C++ error LNK2005: .... already defined

[JohnOne]

I have this error, and I cannot understand why it is happening.

I'm using VS 2010, win32 project.

The error says that the method Full is already defined in MyClass object.

Well yes, I know it is, it is a prototype of the implementation which is defined outside of the class.

If I comment out the implementation code the project compiles fine without issue.

Both class and method are in the same header file.

There are no other functions, methods, or vars with the same name of either class or method.

I've been trying to figure it out too long, and need some help if anyone can offer it.

class MyClass
{
    int cx, cy;
    HDC hdc, hDest;

public:
    MyClass() : cx(0), cy(0)
    {
        hdc = GetDC(NULL);
        hDest = CreateCompatibleDC(hdc);

    }

    MyClass(int vx, int vy)
    {
        hdc = GetDC(NULL);
        hDest = CreateCompatibleDC(hdc);
        cx = vx;
        cy = vy;
    }

    void Full(int, int, int, int, std::string);
    void Region(int, int, int, int, std::string);
};

void MyClass::Full(int x, int y, int w, int h, std::string path){

    return;
}

Edited February 22, 2013 by JohnOne

[JohnOne]

It does not like the implementation code in the same file as the class, the problem goes away when it is moved into .cpp file.

Perhaps it's just me, but that seems a bit daft.

[wraithdu]

Everything would probably work ok in the cpp file, definition and implementation. I guess the compiler doesn't like implementations in header files, not exactly that they are in the *same* file.

[JohnOne]

Really weird, in most tutorials I have read, it says best practice is that a header file should be able to compile and work on it's own, so this is really strange.

[JohnOne]

It has just occurred to me that this project is the first I have made where I specified no precompiled headers.

Setting to use precompiled headers fixes my problem.

Cheers for looking folks.

[JohnOne]

This is cracking me up now.

As I said above setting properties of project to use precompiled headers worked (seemingly)

But after I added to the class in the same manner, the new method showed the same error, so I'm back to square one, being confused and frustrated.

[Ascend4nt]

John,

Because you declared that function outside of the class, for every C++ file that includes the header, duplicate versions of that function will be created - which is correct compiler behavior. That's why you should either declare the function in its own C++ file, declare it as inline, or put it inside the class object definition (which behaves like inline).

[JohnOne]

Cheers, I've just capitulated and put the functions in their own cpp file.

For the record there is only one file that is included in all cpp files and that is stdafx.h, which has header guards and contains the other headers which the project is dependent on.

You know when you just know something is not quite right though?

Thanks for the explanation, glad to put this hiccup behind me.

[Richard Robertson]

You need to make the distinction between declaring a name and defining it. Declarations can occur repeatedly (forward declarations as well as extern (meaning another file)) without problems, but definitions should only occur once.