Re: [boost] static in_class constants

Subject: Re: [boost] static in_class constants
From: Mateusz Loskot (mateusz_at_[hidden])
Date: 2010-03-13 09:39:08

John Maddock wrote:
>> 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.

It is defined in property_base, so the composition looks this way:

template<typename Property>
struct property_base
{
    typedef Property type;
};

struct pnm_image_type : property_base<uint32_>
{
    BOOST_STATIC_CONSTANT( type, _mono_asc  = 1 );
};

> 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?

I suppose it's because of they are need to be composed with
predefined property trait.

Best regards,
-- 
Mateusz Loskot, http://mateusz.loskot.net
Charter Member of OSGeo, http://osgeo.org