Re: [boost] static in_class constants

Subject: Re: [boost] static in_class constants
From: John Maddock (boost.regex_at_[hidden])
Date: 2010-03-13 04:10:45

> Hi there, I'm finishing up some linker problems I have with gcc. One
> area of problems seems to be static in_class constants that I have
> declared in my header file. MSVC doesn't have problem with it but gcc
> is complaining. I'm wondering how other libs in boost are dealing with
> these problems.
>
> For instance in my new gil::io lib I define some tags in pnm_tags.hpp
> as follows:
>
> /// Defines type for image type property.
> struct pnm_image_type : property_base< uint32_t >
> {
>    BOOST_STATIC_CONSTANT( type, _mono_asc  = 1 ); // Monochrome ASCII 
> encoding
>    BOOST_STATIC_CONSTANT( type, _gray_asc  = 2 ); // Gray level ASCII 
> encoding
>    BOOST_STATIC_CONSTANT( type, _color_asc = 3 ); // sRGB color ASCII 
> encoding
>    BOOST_STATIC_CONSTANT( type, _mono_bin  = 4 ); // Monochrome binary 
> encoding
>    BOOST_STATIC_CONSTANT( type, _gray_bin  = 5 ); // Gray level binary 
> encoding
>    BOOST_STATIC_CONSTANT( type, _color_bin = 6 ); // sRGB color binary 
> encoding
> };

Two questions:

1) What is "type", it's not declared anywhere in the snippet above.
2) I notice you saying that you're getting linker errors - that should only 
happen if you're taking the address of the constants somewhere - if that's a 
use case that you want to support, then you need a definition for the 
constants somewhere - suitably guarded by the right Boost.Config macros 
(sorry can't remember what off the top of my head).

And finally, given that your constants look like enumerated values, is there 
actually a pressing need not to make them an anonymous enumerated type in 
this case?

HTH, John.