std::type_info::hash_code

 

C++

Language

Standard library headers

Concepts

Utilities library

Strings library

Containers library

Algorithms library

Iterators library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

Technical Specifications

 

Utilities library

Type support (basic types, RTTI, type traits)

Dynamic memory management

Error handling

Program utilities

Variadic functions

Date and time

Function objects

initializer_list (C++11)

bitset

hash (C++11)

Relational operators

rel_ops::operator!=rel_ops::operator>rel_ops::operator<=rel_ops::operator>=

Pairs and tuples

pair

tuple (C++11)

piecewise_construct_t (C++11)

piecewise_construct (C++11)

integer_sequence (C++14)

Swap, forward and move

swap

exchange (C++14)

forward (C++11)

move (C++11)

move_if_noexcept (C++11)

declval (C++11)

 

Type support

Basic types
Fundamental types
Fixed width integer types (C++11)
ptrdiff_t size_t max_align_t (C++11)	nullptr_t (C++11) offsetof NULL
Numeric limits
numeric_limits
C numeric limits interface
Runtime type information
type_info type_index (C++11)	bad_typeid bad_cast
Type traits
Type categories
is_void (C++11) is_null_pointer (C++14) is_array (C++11) is_pointer (C++11) is_enum (C++11) is_union (C++11) is_class (C++11) is_function (C++11)	is_object (C++11) is_scalar (C++11) is_compound (C++11) is_integral (C++11) is_floating_point (C++11) is_fundamental (C++11) is_arithmetic (C++11)
is_reference (C++11)
is_lvalue_reference (C++11)
is_rvalue_reference (C++11)
is_member_pointer (C++11)
is_member_object_pointer (C++11)
is_member_function_pointer (C++11)

Type properties
is_const (C++11) is_volatile (C++11) is_pod (C++11) is_empty (C++11) is_polymorphic (C++11) is_final (C++14) is_abstract (C++11)	is_trivial (C++11) is_trivially_copyable (C++11) is_standard_layout (C++11) is_literal_type (C++11) is_signed (C++11) is_unsigned (C++11)

Supported operations
is_constructibleis_trivially_constructibleis_nothrow_constructible (C++11)(C++11)(C++11)
is_default_constructibleis_trivially_default_constructibleis_nothrow_default_constructible (C++11)(C++11)(C++11)
is_copy_constructibleis_trivially_copy_constructibleis_nothrow_copy_constructible (C++11)(C++11)(C++11)
is_move_constructibleis_trivially_move_constructibleis_nothrow_move_constructible (C++11)(C++11)(C++11)
is_assignableis_trivially_assignableis_nothrow_assignable (C++11)(C++11)(C++11)
is_copy_assignableis_trivially_copy_assignableis_nothrow_copy_assignable (C++11)(C++11)(C++11)
is_move_assignableis_trivially_move_assignableis_nothrow_move_assignable (C++11)(C++11)(C++11)
is_destructibleis_trivially_destructibleis_nothrow_destructible (C++11)(C++11)(C++11)
has_virtual_destructor (C++11)
Relationships and property queries
is_same (C++11) is_base_of (C++11) is_convertible (C++11)	alignment_of (C++11) rank (C++11) extent (C++11)
Type modifications
remove_cvremove_constremove_volatile (C++11)(C++11)(C++11) add_cvadd_constadd_volatile (C++11)(C++11)(C++11) make_signed (C++11) make_unsigned (C++11)	remove_reference (C++11) add_lvalue_referenceadd_rvalue_reference (C++11)(C++11) remove_pointer (C++11) add_pointer (C++11) remove_extent (C++11) remove_all_extents (C++11)
Type transformations
aligned_storage (C++11) aligned_union (C++11) decay (C++11) enable_if (C++11) void_t (C++17)	conditional (C++11) common_type (C++11) underlying_type (C++11) result_of (C++11)
Type trait constants
integral_constant (C++11)

 

std::type_info

Member functions

type_info::~type_info

type_info::operator==type_info::operator!=

type_info::before

type_info::hash_code (C++11)

type_info::name

 

size_t hash_code() const;		(since C++11)

Returns an unspecified value, which is identical for the type_info objects referring to the same type. No other guarantees are given. For example, the same value may be returned for different types. The value can also change between invocations of the same program.

Parameters

(none)

Return value

Some value, which is identical for the same types.

Example

The following program is an example of an efficient type-value mapping without using std::type_index.

Run this code

#include <iostream>
#include <typeinfo>
#include <unordered_map>
#include <string>
#include <functional>
#include <memory>
 
struct A {
    virtual ~A() {}
};
 
struct B : A {};
struct C : A {};
 
using TypeInfoRef = std::reference_wrapper<const std::type_info>;
 
struct Hasher {
    std::size_t operator()(TypeInfoRef code) const
    {
        return code.get().hash_code();
    }
};
 
struct EqualTo {
    bool operator()(TypeInfoRef lhs, TypeInfoRef rhs) const
    {
        return lhs.get() == rhs.get();
    }
};
 
int main()
{
    std::unordered_map<TypeInfoRef, std::string, Hasher, EqualTo> type_names;
 
    type_names[typeid(int)] = "int";
    type_names[typeid(double)] = "double";
    type_names[typeid(A)] = "A";
    type_names[typeid(B)] = "B";
    type_names[typeid(C)] = "C";
 
    int i;
    double d;
    A a;
 
    // note that we're storing pointer to type A
    std::unique_ptr<A> b(new B);
    std::unique_ptr<A> c(new C);
 
    std::cout << "i is " << type_names[typeid(i)] << '\n';
    std::cout << "d is " << type_names[typeid(d)] << '\n';
    std::cout << "a is " << type_names[typeid(a)] << '\n';
    std::cout << "b is " << type_names[typeid(*b)] << '\n';
    std::cout << "c is " << type_names[typeid(*c)] << '\n';
}

Output:

i is int
d is double
a is A
b is B
c is C

See also

operator==operator!=	checks whether the objects refer to the same type (public member function)
name	implementation defined name of the type (public member function)