class template
<functional>

std::hash

template <class T> struct hash;
Default hash function object class
Unary function object class that defines the default hash function used by the standard library.

The functional call returns a hash value of its argument: A hash value is a value that depends solely on its argument, returning always the same value for the same argument (for a given execution of a program). The value returned shall have a small likelihood of being the same as the one returned for a different argument (with chances of collision approaching 1/numeric_limits<size_t>::max).

Other function object types can be used as Hash for unordered containers provided they behave as defined above and they are at least copy-constructible, destructible function objects.

The default hash is a template class that is not defined for the general case. But all library implementations provide at least the following type-specific specializations:

headertypes
<functional>bool
char
signed char
unsigned char
char16_t
char32_t
wchar_t
short
unsigned short
int
unsigned int
long
unsigned long
long long
unsigned long long
float
double
long double
T* (for any type T)
<string>string
wstring
u16string
u32string
<memory>unique_ptr
shared_ptr
<vector>vector<bool>
<bitset>bitset
<system_error>error_code
<typeindex>type_index
<thread>thread::id

Apart from being callable with an argument of the appropriate types, all objects of hash instantiations are default-constructible, copy-constructible, copy-assignable, destructible and swappable.

Users can provide custom specializations for this template with these same properties.

Member types

member typedefinitionnotes
result_typesize_tThe type of the hash values produced.
argument_typeTThe type of the values taken as argument.

Member functions

operator()
Returns a hash value for its argument, as a value of type size_t.
size_t is an unsigned integral type.

Example

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// hash example
#include <iostream>
#include <functional>
#include <string>

int main ()
{
  char nts1[] = "Test";
  char nts2[] = "Test";
  std::string str1 (nts1);
  std::string str2 (nts2);

  std::hash<char*> ptr_hash;
  std::hash<std::string> str_hash;

  std::cout << "same hashes:\n" << std::boolalpha;
  std::cout << "nts1 and nts2: " << (ptr_hash(nts1)==ptr_hash(nts2)) << '\n';
  std::cout << "str1 and str2: " << (str_hash(str1)==str_hash(str2)) << '\n';

  return 0;
}


Output:
same hashes:
nts1 and nts2: false
str1 and str2: true

Exception safety

No-throw guarantee: no members throw exceptions (this applies to the specializations provided in the library implementation). Custom specializations may provide different guarantees.

C++