Question

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.

Answer 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.