typeid operator

From cppreference.com
< cpp‎ | language
 
 
C++ language
General topics
Flow control
Conditional execution statements
if
Iteration statements (loops)
for
range-for (C++11)
Jump statements
Functions
Function declaration
Lambda function expression
inline specifier
Dynamic exception specifications (until C++17*)
noexcept specifier (C++11)
Exceptions
Namespaces
Types
Specifiers
decltype (C++11)
auto (C++11)
alignas (C++11)
Storage duration specifiers
Initialization
Expressions
Alternative representations
Literals
Boolean - Integer - Floating-point
Character - String - nullptr (C++11)
User-defined (C++11)
Utilities
Attributes (C++11)
Types
typedef declaration
Type alias declaration (C++11)
Casts
Implicit conversions - Explicit conversions
static_cast - dynamic_cast
const_cast - reinterpret_cast
Memory allocation
Classes
Class-specific function properties
explicit (C++11)
static
Special member functions
Templates
Miscellaneous
 
Expressions
General
Value categories (lvalue, rvalue, xvalue)
Order of evaluation (sequence points)
Constant expressions
Potentially-evaluated expressions
Primary expressions
Lambda expressions(C++11)
Literals
Integer literals
Floating-point literals
Boolean literals
Character literals including escape sequences
String literals
Null pointer literal(C++11)
User-defined literal(C++11)
Operators
a=b, a+=b, a-=b, a*=b, a/=b, a%=b, a&=b, a|=b, a^=b, a<<=b, a>>=b
++a, --a, a++, a--
+a, -a, a+b, a-b, a*b, a/b, a%b, ~a, a&b, a|b, a^b, a<<b, a>>b
a||b, a&&b, !a
a==b, a!=b, a<b, a>b, a<=b, a>=b, a<=>b (since C++20)
a[b], *a, &a, a->b, a.b, a->*b, a.*b
a(...), a,b, a?b:c
new-expression
delete-expression
throw-expression
alignof
sizeof
sizeof...(C++11)
typeid
noexcept(C++11)
Fold expressions(C++17)
Alternative representations of operators
Precedence and associativity
Operator overloading
Default comparisons(C++20)
Conversions
Implicit conversions
Usual arithmetic conversions
const_cast
static_cast
reinterpret_cast
dynamic_cast
Explicit conversions: (T)a, T(a), auto(a), auto{a} (since C++23)
User-defined conversion
 

Queries information of a type.

Used where the dynamic type of a polymorphic object must be known and for static type identification.

Syntax

typeid ( type ) (1)
typeid ( expression ) (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.

1) Refers to a std::type_info object representing the type type. If type is a reference type, the result refers to a std::type_info object representing the cv-unqualified version of the referenced type.
2) Examines the expression expression
a) If expression is an lvalue (until C++11)a glvalue (since C++11) expression that identifies an object of a polymorphic type (that is, a class that declares or inherits at least one virtual function), the 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 type std::bad_typeid (until C++14)a type matching handlers of type std::bad_typeid (since C++14) is thrown.
b) If expression is not an lvalue (until C++11)a glvalue (since C++11) expression of polymorphic type, 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

typeid

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

See also

contains some type's information, generated by the implementation.
This is the class returned by the typeid operator.
(class)