String.h specialize hash<string> causes problems for libc++. about folly

[tudor]

Can you please paste this as text somehow? It seems HTML ate a lot of your
angle brackets :)

Also, what is the error that you're getting? Is it that hashstd::string
is already defined?

On Mon, Jul 7, 2014 at 8:02 PM, likan999 [email protected] wrote:

The following simple program will produce error under libc++:

#include
#include
#include

using namespace std;

namespace std {
template
struct hashstd::basic_string : private hash {
size_t operator()(const std::basic_string & s) const {
return hash::operator()(s.c_str());
}
};
}

const static unordered_map m = {
{"abc", 1},
{"xyz", 2},
};

int main() {
assert(m.find("abc") != m.end());
}

String.h shouldn't specialize hash function for libc++.

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[likan999]

@tudor, the specialization is copied from String.h to demonstrate the problem. Simply including "String.h" can also reproduce the same issue. The error is the code successfully compiles under libc++ but at runtime the assertion fails. I guess the culprit is libc++ has a different hash implementation.

I guess the specialization was introduced when c++11 was not yet fully implemented by mainline compilers and hash was not available. But since hash is in C++11 standard, and major compilers supports c++11 faily well, it is no longer needed?

[tudor]

You're right. Our specialization is actually broken! (because we make
hash call hash<const char*> on s.c_str(), which is clearly bad, as
hash<const char*> doesn't hash the strings, only the pointer value)

We haven't seen this at FB because libstdc++ provides full specializations
(as required by the standard), so hash is more specific than
template hash<basic_string>, so the libstdc++ specialization
wins.

Clearly libc++ does something different (and it's not fully standard
compliant), but, regardless, these specializations are unnecessary AND
broken, so I'll fix.

On Tue, Jul 8, 2014 at 9:51 AM, likan999 [email protected] wrote:

@tudor https://github.com/tudor, the specialization is copied from
String.h to demonstrate the problem. By simply including "String.h" can
also reproduce the same issue. The error is the code successfully compiles
under libc++ but at runtime the assertion fails. I guess the culprit is
libc++ has a different hash implementation.

I guess the specialization was introduced when c++11 was not yet fully
implemented by mainline compilers and hash was not available. But since
hash is in C++11 standard, and major compilers supports c++11 faily well,
it is no longer needed?

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