Question

I’m having an issue writing a class as part of a C++ program – in it I have three classes, FirstClass, SecondClass, and ThirdClass – First and Second classes both include ThirdClass.h, and in SecondClass I can declare them normally, however, in FirstClass the first declaration works fine, but any further declarations give me an error that “ThirdClass is not a type name”

here’s a snippet from the class that’s erroring

#include "ThirdClass.h"

class FirstClass
{
public:

    FirstClass(void);

    // This decleration of ThirdClass works fine
    FirstClass(ThirdClass ());
    FirstClass(const FirstClass& rhs);
    ~FirstClass(void);

private:

    //These are the two that're erroring
    ThirdClass nestedClass();

    void Init (ThirdClass ());
    void Copy (FirstClass);
};

I’m assuming that it’s something to do with both of them linking to the same header file, but i’ve been looking far and wide trying to find a solution online to no avail.
I did manage to get it working by placing the include inside a namespace, but I also read that this was very bad practice so I don’t really want to do it that way.

Answer 1

FirstClass(ThirdClass ());

What is this supposed to do?

If the type ThirdClass has been declared then it declares a constructor that takes the address of a function as its argument, that’s not what you wanted, right? ThirdClass () is the type of a function that takes no arguments and returns a ThirdClass, so your constructor argument is (the address of) a function of that type.

If ThirdClass hasn’t been declared (and the error you’re getting implies it hasn’t been) then it is equivalent to:

FirstClass ThirdClass();

i.e. a (non-constructor) function called ThirdClass which returns a FirstClass object.

You probably wanted it to be a constructor taking a ThirdClass object as the argument, which would be:

FirstClass(ThirdClass);

Or to avoid copying the argument (which is usually what you want):

FirstClass(const ThirdClass&);

Similarly for your Init function.

The errors saying ThirdClass is not a type name indicate the type hasn’t been declared. We can only guess because you didn’t show a complete, self-contained example (no cookie for you) but you probably have #include "FirstClass.h" in your ThirdClass.h header, which causes circular reference and only one of the files is processed correctly.

See these questions (and their answers) for more:
cyclic dependency between header files
C++ cyclical header dependency
C++ error: 'Line2' has not been declared