constexpr

Coming from that question: How to build a custom macro that behaves differently when used as constexpr (like assert)? I wonder why it is possible to call a non constexpr function if it is conditional. void bla( ) { std::cout << "bla called!" << std::endl; } constexpr bool check(bool condition) { //bla(); // can not directly be called -> not constexpr! condition ? void (0) : bla(); // compiles and runs even if condition is true or false! // if condition is const, it did not compile because it // directly force execution of non constexpr function true ? void(0): bla(); // also that compiles!, ok

In the standards paper P0092R1, Howard Hinnant wrote: template <class To, class Rep, class Period, class = enable_if_t<detail::is_duration<To>{}>> constexpr To floor(const duration<Rep, Period>& d) { To t = duration_cast<To>(d); if (t > d) --t; return t; } How can this code work? The problem is that operator-- on a std::chrono::duration is not a constexpr operation. It is defined as: duration& operator--(); And yet this code compiles, and gives the right answer at compile time: static_assert(floor<hours>(minutes{3}).count() == 0, "”); What's up with that? The answer is that not all operations

I want to check that a given double/float variable has the actual bit pattern 0x0. Don't ask why, it's used in a function in Qt ( qIsNull() ) that I'd like to be constexpr . The original code used a union: union { double d; int64_t i; } u; u.d = d; return u.i == 0; This doesn't work as a constexpr of course. The next try was with reinterpret_cast : return *reinterpret_cast<int64_t*>(&d) == 0; But while that works as a constexpr in GCC 4.7, it fails (rightfully, b/c of pointer manipulation) in Clang 3.1. The final idea was to go Alexandrescuesque and do this: template <typename T1, typename T2>

Title says it all and both of the usual ways do not work. What am I missing? 1. class Cl { static constexpr double PI; }; constexpr double Cl::PI = 3.14; (26): error C2737: 'private: static double const Cl::PI' : 'constexpr' object must be initialized 2. class Cl { static constexpr double PI = 3.14; }; (26): error C2864: 'Cl::PI' : a static data member with an in-class initializer must have non-volatile const integral type type is 'const double' In both attempts, the error is on the same line inside the class. I am using the VisualStudio/MSVC Nov 2013 CTP compiler. Note that making the

Accessing static class member functions or variables, can be done in two ways: through an object ( obj.member_fun() or obj.member_var ) or through the class ( Class::member_fun() or Class::member_var ). However, in constexpr functions, Clang gives an error on the object access and requires to use class access: struct S { constexpr static auto s_v = 42; constexpr static auto v() { return s_v; } }; #define TEST 1 constexpr auto foo(S const& s [[maybe_unused]]) { #if TEST constexpr auto v = s.v(); // ERROR for clang, OK for gcc #else constexpr auto v = S::v(); // OK for clang and gcc #endif