Re: [Boost-users] [Unordered] template find() request

Daniel James escribió:

2009/3/9 :

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FWIW, Boost.MultiIndex hashed indices do have a notion of "compatible keys" allowing for such heterogeneous lookup operations:

http://www.boost.org/libs/multi_index/doc/tutorial/indices.html#hash_lookup http://www.boost.org/libs/multi_index/doc/reference/hash_indices.html#lookup

In the equivalent to this case:

boost::unordered_setstd::string set;

// ....

set.find("x");

Is a std::string constructed for every hash and equality call? I can't see a way to avoid that without introducing a traits class.

Translating the compatible key functionality of B.MI to the syntax of boost::unordered_set, you'd have two ways of doing this: #1 struct custom_hash:boost::hashstd::string { using boost::hashstd::string::operator(); std::size_t operator()(const char* x)const { return boost::hash_range(x,x+std::strlen(x)); } }; struct custom_equal_to:std::equal_tostd::string { using std::equal_tostd::string::operator(); bool operator()(const std::string& x,const char* y)const { return x==y; } bool operator()(const char* x,const std::string& y)const { return x==y; } }; //... boost::unordered_setstd::string,custom_hash,custom_equal_to set; //... set.find("x"); #2 struct custom_hash { std::size_t operator()(const char* x)const { return boost::hash_range(x,x+std::strlen(x)); } }; struct custom_equal_to { bool operator()(const std::string& x,const char* y)const { return x==y; } bool operator()(const char* x,const std::string& y)const { return x==y; } }; //... boost::unordered_setstd::string set; //... set.find("x",custom_hash(),custom_equal_to()); Joaquín M López Muñoz Telefónica, Investigación y Desarrollo