c++ - std::any without RTTI, how does it work?

Question

If I want to use std::any I can use it with RTTI switched off. The following example compiles and runs as expected also with -fno-rtti with gcc.

int main()
{   
    std::any x;
    x=9.9;
    std::cout << std::any_cast<double>(x) << std::endl;
}

But how std::any stores the type information? As I see, if I call std::any_cast with the "wrong" type I got std::bad_any_cast exception as expected.

How is that realized or is this maybe only a gcc feature?

I found that boost::any did also not need RTTI, but I found also not how that is solved. Does boost::any need RTTI?.

Digging into the STL header itself gives me no answer. That code is nearly unreadable to me.

Answer 1

TL;DR; std::any holds a pointer to a static member function of a templated class. This function can perform many operations and is specific to a given type since the actual instance of the function depends on the template arguments of the class.

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The implementation of std::any in libstdc++ is not that complex, you can have a look at it:

https://github.com/gcc-mirror/gcc/blob/master/libstdc%2B%2B-v3/include/std/any

Basically, std::any holds two things:

• A pointer to a (dynamically) allocated storage;
• A pointer to a "storage manager function":

void (*_M_manager)(_Op, const any*, _Arg*);

When you construct or assign a new std::any with an object of type T, _M_manager points to a function specific to the type T (which is actually a static member function of class specific to T):

template <typename _ValueType, 
          typename _Tp = _Decay<_ValueType>,
          typename _Mgr = _Manager<_Tp>, // <-- Class specific to T.
          __any_constructible_t<_Tp, _ValueType&&> = true,
          enable_if_t<!__is_in_place_type<_Tp>::value, bool> = true>
any(_ValueType&& __value)
  : _M_manager(&_Mgr::_S_manage) { /* ... */ }

Since this function is specific to a given type, you don't need RTTI to perform the operations required by std::any.

Furthermore, it is easy to check that you are casting to the right type within std::any_cast. Here is the core of the gcc implementation of std::any_cast:

template<typename _Tp>
void* __any_caster(const any* __any) {
    if constexpr (is_copy_constructible_v<decay_t<_Tp>>) {
        if (__any->_M_manager == &any::_Manager<decay_t<_Tp>>::_S_manage) {
            any::_Arg __arg;
            __any->_M_manager(any::_Op_access, __any, &__arg);
            return __arg._M_obj;
        }
    }
    return nullptr;
}

You can see that it is simply an equality check between the stored function inside the object you are trying to cast (_any->_M_manager) and the manager function of the type you want to cast to (&any::_Manager<decay_t<_Tp>>::_S_manage).

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The class _Manager<_Tp> is actually an alias to either _Manager_internal<_Tp> or _Manager_external<_Tp> depending on _Tp. This class is also used for allocation / construction of object for the std::any class.