Re: [Boost-users] [multi_index] function object as key extractor?

From: Filip Konvička (filip.konvicka_at_[hidden])
Date: 2007-03-15 07:46:15

>> I've been playing with the problem too, and found this solution...it
>> does not rely on typeof, but it does rely on return_type. I'm a bit
>> unsure about why I had to use "unsigned int" to make it work, so I'd
>> welcome some comments on this.
>>     
>
> Do you mean you couldn't use RetType properly? I've tried with
> the following in GCC 3.2:
>
>   template<class T, typename MemberType, MemberType T::* MemberPtr,
>   typename RetType, RetType (MemberType::*MemberFct)() const>
>   struct extract_and_call {
>       typedef RetType result_type;
>       RetType operator()(T const& t) const {
>           return ((t .* MemberPtr) .* MemberFct)();
>       }
>   };
>   
Aye, I got the following warning that I could not get rid of (VC 7.1)

C:\Boost\include\boost-1_33_1\boost\functional\hash\hash.hpp(132) : 
warning C4267: 'argument' : conversion from 'size_t' to 'unsigned int', 
possible loss of data

Complete error (warning) message:

C:\Boost\include\boost-1_33_1\boost\functional\hash\hash.hpp(132) : 
error C2220: warning treated as error - no object file generated
        
C:\Boost\include\boost-1_33_1\boost\functional\hash\hash.hpp(130) : 
while compiling class-template member function 'size_t 
boost::hash_detail::call_hash<T>::call(const T &)'
        with
        [
            
T=extract_and_call<foo,std::string,pointer-to-member(0x0),std::basic_string<char,std::char_traits<char>,std::allocator<char>>::size_type,std::basic_string<char,std::char_traits<char>,std::allocator<char>>::size_type 
std::basic_string<char,std::char_traits<char>,std::allocator<char>>::size(void) 
const>::result_type
        ]
        
C:\Boost\include\boost-1_33_1\boost\functional\hash\hash.hpp(315) : see 
reference to class template instantiation 
'boost::hash_detail::call_hash<T>' being compiled
        with
        [
            
T=extract_and_call<foo,std::string,pointer-to-member(0x0),std::basic_string<char,std::char_traits<char>,std::allocator<char>>::size_type,std::basic_string<char,std::char_traits<char>,std::allocator<char>>::size_type 
std::basic_string<char,std::char_traits<char>,std::allocator<char>>::size(void) 
const>::result_type
        ]
        
C:\Boost\include\boost-1_33_1\boost\functional\hash\hash.hpp(314) : 
while compiling class-template member function 'size_t 
boost::hash<T>::operator ()(const T &) const'
        with
        [
            
T=extract_and_call<foo,std::string,pointer-to-member(0x0),std::basic_string<char,std::char_traits<char>,std::allocator<char>>::size_type,std::basic_string<char,std::char_traits<char>,std::allocator<char>>::size_type 
std::basic_string<char,std::char_traits<char>,std::allocator<char>>::size(void) 
const>::result_type
        ]
        
C:\Boost\include\boost-1_33_1\boost\multi_index\hashed_index.hpp(980) : 
see reference to class template instantiation 'boost::hash<T>' being 
compiled
        with
        [
            
T=extract_and_call<foo,std::string,pointer-to-member(0x0),std::basic_string<char,std::char_traits<char>,std::allocator<char>>::size_type,std::basic_string<char,std::char_traits<char>,std::allocator<char>>::size_type 
std::basic_string<char,st
d::char_traits<char>,std::allocator<char>>::size(void) const>::result_type
        ]
        
C:\Boost\include\boost-1_33_1\boost\multi_index_container.hpp(80) : see 
reference to class template instantiation 
'boost::multi_index::detail::hashed_index<KeyFromValue,Hash,Pred,SuperMeta,TagList,Category>' 
being compiled
        with
        [
            
KeyFromValue=boost::multi_index::hashed_unique<extract_and_call<foo,std::string,pointer-to-member(0x0),std::basic_string<char,std::char_traits<char>,std::allocator<char>>::size_type,std::basic_string<char,std::char_traits<char>,std::allocator<char>>::size_type 
std::basic_string<char,std::char_traits<char>,std::allocator<char>>::size(void) 
const>>::key_from_value_type,
            
Hash=boost::multi_index::hashed_unique<extract_and_call<foo,std::string,pointer-to-member(0x0),std::basic_string<char,std::char_traits<char>,std::allocator<char>>::size_type,std::basic_string<char,std::char_traits<char>,std::allocator<char>>::size_type 
std::basic_string<char,std::char_traits<char>,std::allocator<char>>::size(void) 
const>>::hash_type,
            
Pred=boost::multi_index::hashed_unique<extract_and_call<foo,std::string,pointer-to-member(0x0),std::basic_string<char,std::char_traits<char>,std::allocator<char>>::size_type,std::basic_string<char,std::char_traits<char>,std::allocator<char>>::size_type 
std::basic_string<char,std::char_traits<char>,std::allocator<char>>::size(void) 
const>>::pred_type,
            
SuperMeta=boost::multi_index::detail::nth_layer<1,foo,boost::multi_index::indexed_by<boost::multi_index::hashed_unique<extract_and_call<foo,std::string,pointer-to-member(0x0),std::basic_string<char,std::char_traits<char>,std::allocator<char>>::size_type,std::basic_string<char,std::char_traits<char>,std::allocator<char>>::size_type 
std::basic_string<char,std::char_traits<char>,std::allocator<char>>::size(void) 
const>>,boost::multi_index::ordered_unique<extract_and_call<foo,std::string,pointer-to-member(0x0),std::basic_string<char,std::char_traits<char>,std::allocator<char>>::siz
e_type,std::basic_string<char,std::char_traits<char>,std::allocator<char>>::size_type 
std::basic_string<char,std::char_traits<char>,std::allocator<char>>::size(void) 
const>>>,std::allocator<foo>>,
            
TagList=boost::multi_index::hashed_unique<extract_and_call<foo,std::string,pointer-to-member(0x0),std::basic_string<char,std::char_traits<char>,std::allocator<char>>::size_type,std::basic_string<char,std::char_traits<char>,std::allocator<char>>::size_type 
std::basic_string<char,std::char_traits<char>,std::allocator<char>>::size(void) 
const>>::tag_list_type,
            Category=boost::multi_index::detail::hashed_unique_tag
        ]
        LCS.cpp(94) : see reference to class template instantiation 
'boost::multi_index::multi_index_container<Value,IndexSpecifierList>' 
being compiled
        with
        [
            Value=foo,
            
IndexSpecifierList=boost::multi_index::indexed_by<boost::multi_index::hashed_unique<extract_and_call<foo,std::string,pointer-to-member(0x0),std::basic_string<char,std::char_traits<char>,std::allocator<char>>::size_type,std::basic_string<char,std::char_traits<char>,std::allocator<char>>::size_type 
std::basic_string<char,std::char_traits<char>,std::allocator<char>>::size(void) 
const>>,boost::multi_index::ordered_unique<extract_and_call<foo,std::string,pointer-to-member(0x0),std::basic_string<char,std::char_traits<char>,std::allocator<char>>::size_type,std::basic_string<char,std::char_traits<char>,std::allocator<char>>::size_type 
std::basic_string<char,std::char_traits<char>,std::allocator<char>>::size(void) 
const>>>
        ]
C:\Boost\include\boost-1_33_1\boost\functional\hash\hash.hpp(132) : 
warning C4267: 'argument' : conversion from 'size_t' to 'unsigned int', 
possible loss of data

Cheers,
Filip