typeid оператор

Пример вывода с использованием одной из реализаций, в которой type_info::name возвращает полные имена типов; если используется gcc или аналогичный компилятор, пропустите вывод через c++filt -t.

#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';
    }
}

Возможный вывод:

======== 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!