The following code gives an error in spite of c being const.

class A {
	static const double c = 1.0;
};


        static const double c = 1.0;
                              ^
te.cpp(5) : Error: initializer for non-const static member must be outside of class def
--- errorlevel 1

I've fallen into this trap as well on more than one occasion, but actually DMC is just following the standard here (you will see similar behaviour using compilers with the EDG front-end). You can only do what you are trying to do for integral or enumeration types.

9.4.2 Static data members [class.static.data] - item 4 says:

If a static data member is of const integral or const enumeration type, its declaration in the class definition can specify a constant-initializer which shall be an integral constant expression (5.19). In that case, the member can appear in integral constant expressions. The member shall still be defined in a namespace scope if it is used in the program and the namespace scope definition shall not contain an initializer.

Sz. Horvát wrote:
> The following code gives an error in spite of c being const.
> 
> class A {
>     static const double c = 1.0;
> };
> 
> 
>         static const double c = 1.0;
>                               ^
> te.cpp(5) : Error: initializer for non-const static member must be outside of class def
> --- errorlevel 1

Anton Sekeris wrote:
> I've fallen into this trap as well on more than one occasion, but actually DMC is just following the standard here (you will see similar behaviour using compilers with the EDG front-end). You can only do what you are trying to do for integral or enumeration types.
> 
> 9.4.2 Static data members [class.static.data] - item 4 says:
> 
> If a static data member is of const integral or const enumeration type, its declaration in the class definition can specify a constant-initializer which shall be an integral constant expression (5.19). In that case, the member can appear in integral constant expressions. The member shall still be defined in a namespace scope if it is used in the program and the namespace scope definition shall not contain an initializer.

Hm ... you are right.  http://www.comeaucomputing.com/tryitout/ gives an error too.  But I really cannot see the reason for not allowing this, it just makes one's life more difficult ... did the people writing the standard simply forget about floating point types?

And DMC's error message is a little bit confusing (telling me that const double is not const).

>> In that case, the member can appear in integral constant
>> expressions.
>
> But I really cannot see the reason for not allowing this,

OK, so should I think about this as a special exception, allowed only to make things similar to the following code possible?

class A {
    static const int len = 3;
    int a[len];
};

But there is no special reason to allow doing the same with float or double (except maybe convenience? ...)

Sz. Horvát wrote:
> But I really cannot see the reason for not allowing this, it just makes one's life more difficult ... did the people writing the standard simply forget about floating point types?

Ours is not to reason why, but to implement the Standard or cry. <g>

Sz. Horvát wrote:
>  >> In that case, the member can appear in integral constant
>>> expressions.
>  >
>  > But I really cannot see the reason for not allowing this,
> 
> OK, so should I think about this as a special exception, allowed only to make things similar to the following code possible?
> 
> class A {
>     static const int len = 3;
>     int a[len];
> };
> 
> But there is no special reason to allow doing the same with float or double (except maybe convenience? ...)

I haven't a clue what the reason was for skipping non-integral built-in types, but your theory sounds quite plausible. From what I've seen in the past these types of standard strangeness are also rather often related to backwards compatibility with C, although in this scenario I don't readily see that.

Sz. Horvát wrote:
> The following code gives an error in spite of c being const.
> 
> class A {
>     static const double c = 1.0;
> };
> 
> 
>         static const double c = 1.0;
>                               ^
> te.cpp(5) : Error: initializer for non-const static member must be outside of class def
> --- errorlevel 1

h/hpp:

class  A
{
   static const double   c;
};

cpp:

const double       A :: c = 1.0;



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