Given two values x and y, if x == y, it is convenient for hash(x) == hash(y).
In this situation, given a floating point value r, complex{r) == r, so it makes sense that hash(complex{r}) == hash(r). This might enable heterogeneous hash table lookup.
Howard
On Oct 16, 2021, at 6:44 PM, Peter Dimov via Boost
At the moment, boost::hash_value for complex<T> is specified as
When T is a built in type and val.imag() == 0, the result is equal to hash_value(val.real()). Otherwise an unspecified value, except that equal arguments shall yield the same result.
This is a bit odd; the natural implementation for a two-argument point/tuple type is
std::size_t seed = 0;
boost::hash_combine( seed, v.real() ); boost::hash_combine( seed, v.imag() );
return seed;
Can anyone offer an opinion on whether or why the original behavior is preferable?
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