static_cast of member pointer in constexpr context on g++

Question

I came across a problem with g++ with upcasting of a member pointer in a constexpr context using static_cast. See code example.

When compiling with g++ version 6.3 and 7.0 they give a compilation error saying reinterpret_cast is not a constant expression. While clang version 4.0 gives no error, which I think is correct since there is no reinterpret_cast here.

Is this a bug in g++ or clang? What is the correct behavior?

struct Base {}; struct Derived : Base { int i; }; struct Ptr { constexpr Ptr(int Derived::* p) : p(static_cast<int Base::*>(p)){} int Base::* p; }; constexpr Ptr constexpr_ptr(&Derived::i);

Compiler output

g++ -c -std=c++14 test.cpp test.cpp:17:40: in constexpr expansion of ‘Ptr(&Derived::i)’ test.cpp:11:41: error: a reinterpret_cast is not a constant expression constexpr Ptr(int Derived::* p) : p(static_cast<int Base::*>(p)){} ^~~~~~~~~~~~~~~~~~~~~~~~~~~

This is not really an upcast.

n. m. could be an AI
– n. m. could be an AI

2017-10-19 10:50:20 +00:00
Commented Oct 19, 2017 at 10:50 — n. m. could be an AI
– n. m. could be an AI, Commented Oct 19, 2017 at 10:50

Columbo · Accepted Answer · 2017-10-19 10:44:01Z

GCC presumably misapprehends [expr.static.cast]/12, which permits your cast and notes that

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.

Since Base is indeed a base of the class containing the member, the behaviour should be defined, and the constructor call a constant expression.

Collectives™ on Stack Overflow

static_cast of member pointer in constexpr context on g++

1 Answer 1

1 Comment

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Related