Question

I was happily working in C++, until compilation time arrived.

I’ve got several classes inside some namespace (let’s call it N); two of these classes correspond to one base class, and other derived from it. Each class has its own pair of .hpp and .cpp files; I think it’d look like:

namespace N{

    class Base{
    };

    class Derived: public Base{
    };

}

However, g++ (maybe linker) keeps telling me:

Derived.hpp:n: error: expected class-name before ‘{’ token

It doesn’t recognize Base as a class, even when I have correctly #include’ed the hpp file corresponding to its definition to Derived’s .hpp!

“It’s something with #includes”, I thought, since these classes’ .hpps are #included in other files, so I added this to Derived declaration in Derived.hpp:

#include "Base.hpp"
namespace N{

    class Base;

    class Derived: public Base{
    };
}

And now g++ complains:

Derived.hpp:n: error: invalid use of incomplete type ‘struct N::Base’

So, I got lost here. Please help me, I will apreciate it a lot. 🙂

(By the way, I’m rather experienced in Python, not C++, so this issues are really strange to me. Also, I changed classes’ names and stuff :).

Edit: A more exact representation of my files is:

File Pieza.hpp
-----------------------
#include "Celda.hpp"

namespace Reglas
{
    class Pieza
    {    
        public:
        Pieza() {}
        virtual ~Pieza() {}

        private: 
        Celda *c; 
    };
}


File Jugador.hpp
-----------------------
#include "Jugada.hpp" 
#include "Excepciones.hpp"
#include "Pieza.hpp"
namespace Reglas
{  
//compiler asked for these :S
class Celda;
class Tablero;
    class Jugador : public Pieza
    {
        public:
        Jugador() {}
        virtual ~Jugador() {}
    };
}

Answer 1

Derived.hpp:n: error: invalid use of incomplete type ‘struct N::Base’

This makes me think that you didn’t #include "Base.hpp in the Derived.cpp source file.

EDIT: In your Derived.cpp, try changing the order of #includes to:

#include "base.hpp"
#include "derived.hpp"

// .. rest of your code ..

Like this:

// Derived.hpp
#pragma once

namespace foo
{
    class Base;

    class Derived : public Base
    {
    public:
        Derived();

        ~Derived();
    };
}

---

// Derived.cpp
#include "base.hpp"
#include "derived.hpp"

namespace foo
{
    Derived::Derived()
    {
    }

    Derived::~Derived()
    {
    }
}

---

So, you’re going to want to edit Jugador.hpp to look like this:

// Jugador.hpp
#include "Pieza.hpp" // move this above Jugada.hpp
#include "Jugada.hpp" 
#include "Excepciones.hpp"
namespace Reglas
{  
//compiler asked for these :S
class Celda;
class Tablero;
    class Jugador : public Pieza
    {
        public:
        Jugador() {}
        virtual ~Jugador() {}
    };
}