std:: type_index

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C++
Utilities library
General utilities
Relational operators (deprecated in C++20)
Type support
std::type_index
Defined in header <typeindex>
class type_index ;
(since C++11)

The type_index class is a wrapper class around a std::type_info object, that can be used as index in associative and unordered associative containers. The relationship with type_info object is maintained through a pointer, therefore type_index is CopyConstructible and CopyAssignable .

Member functions

constructs the object
(public member function)
(destructor)
(implicitly declared)
destroys the type_index object
(public member function)
operator=
(implicitly declared)
assigns a type_index object
(public member function)
(removed in C++20) (C++20)
compares the underlying std::type_info objects
(public member function)
returns hashed code
(public member function)
returns implementation defined name of the type,
associated with underlying type_info object
(public member function)

Helper classes

(C++11)
hash support for std::type_index
(class template specialization)

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 A
    std::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';
}

Output: 

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

See also

contains some type’s information, the class returned by the typeid operator
(class)
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