c++ - constexpr begin of a std::array

Question

I am having trouble understanding why both gcc-8.2.0 and clang-7.0.0 reject the following code (live code here):

#include <array>

int main() {

    constexpr std::array<int,3> v{1,2,3};
    constexpr auto b = v.begin(); // error: not a constexpr 
    return 0;
}

with the error

error: '(std::array<int, 3>::const_pointer)(& v.std::array<int,3>::_M_elems)' 
is not a constant expression (constexpr auto b = v.begin();)

According to en.cppreference.com, the begin() member function is declared constexpr. Is this a compiler bug?

Answer 1

So let's sidestep std::array to make this a bit easier:

template <typename T, size_t N>
struct array {
    T elems[N];

    constexpr T const* begin() const { return elems; }
};

void foo() {
    constexpr array<int,3> v{{1, 2, 3}};
    constexpr auto b = v.begin(); // error
}

constexpr array<int, 3> global_v{{1, 2, 3}};
constexpr auto global_b = global_v.begin(); // ok

Why is b an error but global_b is okay? Likewise, why would b become okay if we declared v to be static constexpr? The problem is fundamentally about pointers. In order to have a constant expression that's a pointer, it has to point to one, known, constant thing, always. That doesn't really work for local variables without static storage duration, since they have fundamentally mutable address. But for function-local statics or globals, they do have one constant address, so you can take a constant pointer to them.

---

In standardese, from [expr.const]/6:

A constant expression is either a glvalue core constant expression that refers to an entity that is a permitted result of a constant expression (as defined below), or a prvalue core constant expression whose value satisfies the following constraints:

• if the value is an object of class type, [...]
• if the value is of pointer type, it contains the address of an object with static storage duration, the address past the end of such an object ([expr.add]), the address of a function, or a null pointer value, and
• [...]

b is none of those things in the second bullet, so this fails. But global_b satisfies the bolded condition - as would b if v were declared static.