How to make an inner class without putting the definition of inner class to parent class?

问题

I'll write an header file,and it's very long.Since it will be too complicated,i don't want to put inner class definition in root class.I mean how can i make a class inner without writing it in root class.

class outer
{

}

class inner
{

}

If i can use like that, The header file will be clearer i think.

回答1:

Like this:

// foo.hpp

class Foo
{
public:
  class Inner;
  Foo();
  void bar();
  Inner zoo();
};

// foo_inner.hpp

#include "foo.hpp"

class Foo::Inner
{
  void func();
};

Then, in the implementation:

#include "foo.hpp"
#include "foo_inner.hpp"

void Foo::bar() { /* ... */ }
void Foo::Inner::func() { /* ... */ }

Note that you can use the incomplete type Foo::Inner inside the class definition of Foo (i.e. in foo.hpp) subject to the usual restrictions for incomplete types, e.g. Inner may appear as a function return type, function argument, reference, or pointer. As long as the member function implementations for the class Foo can see the class definition of Foo::Inner (by including foo_inner.hpp), all is well.

回答2:

You can specify 'outer' as "public class outer", and put both its definition and the "inner" definition into a "class.java" file, and code in outer can instantiate inner just as if inner was in a different source file. It is not clear that is what you're after, because you have not explained why you want an "inner" class.

来源：https://stackoverflow.com/questions/8693590/how-to-make-an-inner-class-without-putting-the-definition-of-inner-class-to-pare