[feature] Support transforming C-style casts

[Ukilele]

Hey Andres,
a (probably not so) small feature request 🙂

The C-style cast is a pretty complex thing in C++: https://en.cppreference.com/w/cpp/language/explicit_cast . It follows a bunch of logical steps to find out what to actually do in the end. It would be convenient if cppsinsights would show which steps and which casts the compiler eventually executes.

Given following code:

struct Base {};
struct Derived : private Base {};

Derived d;
Base& b = (Base&)d;

const Derived d2;
Base& b2 = (Base&)d2;

It would be nice if cppinsights would show something like this:

struct Base
{
  // inline constexpr Base() noexcept = default;
};

struct Derived : private Base
{
  // inline constexpr Derived() noexcept = default;
};


Derived d;
Base & b = /*extended_*/static_cast<Base&>(d);

const Derived d2;
Base & b2 = const_cast<Base&>(/*extended_*/static_cast<const Base&>(d2));

It seems that this feature is already partly implemented. In the following code, the C-style cast gets correctly transformed into a reinterpret_cast:
Code:

struct A { };
struct B { };

int main()
{
    A a;
    ((B*)&a);
}

Tranformation:

struct A
{
  // inline constexpr A() noexcept = default;
};

struct B
{
};


int main()
{
  A a;
  (reinterpret_cast<B *>(&a));
  return 0;
}

Maybe the feature is already partly implemented but only a few bits are missing?

Thanks and best regards,
Kilian

[andreasfertig]

Hello @Ukilele,

please take a look at #639, which is just closed. In your initial example, Base& b2 = (Base&)d2; performs a cast from derived to base, where base is private. Only the explicit type conversion can achieve that (see expr.cast p4.5). Your proposed code would not compile.

I don't see a (good) way of implementing this feature.

Andreas

[Ukilele]

Yes, in my initial example, Base& b2 = (Base&)d2; performs
1.) a cast from derived to base where base is private
2.) a cast from const& to &, i.e. dropping const qualifiers
These are both things, that a c-style-cast can do. A static_cast can not do this. Yet, when I put following code into cppinsights:

struct Base {};
struct Derived : private Base {};

const Derived d2;
Base& b2 = (Base&)d2;

it gets transformed to:

struct Base
{
  // inline constexpr Base() noexcept = default;
};

struct Derived : private Base
{
  // inline constexpr Derived() noexcept = default;
};


const Derived d2;
Base & b2 = static_cast<Base>(d2);

The last line, Base & b2 = static_cast<Base>(d2);, looks not correct, does it?

Thanks for waiting so long and calmly for my response :)