Force compile time evaluation of constexpr with clang

Question

To force the evaluation of a constexpr function at compile time, I should be able to assign its return value to a constexpr variable.

constexpr bool const_d_ref(const double& v) { return false; }

int main() {
  constexpr double dd = 0.0;
  constexpr bool cb = const_d_ref(dd);
}

This seems to work fine with g++ and clang++.

In order to hide the constexpr from the consumer, I move the actual function definition into the namespace detail, create a new function which assigns the return value to a constexpr variable and return it.

namespace detail {
constexpr bool const_d_ref(const double& v) { return false; }
}
constexpr bool const_d_ref(const double& v) {
  constexpr bool b = detail::const_d_ref(v);
  return b;
}
int main() {
  constexpr double dd = 0.0;
  bool b = const_t_ref(dd);
  constexpr bool cb = detail::const_t_ref(dd);
}

It works as expected with g++, but clang++ returns a compiler error:

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

Is what I'm doing allowed? Or is clang being to restrictive? Or is gcc being to permissive?

cpp.godbolt.org: gcc 6.1 & clang 3.8

Howard Hinnant Howard Hinnant · Accepted Answer · 2016-08-13T20:21:27

v in constexpr bool const_d_ref(const double& v) { is not a compile-time expression. Therefore detail::const_d_ref(v) is also not a compile-time expression.

Your example will work if you change b to const:

const bool b = detail::const_d_ref(v);

cpp.godbolt.org

Force compile time evaluation of constexpr with clang

1 Answers