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  1. Qt
  2. QTBUG-134690

qHashMulti and qHashMultiCommutative use the slower seed==0 algorithms for strings

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    • eb1342a3c (dev), db36fea29 (6.9), 1a580ca2a (dev), c05ae82ef (dev)

    Description

      The implementations of qHashMulti() and qHashMultiCommutative() are implemented using calls to qHash() without a seed parameter:

          constexpr result_type operator()(size_t seed, const T &t) const noexcept(noexcept(qHash(t)))
    // combiner taken from N3876 / boost::hash_combine
    { return seed ^ (qHash(t) + 0x9e3779b9 + (seed << 6) + (seed >> 2)) ; }
...
    constexpr result_type operator()(size_t seed, const T &t) const noexcept(noexcept(qHash(t)))
    { return seed + qHash(t); } // don't use xor!

      This results in using the slower implementation of qHashBits() for all of our string classes.

      I don't know if qHashMultiCommutative() is fixable, but qHashMulti() definitely is.

      Requires BC break to fix.

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          Bug - A problem which impairs or prevents the functions of the product. QTBUG-134683 Deprecated one argument version of qHash still required in qHashMulti

          • P3: Somewhat important - Should be fixed, but doesn't affect the release in any way.
          • Closed
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              cnn Qt Core & Network
              thiago Thiago Macieira
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                Created:
                Updated:
                Resolved: