You can’t forward-declare a nested class like that.
Depending on what you’re trying to do, maybe you can use a namespace rather than a class on the outer layer. You can forward-declare such a class no problem:
namespace Outer {
struct Inner;
};
Outer::Inner* sweets; // Outer::Inner is incomplete so
// I can only make a pointer to it
If your Outer absolutely must be a class, and you can’t shoe-horn it into a namespace, then you’ll need for Outer to be a complete type in the context where you forward declare Inner.
class Outer
{
class Inner; // Inner forward-declared
}; // Outer is fully-defined now
Outer yes; // Outer is complete, you can make instances of it
Outer::Inner* fun; // Inner is incomplete, you can only make
// pointers/references to it
class Outer::Inner
{
}; // now Inner is fully-defined too
Outer::Inner win; // Now I can make instances of Inner too