Class declarations in header files - c++

Question

Ive been using c++ for some months now but Ive come across an error when I use header and source code files. I create a source code file
that contains a class gun(example not actual program):
class gun
{
private:
int stuff;
public:
void doStuff();
};
void Gun::doStuff()
{
cout << stuff << endl;
}
and then i created a header file and declared the class like this:
class gun;
then in my main source file i do this:
int main()
{
gun *mygun = new gun;
mygun->doStuff();
return 0;
}
however when i try to execute it i get the error:
invalid use of incomplete type 'class gun' and i think the problem is how i declared it in the header, did i do it wrong? how was i meant to do it? thanks.
Thanks Everyone that helped! I understand now, i thought that only the forward declaration
went into the header file, thanks for all your answers!

Answer 1

You seem to be going about seperating the implementation and header file the wrong way. Forward declarations should not go in the header file. The entire declaration should! This is how your code should be structured
Gun.hpp
#pragma once
class Gun
{
private:
int stuff;
public:
void doStuff();
};
Gun.cpp
#include "Gun.hpp"
#include <iostream>
using std::cout;
using std::endl;
void Gun::doStuff()
{
cout << stuff << endl;
}
main.cpp
int main()
{
Gun *mygun = new Gun;
mygun->doStuff();
delete mygun; // <-- remember this!
return 0;
}
the separation of header and implementation is crucial in C++ and other languages! You should only declare the class in the header along with its full interface (as above) and include all implementation details in the .cpp file (as above :)

Answer 2

The entire declaration of the gun class needs to be in the header file. What you declared in the header file is a forward declaration, which is not enough by itself to create an instance of the class. Forward declarations are useful for allowing other code to declare pointers only, since the compiler does not need to know the full details just to declare a pointer. But a forward declaration can't be used for creating actual object instances of the class. That is why you are getting errors about an incomplete type. From main()'s perspective, it has no idea what gun actually looks like, so it can't create a full instance of it.
The implementation of the methods for the gun class needs to be in the gun's source file, which can #include the header file to validate and access the class members.
gun.h
#ifndef gun_h
#define gun_h
class gun
{
private:
int stuff;
public:
void doStuff();
};
#endif
gun.cpp
#include "gun.h"
#include <iostream>
void gun::doStuff()
{
std::cout << stuff << std::endl;
}
Now, in main() (or any other source file), you can #include the header file and use the class as needed:
#include "gun.h"
int main()
{
gun *mygun = new gun;
mygun->doStuff();
delete mygun;
return 0;
}

Answer 3

In gun.h
#ifndef GUN_H_
#define GUN_H_
// You can use #pragma once too here
class gun
{
private:
int stuff;
public:
void doStuff();
};
#endif
In gun.cpp file
#include "gun.h"
void gun::doStuff()
{
cout << stuff << endl;
}
and then main.cpp
#include "gun.h"
int main() {
//your code using the class
gun *mygun = new gun;
mygun->doStuff();
return 0;
}
and you can compile and test using
g++ -o prg_name gun.cpp main.cpp && ./prg_name