Why should C++ ensure that the hash will not throw an exception?

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C.89: make sure that the hash does not throw an exception

Reason (reason)

Users of the hash container indirectly use the hash function and do not want an exception to occur in a simple operation. This is the requirement of the standard library.

Example, bad (negative example)

Template

Struct hash {/ / thoroughly bad hash specialization

Using result_type = size_t

Using argument_type = My_type

Size_t operator () (const My_type & x) const

{

Size_t xs = x.s.size ()

If (xs < 4) throw Bad_My_type {}; / / "Nobody expects the Spanish inquisition!"

Return hash () (x.s.size ()) ^ trim (x.s)

}

}

Int main ()

{

Unordered_map m

My_type mt {"asdfg"}

M [mt] = 7

Cout