Constexpr pointer to data member conversion

Question

GCC 8.2.1 and MSVC 19.20 compile the below code but Clang 8.0.0 and ICC 19.0.1 fail to do so.

// Base class.
struct Base {};

// Data class.
struct Data { int foo; };

// Derived class.
struct Derived : Base, Data { int bar; };

// Main function.
int main()
{
  constexpr int Data::* data_p{ &Data::foo };
  constexpr int Derived::* derived_p{ data_p };
  constexpr int Base::* base_p{ static_cast<int Base::*>(derived_p) };

  return (base_p == nullptr);
}

The error message with Clang 8.0.0 is the following:

case.cpp:16:33: error: constexpr variable 'base_p' must be initialized by a constant expression
  constexpr int Base::* base_p{ static_cast<int Base::*>(derived_p) };
                              ~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

I noticed that it compiles fine with Clang in two cases:

remove the constexpr from the last definition
replace the line constexpr int Derived::* derived_p{ data_p }; with constexpr int Derived::* derived_p{ &Derived::bar }; .

Should the constexpr expression (the one that makes Clang and ICC fail) compile?

Michael Kenzel Michael Kenzel · Accepted Answer · 2019-04-25T08:14:29

I believe GCC and MSVC are correct, this code should compile.

data_p points to the member foo of Data. derived_p points to the member foo of the Data base class subobject of a Derived via implicit pointer to member conversion [conv.mem]/2.

From [expr.static.cast]/12

A prvalue of type “pointer to member of D of type cv1 T” can be converted to a prvalue of type “pointer to member of B of type cv2 T”, where B is a base class of D, if cv2 is the same cv-qualification as, or greater cv-qualification than, cv1. […] If class B contains the original member, or is a base or derived class of the class containing the original member, the resulting pointer to member points to the original member. Otherwise, the behavior is undefined. [ Note: Although class B need not contain the original member, the dynamic type of the object with which indirection through the pointer to member is performed must contain the original member; see [expr.mptr.oper]. — end note ]

As pointed out by @geza in his comment below, the class Base is a base class of Derived, the latter of which contains the original member Data::foo in its Data base class subobject (the Note in the quote above would seem to be further evidence in support of this interpretation). Thus, the static_cast used to initialize base_pis well-formed and has well-defined behavior. The resulting pointer points to the Data::foo member of a Derived object from the perspective of the Base base class subobject of that Derived object.

To initialize a constexpr object, a constant expression is required [dcl.constexpr]/9. Our expression (the result of the static_cast) is a core constant expression because there is nothing in [expr.const]/2 that would say otherwise. And it is also a constant expression because it is a prvalue that satisfies all the constraints laid out in [expr.const]/5.

Constexpr pointer to data member conversion

2 Answers