variable-templates

问题 Consider the following example: #include <cstdio> template <int N> int fib = fib<N - 1> + fib<N - 2>; template <> int fib<2> = 1; template <> int fib<1> = 1; int main() { std::printf("%d %d %d", fib<4>, fib<5>, fib<6>); } GCC 7.x, 8.x, 9.x, and 10.x all print out the expected result of 3 5 8 . Clang 5.x, 6.x, 7.x, 8.x, 9.x, and 10.x all print out 1 3 4 as a result. live example on godbolt.org Clang's behavior is surprising. Is there any subtle interaction between variable template

问题 The following nonsensical example does not compile, but is there some other way to pass a variable template as a template template argument? template<typename T> constexpr auto zero = T{0}; template<typename T, template<typename> auto VariableTemplate> constexpr auto add_one() { return VariableTemplate<T> + T{1}; } int main() { return add_one<int, zero>(); } Try on Compiler Explorer 回答1: Short answer: No. Long answer: Yes you can using some indirection through a class template: template

问题 I have a function which looks like this: template <typename T, std::size_t... I> std::ostream& vector_insert(std::ostream& lhs, const char* delim, const T& rhs, std::index_sequence<I...>) { std::ostream_iterator<float> it(lhs, delim); ((*it++ = at(rhs, I)), ...); return lhs; } This is my final attempt and I'm still failing on my expansion of the integer_sequence I'm hoping someone can tell me how to write a line that will effectively expand to: *it++ = at(rhs, 0U), *it++ = at(rhs, 1U), *it++

问题 Using N3651 as a basis, A variable template at class scope is a static data member template . The example given is: struct matrix_constants { template <typename T> using pauli = hermitian_matrix<T, 2>; Yet all of the following definitions give an error: struct foo { template <typename T> T pi = T{3.14}; }; template <typename T> struct foo2 { template <typename U = T> U pi = U{3.14}; }; template <typename T> struct foo3 { template <T> T pi = 42; }; error: member 'pi' declared as a template

问题 Using N3651 as a basis, A variable template at class scope is a static data member template . The example given is: struct matrix_constants { template <typename T> using pauli = hermitian_matrix<T, 2>; Yet all of the following definitions give an error: struct foo { template <typename T> T pi = T{3.14}; }; template <typename T> struct foo2 { template <typename U = T> U pi = U{3.14}; }; template <typename T> struct foo3 { template <T> T pi = 42; }; error: member 'pi' declared as a template

问题 I tried to forward-declare a constexpr variable template like this: template<typename> constexpr std::size_t iterator_category_value; The goal was to document that every specialization should be constexpr but I have to admit that I never checked whether it was legal or not and g++ was happy with it. However, when I tried to compile this spinnet with clang++ instead, I got the following error: error: default initialization of an object of const type 'const std::size_t' (aka 'const unsigned

问题 Having: struct Value { template<class T> static constexpr T value{0}; }; (0) ideone template<typename TValue> struct Something { void x() { static_assert(TValue::template value<int> == 0, ""); } }; int main() { Something<Value>{}.x(); return 0; } Does not compile with clang++ 3.6. error: cannot refer to variable template 'value' without a template argument list Does not compile with g++ 5.2. error: ‘template constexpr const T Value::value’ is not a function template (1) ideone Compiles with

问题 This is probably a visual-studio-2017 bug related to this question: Templated Variables Bug With Lambdas in Visual Studio? And as mentioned in the comments there seems to be optimizer related. Division in the definition of a variable template seems to have a bug in Visual Studio 2017. So this code for example: template <typename T> const T PI = std::acos(static_cast<T>(-1)); template <typename T> const T ONE_EIGHTY = 180; template <typename T> const T DEG_TO_RAD = PI<T> / ONE_EIGHTY<T>; int

问题 c++14 provides variable templates Which work fine in visual-studio-2017, but within lambdas they seem to fall apart. For example: template <typename T> const auto PI = std::acos(static_cast<T>(-1)); int main() { auto func = []() { cout << PI<float> << endl; }; func(); } On gcc 6.3 this outputs: 3.14159 On Visual Studio 2017 this outputs: 0.0 回答1: Wierd bug, but it seems to have a reliable workaround, which works for both, this case and the related case. In both cases forcefully activating

问题 Having seen this answer, I tried to come up with a variable template utility to the code from it: template <class T, template <class...> class Template> struct is_specialization : std::false_type {}; template <template <class...> class Template, class... Args> struct is_specialization<Template<Args...>, Template> : std::true_type {}; And implement it like so: template <template <class...> class Template, class... Args> constexpr bool is_specialization_v = is_specialization<Template<Args...>,