typeid operator
Queries information of a type.
Used where the dynamic type of a polymorphic object must be known and for static type identification.
Contents |
Syntax
typeid ( type )
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(1) | ||||||||
typeid ( expression )
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(2) | ||||||||
The header <typeinfo> must be included or imported(since C++20) or a standard library module imported(since C++23) before using typeid, otherwise the program is ill-formed.
The typeid expression is an lvalue expression which refers to an object with static storage duration, of const-qualified version of the polymorphic type std::type_info or some type derived from it.
Explanation
If the type of type or expression is a class type or a reference to a class type, then that class type cannot be an incomplete type.
typeid expression evaluates the expression and then refers to the std::type_info object that represents the dynamic type of the expression. If expression is obtained by applying the unary * operator to a pointer and the pointer is a null pointer value, an exception of a type matching handlers of type std::bad_typeid is thrown.typeid does not evaluate the expression, and the std::type_info object it identifies represents the static type of the expression. Lvalue-to-rvalue, array-to-pointer, or function-to-pointer conversions are not performed. Temporary materialization, however, is (formally) performed for prvalue arguments: the argument must be destructible in the context in which the typeid expression appears.(since C++17)If the type of type or expression is cv-qualified, the result of the typeid refers to a std::type_info object representing the cv-unqualified type (that is, typeid(const T) == typeid(T)).
If typeid is used on an object under construction or destruction (in a destructor or in a constructor, including constructor's initializer list or default member initializers), then the std::type_info object referred to by this typeid represents the class that is being constructed or destroyed even if it is not the most-derived class.
Keywords
Notes
When applied to an expression of polymorphic type, evaluation of a typeid expression may involve runtime overhead (a virtual table lookup), otherwise typeid expression is resolved at compile time.
It is unspecified whether the destructor for the object referred to by typeid is executed at the end of the program.
There is no guarantee that the same std::type_info instance will be referred to by all evaluations of the typeid expression on the same type, although they would compare equal, std::type_info::hash_code of those type_info objects would be identical, as would be their std::type_index.
const std::type_info& ti1 = typeid(A); const std::type_info& ti2 = typeid(A); assert(&ti1 == &ti2); // not guaranteed assert(ti1 == ti2); // guaranteed assert(ti1.hash_code() == ti2.hash_code()); // guaranteed assert(std::type_index(ti1) == std::type_index(ti2)); // guaranteed
Example
The example showing output using one of the implementations where type_info::name returns full type names; filter through c++filt -t if using gcc or similar.
#include <iostream> #include <string> #include <typeinfo> struct Base {}; // non-polymorphic struct Derived : Base {}; struct Base2 { virtual void foo() {} }; // polymorphic struct Derived2 : Base2 {}; int main() { int myint = 50; std::string mystr = "string"; double *mydoubleptr = nullptr; std::cout << "myint has type: " << typeid(myint).name() << '\n' << "mystr has type: " << typeid(mystr).name() << '\n' << "mydoubleptr has type: " << typeid(mydoubleptr).name() << '\n'; // std::cout << myint is a glvalue expression of polymorphic type; it is evaluated const std::type_info& r1 = typeid(std::cout << myint); // side-effect: prints 50 std::cout << '\n' << "std::cout<<myint has type : " << r1.name() << '\n'; // std::printf() is not a glvalue expression of polymorphic type; NOT evaluated const std::type_info& r2 = typeid(std::printf("%d\n", myint)); std::cout << "printf(\"%d\\n\",myint) has type : " << r2.name() << '\n'; // Non-polymorphic lvalue is a static type Derived d1; Base& b1 = d1; std::cout << "reference to non-polymorphic base: " << typeid(b1).name() << '\n'; Derived2 d2; Base2& b2 = d2; std::cout << "reference to polymorphic base: " << typeid(b2).name() << '\n'; try { // dereferencing a null pointer: okay for a non-polymorphic expression std::cout << "mydoubleptr points to " << typeid(*mydoubleptr).name() << '\n'; // dereferencing a null pointer: not okay for a polymorphic lvalue Derived2* bad_ptr = nullptr; std::cout << "bad_ptr points to... "; std::cout << typeid(*bad_ptr).name() << '\n'; } catch (const std::bad_typeid& e) { std::cout << " caught " << e.what() << '\n'; } }
Possible output:
======== output from Clang ========
myint has type: i
mystr has type: NSt3__112basic_stringIcNS_11char_traitsIcEENS_9allocatorIcEEEE
mydoubleptr has type: Pd
50
std::cout<<myint has type : NSt3__113basic_ostreamIcNS_11char_traitsIcEEEE
printf("%d\n",myint) has type : i
reference to non-polymorphic base: 4Base
reference to polymorphic base: 8Derived2
mydoubleptr points to d
bad_ptr points to... caught std::bad_typeid
======== output from MSVC ========
myint has type: int
mystr has type: class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >
mydoubleptr has type: double * __ptr64
50
std::cout<<myint has type : class std::basic_ostream<char,struct std::char_traits<char> >
printf("%d\n",myint) has type : int
reference to non-polymorphic base: struct Base
reference to polymorphic base: struct Derived2
mydoubleptr points to double
bad_ptr points to... caught Attempted a typeid of nullptr pointer!Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| CWG 492 | C++98 | when typeid is applied to a reference to cv-qualified type,the result represents the referenced type |
the result represents the cv-unqualified referenced type |
| CWG 1416 | C++98 | the wording regarding top-level cv-qualification might be misinterpreted | improved the wording |
| CWG 1431 | C++98 | typeid was only allowed to throw std::bad_typeid
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allowed to throw matchable derived classes |
See also
| contains some type’s information, the class returned by the typeid operator (class) |
