std::hash value on char* value and not on memory address? - c++

Question

As stated in this link:
There is no specialization for C strings. std::hash produces a hash of the value of the pointer (the memory address), it does not examine the contents of any character array.
Which means that with the same char* value, different hashcodes could be produced. For example, having this code:
//MOK and MOV are template arguments
void emit(MOK key, MOV value) {
auto h = hash<MOK>()(key);
cout<<"key="<<key<<" h="<<h<<endl;
...
This is the output produced by calling 4 times emit() on the same key (with MOK=char*) value (but 4 different tokens/string objects):
key=hello h=140311481289184
key=hello h=140311414180320
key=hello h=140311414180326
key=hello h=140311481289190
How can I obtain the same hash code for char*? I'd prefer not to use boost

Answer 1

There is of course the trivial (and slow) solution of creating a temporary std::string and hashing that one. If you don't want to do this, I'm afraid you will have to implement your own hash function. Sadly enough, the current C++ standard library doesn't provide general purpose hash algorithms disentangled from object-specific hash solutions. (But there is some hope this could change in the future.)
Suppose you had a function
std::size_t
hash_bytes(const void * data, std::size_t size) noexcept;
that would take an address and a size and return you a hash computed from the that many bytes following that address. With the help of that function, you could easily write
template <typename T>
struct myhash
{
std::size_t
operator()(const T& obj) const noexcept
{
// Fallback implementation.
auto hashfn = std::hash<T> {};
return hashfn(obj);
}
};
and then specialize it for the types you're interested in.
template <>
struct myhash<std::string>
{
std::size_t
operator()(const std::string& s) const noexcept
{
return hash_bytes(s.data(), s.size());
}
};
template <>
struct myhash<const char *>
{
std::size_t
operator()(const char *const s) const noexcept
{
return hash_bytes(s, std::strlen(s));
}
};
This leaves you only with the exercise of implementing hash_bytes. Fortunately, there are some fairly good hash functions that are rather easy to implement. My go-to algorithm for simple hashing is the Fowler-Noll-Vo hash function. You can implement it in five lines of code; see the linked Wikipedia article.
If you want to get a bit fancy, consider the following implementation. First, I define a generic template that can be specialized for any version of the FNV-1a hash function.
template <typename ResultT, ResultT OffsetBasis, ResultT Prime>
class basic_fnv1a final
{
static_assert(std::is_unsigned<ResultT>::value, "need unsigned integer");
public:
using result_type = ResultT;
private:
result_type state_ {};
public:
constexpr
basic_fnv1a() noexcept : state_ {OffsetBasis}
{
}
constexpr void
update(const void *const data, const std::size_t size) noexcept
{
const auto cdata = static_cast<const unsigned char *>(data);
auto acc = this->state_;
for (auto i = std::size_t {}; i < size; ++i)
{
const auto next = std::size_t {cdata[i]};
acc = (acc ^ next) * Prime;
}
this->state_ = acc;
}
constexpr result_type
digest() const noexcept
{
return this->state_;
}
};
Next, I provide aliases for the 32 and 64 bit versions. The parameters were taken from Landon Curt Noll's website.
using fnv1a_32 = basic_fnv1a<std::uint32_t,
UINT32_C(2166136261),
UINT32_C(16777619)>;
using fnv1a_64 = basic_fnv1a<std::uint64_t,
UINT64_C(14695981039346656037),
UINT64_C(1099511628211)>;
Finally, I provide type meta-functions to select a version of the algorithm given the wanted number of bits.
template <std::size_t Bits>
struct fnv1a;
template <>
struct fnv1a<32>
{
using type = fnv1a_32;
};
template <>
struct fnv1a<64>
{
using type = fnv1a_64;
};
template <std::size_t Bits>
using fnv1a_t = typename fnv1a<Bits>::type;
And with that, we're good to go.
constexpr std::size_t
hash_bytes(const void *const data, const std::size_t size) noexcept
{
auto hashfn = fnv1a_t<CHAR_BIT * sizeof(std::size_t)> {};
hashfn.update(data, size);
return hashfn.digest();
}
Note how this code automatically adapts to platforms where std::size_t is 32 or 64 bits wide.

Answer 2

I've had to do this before and ended up writing a function to do this, with essentially the same implementation as Java's String hash function:
size_t hash_c_string(const char* p, size_t s) {
size_t result = 0;
const size_t prime = 31;
for (size_t i = 0; i < s; ++i) {
result = p[i] + (result * prime);
}
return result;
}
Mind you, this is NOT a cryptographically secure hash, but it is fast enough and yields good results.

Answer 3

In C++17 you should use std::hash<std::string_view> which works seamlessly since const char* can be implicitly converted to it.

Answer 4

Since C++17 added std::string_view including a std::hash specialization for it you can use that to compute the hash value of a C-string.
Example:
#include <string_view>
#include <cstring>
static size_t hash_cstr(const char *s)
{
return std::hash<std::string_view>()(std::string_view(s, std::strlen(s)));
}
If you have to deal with a pre-C++17 compiler you can check your STL for an implementation defined hash function and call that.
For example, libstdc++ (which is what GCC uses by default) provides std::_Hash_bytes which can be called like this:
#include <functional>
// -> which finally includes /usr/include/c++/$x/bits/hash_bytes.h
#include <cstring>
static size_t hash_cstr_gnu(const char *s)
{
const size_t seed = 0;
return std::_Hash_bytes(s, std::strlen(s), seed);
}

Answer 5

You can use std::collate::hash
e.g. https://www.cplusplus.com/reference/locale/collate/hash/