Thread: Multiple definition linker error

Why does code like this

Code:

#ifndef _foo_h
#define _foo_h

class Foo
{
  public:
  static int sInt;
};

int Foo::sInt = 1;

#endif

cause linker error "multiple definition of 'Foo::sInt' when foo.h is included in several source files? I was under the impression that the purpose of header guards was precisely to prevent such an error.

Nope, you need to put the line:

Code:

int Foo::sInt = 1;

in a .cpp file.

Originally Posted by bithub

Nope, you need to put the line:

Code:

int Foo::sInt = 1;

in a .cpp file.

Yes, I already figured that out. My question is why? (If the guards prevent multiple definition of Foo, why don't they prevent multiple definition of Foo::sInt?)

Originally Posted by brewbuck

It prevents multiple inclusion in any one source module. It doesn't prevent anything at all across different modules.

Good to know. Thanks.

It prevents multiple inclusion in any one source module. It doesn't prevent anything at all across different modules.

Damn really? That's crazy

Originally Posted by brewbuck

It prevents multiple inclusion in any one source module. It doesn't prevent anything at all across different modules.

Wait a minute. How can that be? Of course I can have

Code:

#ifndef _foo_h
#define _foo_h

class Foo
{
  public:
  int bar;
};

#endif

and #include "foo.h" in several different source files with no problem. It's only the initialization of the static member that causes a problem. Or does "different modules" mean something else?

Originally Posted by brewbuck

Uh... Yeah, because in this case you're declaring something, not creating an object/variable...

[smacks self in head...] Yeah, I lost sight of the declaration/definition distinction.