Can `reinterpret_cast` be used to initialize a `constexpr` variable?

constexpr Variable Initialization with reinterpret_cast and Compiler Compatibility

Consider the following code snippet:

struct foo {
  static constexpr const void* ptr = reinterpret_cast<const void*>(0x1);
};

When compiled with g v4.9, this code compiles successfully. However, clang v3.4 fails to compile, issuing the error:

error: constexpr variable 'ptr' must be initialized by a constant expression

Compiler Correctness

According to the C 11 draft standard (section 5.19, paragraph 2), a conditional-expression is not considered a constant expression if it involves a reinterpret_cast. Therefore, clang is correct in its interpretation that the initialization of ptr is not valid.

Proper Declaration

To properly declare a constant expression of this nature, one should use intptr_t instead and cast when necessary:

static constexpr intptr_t ptr = 0x1;

Alternatively, a workaround supported by both gcc and clang involves using the undocumented __builtin_constant_p macro:

static constexpr const void* ptr =
  __builtin_constant_p(reinterpret_cast<const void*>(0x1)) ?
    reinterpret_cast<const void*>(0x1) : reinterpret_cast<const void*>(0x1);

This expression is accepted by both compilers due to the __builtin_constant_p check, which forces the expression to be constant-folded.

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