Is there reason we are able to define [static const int] in a class definition, but not other static const types?

Question

Possible Duplicate:
Why aren't static const floats allowed?

Is there any reason why this is not possible in C++? It confuses me.

static const int A = 100; //no error
static const float B = 2.0f; //error, can't define this type in class definition.

[One Definition Rule](http://en.wikipedia.org/wiki/One_Definition_Rule). The details surrounding "ODR-use" are foggy but can be mostly generalized to needing an object's address. **EDIT:** Sorry, I misunderstood the OP to mean static data members. — ildjarn, Jan 10 '12 at 23:12
Please show a complete minimal example that is failing for something of the form of `B`, because I'm unable to reproduce this. — bitmask, Jan 10 '12 at 23:13

score 1 · Accepted Answer · answered Jan 10 '12 at 23:14

1

Static constants of integral types can be initialized inside a class definition. That doesn't mean that the object actually exists, since you haven't provided a definition yet, but because the compiler knows the value of the object, you can sometimes get away with it.

That is, if you're not attempting to take the address of the variable or pass it by reference, but only use its value, then you don't need to provide a defintion at all, and the compiler simply substitutes the value wherever you use the variable.

C++11 introduces the constexpr keyword which allows you to do the same for a much wider variety of types.

answered Jan 10 '12 at 23:14

Kerrek SB

464,522
92
875
1,084

so, static const int defined in a header behaves like a literal? – xcrypt Jan 10 '12 at 23:17
@xcrypt: Not "defined in a header", but "inside a class definition" (as Kerrek correctly said). There is NOTHING special about headers or `#include`. – Ben Voigt Jan 10 '12 at 23:18
@BenVoigt, where did I mention #include? Please calm down. :) – xcrypt Jan 10 '12 at 23:20
@xcrypt: Then what exactly distinguishes a header from any other file, in your mind? – Ben Voigt Jan 10 '12 at 23:22
@BenVoigt I get your point. I am however taught a coding style where I nearly always use headers for class definitions, so I mixed up. – xcrypt Jan 10 '12 at 23:24
@Ben: ... the extension ... or lack thereof ... – Xeo Jan 10 '12 at 23:25
@Xeo: Don't you mean "lack of extension"? – Ben Voigt Jan 10 '12 at 23:29

Is there reason we are able to define [static const int] in a class definition, but not other static const types?

1 Answers1