Is casting a null pointer to a reference defined behavior?

Question

I have this piece of C++ code:

int main() {
    int* ptr = nullptr;
    int& ref = reinterpret_cast<int&>(ptr);
    return ref;
}

While compiling with -Wall -Wextra -pedantic on both compilers, neither Clang nor GCC diagnose the cast, and only GCC diagnoses that I dereference a null pointer.

Is the cast from ptr to ref undefined behaviour? Or is only the read from ref in the return statement UB?

Answer 1

only GCC diagnoses that I dereference a null pointer.

That is not the warning I get when compiling your code. Instead I get

warning: casting 'int*' to 'int&' does not dereference pointer

This is a warning that there is no dereference, from a null pointer or otherwise.

Is the cast from ptr to ref undefined behaviour?

The cast is perfectly valid by itself. This falls under point 6 of cppreference.com's documenation. A glvalue of type int* can be converted to a reference to another type, in this case int. The result is that of *reinterpret_cast<int*>(&ptr). That is, this cast causes ref to refer to the value of ptr, not (as you seem to think) to what that value points to.

If it was safe to access the reference, ref would start with the value zero (assuming the common case where a null pointer has all zero bits). If a new pointer was assigned to ptr, then the value of ref would also change, still assuming access was safe.

Or is only the read from ref in the return statement UB?

An int is not allowed to alias a int*, so, yes, accessing the reference in the return statement yields undefined behavior. The same would be true if ptr had been initialized with int* ptr = new int; – whether or not the pointer is null has no bearing.