how to get rid of nasty compiler warning in boost/property_tree/detail/rapidxml.hpp

[Dieter Stach <Dieter.Stach@…>]

compiling e.g libs/graph/src/graphml.cpp warns ...

/boost/property_tree/detail/rapidxml.hpp: In function `size_t boost::property_t
ree::detail::rapidxml::internal::get_index(Ch) [with Ch = char]':
./boost/property_tree/detail/rapidxml.hpp:1413:   instantiated from `static unsi
gned char boost::property_tree::detail::rapidxml::xml_document<Ch>::whitespace_p
red::test(Ch) [with Ch = char]'
./boost/property_tree/detail/rapidxml.hpp:1542:   instantiated from `static void
 boost::property_tree::detail::rapidxml::xml_document<Ch>::skip(Ch*&) [with Stop
Pred = boost::property_tree::detail::rapidxml::xml_document<char>::whitespace_pr
ed, int Flags = 3072, Ch = char]'
./boost/property_tree/detail/rapidxml.hpp:1377:   instantiated from `void boost:
:property_tree::detail::rapidxml::xml_document<Ch>::parse(Ch*) [with int Flags =
 3072, Ch = char]'
./boost/property_tree/detail/xml_parser_read_rapidxml.hpp:116:   instantiated fr
om `void boost::property_tree::xml_parser::read_xml_internal(std::basic_istream<
typename Ptree::key_type::value_type, std::char_traits<typename Ptree::key_type:
:value_type> >&, Ptree&, int, const std::string&) [with Ptree = boost::property_
tree::basic_ptree<std::string, std::string, std::less<std::string> >]'
./boost/property_tree/xml_parser.hpp:52:   instantiated from `void boost::proper
ty_tree::xml_parser::read_xml(std::basic_istream<typename Ptree::key_type::value
_type, std::char_traits<typename Ptree::key_type::value_type> >&, Ptree&, int) [
with Ptree = boost::property_tree::ptree]'
libs/graph/src/graphml.cpp:49:   instantiated from here
./boost/property_tree/detail/rapidxml.hpp:317: warning: comparison is always fal
se due to limited range of data type

Caused by the template:

template<class Ch>
        inline size_t get_index(const Ch c)
        {
            // If not ASCII char, its sematic is same as plain 'z'
            if (c > 255)
            {
                return 'z';
            }
            return c;
        }

How to avoid ? Just specify additionally a user defined implementation of get_index for the "char" type :

        inline size_t get_index(const char c)
        {
            return c;
        }

        template<class Ch>
        inline size_t get_index(const Ch c)
        {
            // If not ASCII char, its sematic is same as plain 'z'
            if (c > 255)
            {
                return 'z';
            }
            return c;
        }

I checked the code size (using gcc4.0) : same and the difference of the ASM code: "same"

14972c14972
<       cmpq    (%r15), %rdi
---
>       cmpq    %rdi, (%r15)
14974c14974
<       jae     .L3637
---
>       jbe     .L3637
15002c15002
<       cmpq    -16(%rsi), %rdi
---
>       cmpq    %rdi, -16(%rsi)
15004c15004
<       jae     .L3643
---
>       jbe     .L3643
15017c15017
<       cmpq    -16(%rdx), %rdi
---
>       cmpq    %rdi, -16(%rdx)
15019c15019
<       jb      .L3796
---
>       ja      .L3796
15487c15487
---