C++ Linker Error

c++linkervisual-studio

I had a function like this, that wasn't within a class:

// Gets the maximum number of tracks displayable
const utils::uint32 GetConstMaxSystemRange()
{
    return constMaxSystemNumber - constMinSystemNumber + 1;
}

It compiled fine in VS2005, but then I got linker errors for each file after the first one to include it, even though I was using Compile Guards. On a hunch, I surrounded it with a class like so:

class CDSLimitsAccess
{
public:
    // Gets the maximum number of tracks displayable
    static const utils::uint32 GetConstMaxSystemRange()
    {
        return constMaxSystemNumber - constMinSystemNumber + 1;
    }

protected:
    CDSLimitsAccess(){}
};

And bang! Fixed.

Question: WHY?

Best Solution

Include guards only protect that function from being included into the same translation unit twice. But it won't protect that function from being included into the whole program twice.

As the function is non-inline, it violates the One Definition Rule if it appears more than once in the whole program. Make it inline to solve the problem, or put the definition into a implementation file, putting only its declaration in the header. 

// Gets the maximum number of tracks displayable. possible fix:
inline utils::uint32 GetConstMaxSystemRange() {
    return constMaxSystemNumber - constMinSystemNumber + 1;
}

On a side note - better don't put const on a built-in return value. Temporaries of them can't be qualified with const / volatile. But that's also not necassary: You can't modify them anyway. It's ignored as far as i know. But it won't ever have an effect.

Related Question