Is type_info not a part of RTTI?

Question

I had asked a question Do C++ POD types have RTTI? and someone told me in the comments:

POD types do have type_info, but don't have RTTI, and that's possible because type_info isn't always RTTI.

and it seems right as i could get the type_info of a POD (non-polymorphic) type.

But while I compile this simple program:

#include <iostream>

struct X
{
    int a;
};

int main()
{
    using namespace std;

    std::cout << typeid(X) << std::endl;

    return 0;
}

with flag -fno-rtti of GCC:

$ g++ -fno-rtti main.cpp && ./main

It won't compile:

main.cpp: In function ‘int main()’:
main.cpp:12:26: error: cannot use typeid with -fno-rtti
     std::cout << typeid(X) << std::endl;
                          ^

Does that mean type_info is a part of RTTI, or is it just a behavior of GCC?

Answer 1

Abstract

RTTI per se is not something really formally defined: C++ only says what typeid and dynamic_cast do, not how they're implemented. However, it is convenient indeed to group such kind of operations under a common name which is RTTI.

Notice an implementation is not required to strictly obtain this information at runtime i.e.

if ( typeid(int) == typeid(double) )

could also be determined during the program evaluation, much like std::is_same. int is undeniably non-polymorphic (it has no 'dynamic' type). cppreference even claims:

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.

But it's to be taken cautiously.

---

Does that mean type_info is a part of RTTI, or is it just a behavior of GCC?

type_info is a class. You may not construct any object of that type - you only can through typeid.

-fno-rtti disable RTTI under GCC: you can't use typeid, and thereby neither can be type_info. They're very close each other.

To conclude, the original quote is totally right:

POD types do have type_info, but don't have RTTI, and that's possible because type_info isn't always RTTI.

The runtime information is available through typeid. There is just nothing dynamic to consider (indeed, dynamic_cast would make no sense).