Static constant class member declaration

Question

in Foo.h:

class Foo
{
public:
    Foo();
    static const unsigned int FOOBAR = 10;
    static const unsigned int BARFOO = 20;

private:
    unsigned int m_FooBar;
    bool m_Bar;
    void Bar();
};

in Foo.cpp:

Foo::Foo()
    : m_FooBar(FOOBAR), // this works
      m_Bar(false)
{
}

void Foo::Bar()
{
    //m_FooBar = m_Bar ? FOOBAR : BARFOO; // linker fails *1
    m_FooBar = FOOBAR; // ok
}

I'm compiling with GCC 4.5.3. Is there any reason why the linker would fail when line *1 is uncommented?

Foo.o: In function 'Foo::Bar' (name unmangled):
Foo.cpp: undefined reference to `Foo::FOOBAR'
Foo.cpp: undefined reference to `Foo::BARFOO'

Tried with VC2005, 2008, 2010 and CB2010. They all compiled and linked fine. Why does GCC fail in this case?

Given the answer here , why doesn't the other popular compilers fail like GCC? One way or another, it has to be a bug, either for GCC or the other popular compilers. Or is there a more reasonable explanation?

Answer 1

Formally, the header only declares the static constants, and they also have to be defined (at least in C++03). However, if you only use their values, you most often get away with this.

In C++11 this is more formally specified as requiring a definition when the static is "odr-used". The *1 line is an example of this. The triadic operator tries to form a reference to the values, and the compiler (or linker actually) realizes that it cannot.

---

The C++11 standard says

9.4.2 Static data members
§3...
The member shall still be defined in a namespace scope if it is odr-used (3.2) in the program and the namespace scope definition shall not contain an initializer .

Answer 2

Try defining these members:

static const unsigned int FOOBAR = 10;
static const unsigned int BARFOO = 20;

Outside the class declaration.

Foo::FOOBAR = 10;
Foo::BARFOO = 20;