Question

Possible Duplicate:
Static members class vs. normal c-like interface

I am looking at somebodies code and there are several dozen constants defined in a class like this:

// header file

class Defines
{
public:
    static const int Val1;
    static const int ValN;
    static const char* String1;
    static const char* StringN;
...
}

// .CPP
const char* Defines::String1 = "some value"
etc.

Is there some reason to do things this was as opposed to using a namespace instead?
Are there advantages/disadvantages of one over the other

——— Edit ———-

I’m sorry, I obviously should have pointed this out explicitly, as nobody has inferred it from the name of the class – which is "Defines". i.e. these constants are not associated with a particular class, there has been a class created specifically just to hold constants and nothing else, that is all the class Defines contains.

My question is not why should you place constants in a particular class, the question is is there any value in collecting dozens of them together and placing them in a class whose only purpose is to collect together constants, as opposed to collecting them together in a namespace, or just collecting them together in a header file specifically for that purpose etc.

(There is no currently existing namespace in the project therefore potential issues of polluting the namespace as mentioned in answers are not relevant in this case.)

—– 32nd edit ———–

and a follow up question —
is placing
const char* Defines::StringN = "Somevalue"

in the .h file inefficient versus placing it in the .cpp file?

Answer 1

In general, there’s no reason to use a type this way. I have seen it argued that if the “collection of constants” evolves into a concrete object, starting this way makes the transition easier. In practice, I’ve never seen this happen. It just hides intent, and potentially flutters the code with private constructors.

One could argue that classes work with templates, while namespaces do not. So something like the following would only work if Defines is a class:

template<typename T> int function() {
    return T::x + T::y;
}
//later
cout << function<Defines>() << function<OtherDefines>() << endl;

In most cases, there’s probably a redesign that would work better, particularly if all you have are “constants” that aren’t really. Occasionally, this may come in handy, though.

At times, it can also fight argument dependent lookup. In brief, the compiler is allowed to expand it’s search for eligable function names to different name namespaces based on the parameters passed to the function. This does not extend to static functions of a class. This applies more to the general case, though, where the “static class” also includes nested types and functions in addition to the collection of constants.

Why people do this varies. Some come from languages where can’t be used this way, others just don’t know any better.