Question

I am experiencing some strange behavior with Visual Studio 2010 and C++.
I have a header file in which I declare some global constants

#ifndef CONSTANTS_H
#define CONSTANTS_H

#define WIN32_LEAN_AND_MEAN

// Macros...
#define SAFE_RELEASE(ptr) { if (ptr) { ptr->Release(); ptr = NULL } }
#define SAFE_DELETE(ptr) { if (ptr) { delete ptr; ptr = NULL; } }

// Constants...
const char* CLASS_NAME = "WinMain";
const char GAME_TITLE[] = "DirectX Window";

const int GAME_WIDTH = 640;
const int GAME_HEIGHT = 480;

#endif

My problem comes in with the following line:

const char* CLASS_NAME = "WinMain";

When it’s like this, and I build my solution I get the following 2 errors:

error LNK1169: one or more multiply defined symbols found, and

error LNK2005: "char const * const CLASS_NAME" (?CLASS_NAME@@3PBDB) already defined in graphics.obj

Now is strange since a ran a ‘find in files’ and I definitely do not declare it somewhere else ie no duplicate declarations.

Should I change it to:

const char* const CLASS_NAME = "WinMain";

OR

const char CLASS_NAME[] = "WinMain";

It compiles just fine! But as far as I know char* x is equivalent to char x[], and the fact that I’m enforcing ‘const-ness’ on both the pointer and the pointed-to value should make no difference…. or does it?

I’m a bit new to C++ development on the Windows platform, so any help will be greatly appreciated!

Answer 1

You mistake is that you did not declare your constants as constants. In C++ syntax (as well as in C) in order to declare a constant pointer you have to do this

const char* const CLASS_NAME = "WinMain";

Note that your GAME_TITLE, GAME_WIDTH and GAME_HEIGHT are correctly declared as constants, which is why they give you no trouble. Only the CLASS_NAME is declared incorrectly, i.e. as non-constant.

Constants in C++ have internal linkage by default, which is why you can define them in header files without violating the One Definition Rule.