I have a problem when I want to check certain template-parameters for their validity using some helper struct and constepxr functions. As long as there is no reference to the static constexpr member I want to initialize the compiler decides not to evaluate the expression. The code I use is the following:
#include <cstddef> #include <iostream> #define CONSTEXPR static constexpr using namespace std; template<size_t ... Sizes> struct _size_check_impl { static_assert(sizeof...(Sizes) != 0, "Dimension has to be at least 1"); CONSTEXPR size_t dimension = sizeof...(Sizes); }; template<size_t ... Sizes> constexpr size_t check_sizes() { return _size_check_impl<Sizes...>::dimension; } template<size_t ... Sizes> struct Test { static constexpr size_t Final = check_sizes<Sizes...>(); }; int main() { Test<> a; // This shouldn't get through the static assert Test<1, 2> b; // Passing Test<2> c; // Passing // cout << Test<>::Final; // With this it works just fine, bc Final is accessed return 0; } Is there a way I can do this, some proxy dependecy that forces the compiler to evaluate the Final value if constexpr are evaluated? Is there another, clean way to check this property clean and quickly?
Sizes...parameter, which isn't instantiated until phase two of two phase lookup.Test<...>::Finalis used anywhere, then its value will be evaluated. If not, it won't. Then you shouldn't care whether it is evaluated or not.