Re: [boost] [interprocess] boost::hash - different values forthesame input?

Daniel James wrote:

On 14 January 2016 at 04:54, Peter Dimov wrote:

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Daniel James wrote:

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The reason why I originally wrote that note was because it can generate different hash values when compiled for different platforms or architectures, e.g. a 32-bit executable might generate a different hash value to a 64-bit executable.

Is this just because of pointers and floats, or are there other reasons? size_t hashing to different values perhaps?

Anything with a bigger range than size_t will have to, e.g. int64_t will hash differently on 32 and 64 bit machines.

Yes, my question was a bit stupid because the return value of hash<> is size_t. A better way to put it would be whether the lower 32 bits of the return value need to differ on 32 and 64 bit platforms. I realize that there's a way to hash uint64_t to different values, but there are also ways to make it hash to the same value (mod 2^32). Apart from that, people have argued recently that hash values should vary for each process run. That's because of security considerations with hash tables - an attacker can supply input that is known to create collisions and can so degrade the hash table's performance and effect a denial of service. When this came up on the std reflectors, I argued that the proper way to tackle this issue was not to make the standard hash function nondeterministic, but to have the containers use a random seed when hashing an element.