constexpr - w3toppers.com

Is is_constexpr possible in C++11?

I once wrote it (EDIT: see below for limitations and explanations). From https://stackoverflow.com/a/10287598/34509 : template<typename T> constexpr typename remove_reference<T>::type makeprval(T && t) { return t; } #define isprvalconstexpr(e) noexcept(makeprval(e)) However there are many kinds of constant expressions. The above answer detects prvalue constant expressions. Explanation The noexcept(e) expression gives false iff e contains a potentially … Read more

Create N-element constexpr array in C++11

In C++14 it can be easily done with a constexpr constructor and a loop: #include <iostream> template<int N> struct A { constexpr A() : arr() { for (auto i = 0; i != N; ++i) arr[i] = i; } int arr[N]; }; int main() { constexpr auto a = A<4>(); for (auto x : a.arr) … Read more

const vs constexpr on variables

I believe there is a difference. Let’s rename them so that we can talk about them more easily: const double PI1 = 3.141592653589793; constexpr double PI2 = 3.141592653589793; Both PI1 and PI2 are constant, meaning you can not modify them. However only PI2 is a compile-time constant. It shall be initialized at compile time. PI1 … Read more

When should you use constexpr capability in C++11?

Suppose it does something a little more complicated. constexpr int MeaningOfLife ( int a, int b ) { return a * b; } const int meaningOfLife = MeaningOfLife( 6, 7 ); Now you have something that can be evaluated down to a constant while maintaining good readability and allowing slightly more complex processing than just … Read more

Why do constant expressions have an exclusion for undefined behavior?

The wording is actually the subject of defect report #1313 which says: The requirements for constant expressions do not currently, but should, exclude expressions that have undefined behavior, such as pointer arithmetic when the pointers do not point to elements of the same array. The resolution being the current wording we have now, so this … Read more

When does a constexpr function get evaluated at compile time?

constexpr functions will be evaluated at compile time when all its arguments are constant expressions and the result is used in a constant expression as well. A constant expression could be a literal (like 42), a non-type template argument (like N in template<class T, size_t N> class array;), an enum element declaration (like Blue in … Read more

Undefined reference to static constexpr char[]

Add to your cpp file: constexpr char foo::baz[]; Reason: You have to provide the definition of the static member as well as the declaration. The declaration and the initializer go inside the class definition, but the member definition has to be separate.

What’s the difference between constexpr and const?

Basic meaning and syntax Both keywords can be used in the declaration of objects as well as functions. The basic difference when applied to objects is this: const declares an object as constant. This implies a guarantee that once initialized, the value of that object won’t change, and the compiler can make use of this … Read more