[jira] [Created] (THRIFT-2062) cpp flag for using unordered_set instead of set

["Vitali Lovich (JIRA)" <ji...@apache.org>]

Vitali Lovich created THRIFT-2062:
-------------------------------------

             Summary: cpp flag for using unordered_set instead of set
                 Key: THRIFT-2062
                 URL: https://issues.apache.org/jira/browse/THRIFT-2062
             Project: Thrift
          Issue Type: Improvement
          Components: C++ - Compiler
            Reporter: Vitali Lovich


It would be nice if the cpp compiler could generate code to use unordered_set instead of set.  unordered_set is faster & usually an unordered_set is wanted rather than an ordered one.  Furthermore, since unordered_set only needs a hash function, we could generate one for every type.

The hash code generated can be along the lines of what Eclipse does for auto-generated hash codes:

struct ThriftObj {
   1: string f1;
   2: i32 f2;
   3: SomeOtherThriftObj f3;
}

// header file:
namespace std {
    template <> struct hash<ThriftObj> {
        size_t operator()(const ThriftObj& o);
    }
}

// cpp file:
namespace std {
    template <> struct hash<ThriftObj> {
        size_t operator()(const ThriftObj& o)
            constexpr size_t prime = 31;
            size_t hashcode = 1;
            hashcode += o.__isset.f1 ? prime * std::hash<decltype(o.f1)>()(o.f1) : 0;
            hashcode += o.__isset.f2 ? prime * std::hash<decltype(o.f1)>()(o.f2) : 0;
            hashcode += o.__isset.f3 ? prime * std::hash<decltype(o.f1)>()(o.f3) : 0;
        }
    }
}


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