Why does static_cast not work as the cppref site says in such a case?

Question

Consider the following code:

struct A {
    int    n;
    double d;
};

int main() {
    auto a = A{.n = 1, .d = 3.14};
    
    // error: static_cast from 'int*' to 'A*' is not allowed
    auto _ = static_cast<A*>(&a.n); 
}

See https://godbolt.org/z/8814sEh79

However, the documentation of static_cast in cppref says: (emphasis mine)

Two objects a and b are pointer-interconvertible if:

they are the same object, or

one is a union object and the other is a non-static data member of that object, or

one is a standard-layout class object and the other is the first non-static data member of that object or any base class subobject of that object, or

there exists an object c such that a and c are pointer-interconvertible, and c and b are pointer-interconvertible.

According to the citation above, &a and &a.n should be pointer-interconvertible, so why is static_cast<A*>(&a.n) ill-formed?

pointer-interconvertible is irrelevant to static_cast. Note that the link "the C++ standard" references the cppreference site that is a wiki-like site and not the C++ standard. — 3CEZVQ
– 3CEZVQ, Commented Jul 10 at 5:03
In the standard (if there is a conflict between content of the standard, and any other documentation - including cppreference - then the standard is correct, as that is a primary purpose of a standard), the text you quoted is not part of the section on static_cast — Peter
– Peter, Commented Jul 10 at 5:25
@Peter, Yes. What I cited is listed under static_cast, rather than reinterpret_cast. — xmllmx
– xmllmx, Commented Jul 10 at 5:43

Jarod42 · Accepted Answer · 2025-07-10 07:38:14Z

Pointer-interconvertible is only mentioned in 7.g (in the since C++17 part) (emphasis mine):

Otherwise, lvalue-to-rvalue, array-to-pointer, and function-to-pointer conversions are applied to expression. After these conversions, only the following conversions can be performed by static_cast:

[..]

g) An rvalue(until C++11)A prvalue(since C++11) of type “pointer to cv1 void” can be explicitly converted to the type “pointer to cv2 T”

[..]

If expression represents the address A of a byte in memory but A does not satisfy the alignment requirement of T, then the resulting pointer value is unspecified.

Otherwise, if expression points to an object a, and there is an object b of type T (ignoring cv-qualification) that is pointer-interconvertible (see below) with a, the result is a pointer to b.

Otherwise, the pointer value is unchanged by the conversion.

You don't use void* here, so pointer-interconvertible is irrelevant. No mistakes from cppreference.

Raildex · Accepted Answer · 2025-07-10 05:31:43Z

2

What you are referring to is §6.8.4 [basic.compound], paragraph 4:

Two objects a and b are pointer-interconvertible if:

[...]

—(4.3) one is a standard-layout class object and the other is the first non-static data member of that object or

[...]

However, static_cast is not the tool for such a conversion, since for static_cast to work, the types need to be related. in your case, they are not. int is not related to A and vice versa. the only way you can achieve what you want is using a double static_cast over void* :

static_cast<int*>(static_cast<void*>(a));

which is equivalent to reinterpret_cast, which is the correct tool:

If two objects are pointer-interconvertible, then they have the same address, and it is possible to obtain a pointer to one from a pointer to the other via a reinterpret_cast (7.6.1.10)

edited Jul 10 at 5:31

answered Jul 10 at 5:25

Raildex

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3 Comments

xmllmx Jul 10 at 5:43

However, what I cited is is listed under static_cast, rather than reinterpret_cast.

Raildex Jul 10 at 8:22

you can use static_cast, since void* is implicitly interconvertible to every other pointer. Hence you can use a double static_cast as I demonstrated.

Alan Birtles Jul 10 at 6:11

@xmllmx yep and just under that statement they give an example of casting via void*. The statement says the pointers are equivalent, it doesn't say it's possible to directly static_cast between them

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