
     class NestedClass{ 
     public: 
        int a; 
        static int test(){ return 100; }
     };
     NestedClass nc;
};

int main(){
 SomeClass sc;
 sc.nc.a = 10;
 std::cout<< sc.nc.a << std::endl;
 std::cout<< sc.nc.test() << std::endl;
}

And, I get the following output: 10 100

But, I guess what the author intended (like @jon suggested) is actually this:

class SomeClass::NestedClass;
template<class T> class TemplateClass{};
class SomeClass : public TemplateClass<SomeClass::NestedClass>
{
public:
     class NestedClass{};
     NestedClass nc;
};

And this does not work. The reason being that to be able to declare SomeClass::NestedClass in the template specification, SomeClass should have been declared. But, we are trying to do exactly that - hence we get a cyclic dependency.

So I guess @jon's answer best solves this problem.