In what cases does the standard forbid taking pointers to functions/objects?

Question

This article lists all the different categories of pointers. I have tested the explicit conversion of different types of pointers to const void* in the following snippet (live):

#include <print>


int var {};

void foo()
{
}

struct Bar
{
    char mem;

    void func()
    {
    }
};

Bar b {};

int main()
{
    // nullptr
    std::println( "{}", static_cast<const void*>( nullptr ) );
    // pointer to object
    std::println( "{}", static_cast<const void*>( &var ) );
    // pointer to function
    std::println( "{}", reinterpret_cast<const void*>( &foo ) );
    // pointer to member variable
    std::println( "{}", static_cast<const void*>( &Bar::mem ) ); // doesn't compile
    std::println( "{}", static_cast<const void*>( &(b.mem) ) );
    std::println( "{}", static_cast<const void*>( &(b.*(&Bar::mem)) ) );
    // pointer to member function
    std::println( "{}", reinterpret_cast<const void*>( &Bar::func ) ); // compiles with a warning
    std::println( "{}", reinterpret_cast<const void*>( &(b.func) ) ); // doesn't compile
    std::println( "{}", reinterpret_cast<const void*>( &(b.*(&Bar::func)) ) ); // doesn't compile

    // Did I miss any other category?
}

Now I have a few questions:

1. I want to know why static_cast<const void*>( &Bar::mem ) ); does not compile (since &(b.*(&Bar::mem)) does). The compiler says:

error: invalid 'static_cast' from type 'char Bar::*' to type 'const void*'

2. Also I want to know why it's not allowed to take the address of member functions (as seen in the last three statements). The compiler says:

ISO C++ forbids taking the address of a bound member function to form a pointer to member function.  Say '&Bar::func'

3. Finally, which pointer types are we allowed to take (at least for trivial tasks like printing) without invoking errors or UB? It seems to me that pointer to member function is not allowed.

Answer 1

A pointer to member is different from other pointers. Its not bound to a specific instance, but its rather an offset into a class. Given an instance you can use a pointer to member to get the member. Casting it to void* is of no great use, hence C++ does not allow it. Generally C++ is less permissive than C concering casts to void*. Hence, static_cast<const void*>( &Bar::mem ) ); does not compile.

On the other hand (b.*(&Bar::mem) uses the instance and the pointer to member to get a reference to the member mem of b. And &(b.*(&Bar::mem) is just the same as &(b.mem). Its a pointer to b.mem, a char*. You can cast char* to void*.

(2) and (3) is just about how the syntax is defined.