constexpr

I want to experiment with migrating a project from gcc to clang++. I admit ignorance on my part, I'm not sure why the following bit of code template <typename T> constexpr T pi{std::acos(T(-1.0))}; compiles silently with g++ but clang++ produces the error trig.hpp:3:13: error: constexpr variable 'pi<float>' must be initialized by a constant expression constexpr T pi{std::acos(T(-1.0))}; and I was hoping someone who knows more about it than I do could enlighten me. NB: Tried with -std=C++14 and C++1y. Fails under clang version 3.6.2 (tags/RELEASE_362/final). Works with g++ (GCC) 5.2.0. Shafik

Here is my working code example: #include <iostream> template<typename B> class b { public: int y; constexpr b(int x) : y(x) { } constexpr void sayhi() { std::cout << "hi" << std::endl; } }; template<int x> struct A { static constexpr b<int> bee = x; static constexpr int y = x; // this one is fine and usable already, I don't have to do something like what I did on member bee inline static void sayhi() { std::cout << y << std::endl; } }; template<int x> constexpr b<int> A<x>::bee; // why do I have to do something like this, it will cause errors if I don't. int main(int argc, char** argv) { A<30

Consider this code: #include <vector> struct A { static constexpr int kDefaultValue = -1; std::vector<int> v; A(int n): v(n, A::kDefaultValue) {} }; int main() { A(10); return 0; } It fails to link (llvm clang, gcc 4.9, both on OS X): Undefined symbols for architecture x86_64: "A::kDefaultValue", referenced from: A::(int) in main.cpp.o ld: symbol(s) not found for architecture x86_64 The question is what's wrong with it? It can be fixed by static_cast -ing A::kDefaultValue to int . Or by moving kDefaultValue out of A . Both cases seem to be ugly. Is this another way to make it link? This

I just noticed that the new standard defines min(a,b) and max(a,b) without constexpr . Examples from 25.4.7, [alg.min.max]: template<class T> const T& min(const T& a, const T& b); template<class T> T min(initializer_list<T> t); Isn't this a pity? I would have liked to write char data[ max(sizeof(A),sizeof(B)) ]; instead of char data[ sizeof(A) > sizeof(B) ? sizeof(A) : sizeof(B) ]; char data[ MAX(sizeof(A),sizeof(B)) ]; // using a macro Any reason why those can not be constexpr ? Critical Update The below analysis is wrong, because it confuses one important thing . The following statement I

Beating the dead horse here. A typical (and fast) way of doing integer powers in C is this classic: int64_t ipow(int64_t base, int exp){ int64_t result = 1; while(exp){ if(exp & 1) result *= base; exp >>= 1; base *= base; } return result; } However I needed a compile time integer power so I went ahead and made a recursive implementation using constexpr: constexpr int64_t ipow_(int base, int exp){ return exp > 1 ? ipow_(base, (exp>>1) + (exp&1)) * ipow_(base, exp>>1) : base; } constexpr int64_t ipow(int base, int exp){ return exp < 1 ? 1 : ipow_(base, exp); } The second function is only to

This used to work some weeks ago: template <typename T, T t> T tfunc() { return t + 10; } template <typename T> constexpr T func(T t) { return tfunc<T, t>(); } int main() { std::cout << func(10) << std::endl; return 0; } But now g++ -std=c++0x says: main.cpp: In function ‘constexpr T func(T) [with T = int]’: main.cpp:29:25: instantiated from here main.cpp:24:24: error: no matching function for call to ‘tfunc()’ main.cpp:24:24: note: candidate is: main.cpp:16:14: note: template<class T, T t> T tfunc() main.cpp:25:1: warning: control reaches end of non-void function [-Wreturn-type] clang++ -std