CODEWITHSUNDEEP
c++language-lawyer
Brian Bi
Brian Bi

Reputation: 119099

In C++, can a C-style cast invoke a conversion function and then cast away constness?

GCC and Clang both reject the C-style cast in the following code.

http://coliru.stacked-crooked.com/a/c6fb8797d9d96a27

struct S {
    typedef const int* P;
    operator P() { return nullptr; }
};
int main() {
    int* p1 = const_cast<int*>(static_cast<const int*>(S{}));
    int* p2 = (int*)(S{});
}

main.cpp: In function 'int main()':
main.cpp:7:25: error: invalid cast from type 'S' to type 'int*'
     int* p2 = (int*)(S{});
main.cpp:7:15: error: cannot cast from type 'S' to pointer type 'int *'
    int* p2 = (int*)(S{});
              ^~~~~~~~~~~

However, according to the standard, a C-style cast can perform the conversions performed by a static_cast followed by a const_cast. Is this code well-formed? If not, why not?

Upvotes: 27

Views: 725

Answers (1)

Columbo
Columbo

Reputation: 60979

This is core issue 909:

According to 5.4 [expr.cast] paragraph 4, one possible interpretation of an old-style cast is as a static_cast followed by a const_cast. One would therefore expect that the expressions marked #1 and #2 in the following example would have the same validity and meaning:

struct S {
  operator const int* ();
};

void f(S& s)  {
  const_cast<int*>(static_cast<const int*>(s));  // #1
  (int*) s;  // #2
}

However, a number of implementations issue an error on #2.

Is the intent that (T*)x should be interpreted as something like

const_cast<T*>(static_cast<const volatile T*>(x))

Rationale (July, 2009):

According to the straightforward interpretation of the wording, the example should work. This appears to be just a compiler bug.

This was apparently never resolved by neither Clang nor GCC. Time to open tickets.

Upvotes: 10

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