Is type_info not a part of RTTI?

问题

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?

回答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).

回答2:

There is no concept of "RTTI" in the standard. Instead, it's said in different words.

<typeinfo> is referred to as "dynamic type identification" in [support.general]
[intro.object] says: "Some objects are polymorphic (10.3); the implementation generates information associated with each such object that makes it possible to determine that object’s type during program execution"
[expr.dynamic.cast] talks about checks that happen at "run-time". All other uses of "runtime" in the standard refer to something else.

[expr.typeid] explains what typeid does:

2 When typeid is applied to a glvalue expression whose type is a polymorphic class type (10.3), the result refers to a std::type_info object representing the type of the most derived object (1.8) (that is, the dynamic type) to which the glvalue refers. ...

The latter is typically referred to a "run-time" operation.

3 When typeid is applied to an expression other than a glvalue of a polymorphic class type, the result refers to a std::type_info object representing the static type of the expression. ... The expression is an unevaluated operand (Clause 5).

While the former can be seen as a "compile-time" operation.

Regardless, GCC doesn't really care and disables typeid and dynamic_cast altogether if you use the -fno-rtti flag:

rtti.c:

  if (! flag_rtti)
    {
      error ("cannot use typeid with -fno-rtti");
      return false;
    }

Could it disable typeid for polymorphic types only? Sure. But we're going to go with occam's razor in that it's much easier developmentally-wise to prevent the use of typeid altogether.

来源：https://stackoverflow.com/questions/37699188/is-type-info-not-a-part-of-rtti

标签

c++

gcc

rtti