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:

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int Foo::sInt = 1;

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in a .cpp file.

Quote:

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Originally Posted by bithub

Nope, you need to put the line:

Code:

---

int Foo::sInt = 1;

---

in a .cpp file.

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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?)

Quote:

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Originally Posted by R.Stiltskin

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?)

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

Quote:

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Originally Posted by brewbuck

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

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Good to know. Thanks.

Quote:

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

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Damn really? That's crazy

Quote:

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Originally Posted by brewbuck

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

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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?

Quote:

---

Originally Posted by R.Stiltskin

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?

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

You can think of the headers as basically being copy/pasted into the source files where they are included. So if you put global declarations inside a header, the linker will find them in each module which had it included. This is why you separate .h and .c files so the .c is compiled once, and any other modules which would need something from it include the header files to get references. I'm not sure about C++ static vars, but you might be able to declare this in the .cpp file and then declare it extern in the .h file. Anyone know this off hand? I'd experiment but have to go.

Quote:

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Originally Posted by brewbuck

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

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

Quote:

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Originally Posted by neandrake

I'm not sure about C++ static vars, but you might be able to declare this in the .cpp file and then declare it extern in the .h file. Anyone know this off hand? I'd experiment but have to go.

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It's the same as any global var. Define in one source file, declare extern in a header.