Re: [Boost-users] multi_index::composite_key with char[]

leyiliu <595657419 <at> qq.com> writes:

Hi,All: When I use the multi_index::composite_key with char[],I meet a error,when find a element。It seems to the make_tuple can't support the char[].Because the char[] more faster the std::string。So our project choose char[] as the column instead of std::string.So How should I do

Hi leyiliu, I'm not totally sure I'm getting your question, so allow to me to make a guess: I assume you have something like struct X { int x; char str[20]; }; using multi_index_t=multi_index_container< X, indexed_by< ordered_unique< composite_key< X, member, member > > > >; ... multi_index_t m=...; auto it=m.lower_bound(std::make_tuple(1,"hello")); which fails with error: no match for call to '(const std::less) (const char* const&, const char [20]) because std::less won't accept a const char* (the type of "hello") as an argument. There's two dixes for this: 1. Provide a comparison for str that does the work using multi_index_t=multi_index_container< X, indexed_by< ordered_unique< composite_key< X, member, member >, composite_key_compare< std::less<int>, std::less > > > >; std::less works equally well for const char* and char[20] (technically, the latter *decays* to const char* when passed to std::less::operator()). 2. Provide a key extractor that returns a const char* rather than a reference to char[20]: inline const char* get_str(const X& x){return x.str;} using multi_index_t=multi_index_container< X, indexed_by< ordered_unique< composite_key< X, member, global_fun > > > >; Please report back whether this is your problem and any of 1,2 solves it. Best regards, Joaquín M López Muñoz Telefónica