Re: [Boost-users] BOOST_STATIC_CONST

From: John Maddock (john_at_[hidden])
Date: 2006-12-01 05:04:45

Michael Held wrote:
> hi,
>
> I am using BOOST_STATIC_CONST for static class members of a base
> class. I have following situation:
>
> template <int T>
> class A
> {
>   typedef typename MyType<T>::value_type value_type;
>   BOOST_STATIC_CONST(typename value_type::some_type, MY_CONSTANT = 0);
> };
>
> template <int T>
> class B : public A<T>
> {
>   typedef A<T> Base;
>   typedef typename Base::value_type value_type;
>   BOOST_STATIC_CONST(typename value_type::some_type, MY_CONSTANT =
> Base::MY_CONSTANT);
> };
>
> or
>
> template <int T>
> class B : public A<T>
> {
>   typedef A<T> Base;
>   typedef typename Base::value_type value_type;
>   using Base::MY_CONSTANT;
> };
>
>
> for both cases I got a linker error:
> /usr/bin/ld: Undefined symbols:
> B<0>::MY_CONSTANT
>
> what am I doing wrong?
> I am using darwin gcc4.0.1 and Boost from CVS

In theory you are doing nothing wrong, linker errors like that should only 
occur if you take the address of the constant: in that case you would need 
an out-of-line definition as well:

template<int T>
type_of_constant B<T>::MY_CONSTANT;

I'll leave it to you to figure out what "type_of_constant" is ! :-)

You would need to wrap this in

#ifndef BOOST_NO_INCLASS_MEMBER_INITIALIZATION

#endif

if you want the code to be portable, but if you really are taking the 
constant's address then it won't be portable to older compilers anyway 
(since you can't take the address of an enum which is what 
BOOST_STATIC_CONSTANT uses in those cases).

HTH, John.