std::type_index

From cppreference.com

Compiler support
Freestanding and hosted
Language
Standard library
Standard library headers
Named requirements
Feature test macros(C++20)
Language support library
Concepts library(C++20)
Diagnostics library
Memory management library
Metaprogramming library(C++11)
General utilities library
Containers library
Iterators library
Ranges library(C++20)
Algorithms library
Strings library
Text processing library
Numerics library
Date and time library
Input/output library
Filesystem library(C++17)
Concurrency support library(C++11)
Execution control library(C++26)
Technical specifications
Symbols index
External libraries

[edit]

Utilities library

Language support

Type support (basic types, RTTI)

Library feature-test macros(C++20)

Program utilities

Variadic functions

initializer_list

(C++11)

is_constant_evaluated

(C++20)

is_within_lifetime

(C++26)

source_location

(C++20)

Coroutine support(C++20)

Contract support(C++26)

Three-way comparison

three_way_comparablethree_way_comparable_with

(C++20)(C++20)

strong_ordering

(C++20)

weak_ordering

(C++20)

partial_ordering

(C++20)

common_comparison_category

(C++20)

compare_three_way_result

(C++20)

(C++20)

(C++20)

(C++20)

(C++20)

compare_strong_order_fallback

(C++20)

compare_weak_order_fallback

(C++20)

compare_partial_order_fallback

(C++20)

is_eqis_ltis_lteq

(C++20)(C++20)(C++20)

is_neqis_gtis_gteq

(C++20)(C++20)(C++20)

General utilities

Function objects
Bit manipulation(C++20)
C-style bit manipulation(C++26)
bitset
hash (C++11)

Relational operators(deprecated in C++20)

rel_ops::operator!=rel_ops::operator>

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

Integer comparison functions

cmp_equalcmp_lesscmp_less_than

(C++20)(C++20)(C++20)

cmp_not_equalcmp_greatercmp_greater_than

(C++20)(C++20)(C++20)

in_range

(C++20)

Swap andtype operations

swap
ranges::swap (C++20)
exchange (C++14)
declval (C++11)
to_underlying (C++23)

forward (C++11)
forward_like (C++23)
move (C++11)
move_if_noexcept (C++11)
as_const (C++17)

Common vocabulary types

pair
tuple (C++11)
optional (C++17)
any (C++17)
variant (C++17)

tuple_size (C++11)
tuple_element (C++11)
apply (C++17)
make_from_tuple (C++17)
expected (C++23)

[edit]

Type support

Basic types

Fixed width integer types(C++11)

Fixed width floating-point types(C++23)

ptrdiff_t
size_t
max_align_t (C++11)
byte (C++17)

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

[edit]

std::type_index

Member functions
type_index::type_index
type_index::operator=type_index::operator!=type_index::operator<type_index::operator<=type_index::operator>type_index::operator>=type_index::operator<=> (until C++20)(C++20)
type_index::hash_code
type_index::name
Helper classes
hash<std::type_index> (C++11)

[edit]

Defined in header`<typeindex>`
class type_index;		(since C++11)

Thetype_index class is a wrapper class around astd::type_info object, that can be used as index in associative and unordered associative containers. The relationship withtype_info object is maintained through a pointer, thereforetype_index isCopyConstructible andCopyAssignable.

[edit]Member functions

(constructor)	constructs the object (public member function)[edit]
(destructor) (implicitly declared)	destroys the`type_index` object (public member function)
operator= (implicitly declared)	assigns a`type_index` object (public member function)
operator==operator!=operator<operator<=operator>operator>=operator<=> (removed in C++20)(C++20)	compares the underlyingstd::type_index objects (public member function)[edit]
hash_code	returns hashed code (public member function)[edit]
name	returns implementation defined name of the type, associated with underlying`type_info` object (public member function)[edit]

[edit]Helper classes

std::hash<std::type_index>

(C++11)

hash support forstd::type_index
(class template specialization)[edit]

[edit]Example

The following program is an example of an efficient type-value mapping.

Run this code

#include <iostream>#include <memory>#include <string>#include <typeindex>#include <typeinfo>#include <unordered_map> struct A{virtual ~A(){}}; struct B: A{};struct C: A{}; int main(){std::unordered_map<std::type_index,std::string> type_names;     type_names[std::type_index(typeid(int))]="int";    type_names[std::type_index(typeid(double))]="double";    type_names[std::type_index(typeid(A))]="A";    type_names[std::type_index(typeid(B))]="B";    type_names[std::type_index(typeid(C))]="C"; int i;double d;    A a; // note that we're storing pointer to type Astd::unique_ptr<A> b(new B);std::unique_ptr<A> c(new C); std::cout<<"i is "<< type_names[std::type_index(typeid(i))]<<'\n';std::cout<<"d is "<< type_names[std::type_index(typeid(d))]<<'\n';std::cout<<"a is "<< type_names[std::type_index(typeid(a))]<<'\n';std::cout<<"*b is "<< type_names[std::type_index(typeid(*b))]<<'\n';std::cout<<"*c is "<< type_names[std::type_index(typeid(*c))]<<'\n';}