For the record, the static constexpr version will work like you’d expected in C++17. From N4618 Annex D.1 [depr.static_constexpr]: D.1 Redeclaration of static constexpr data members [depr.static_constexpr] For compatibility with prior C++ International Standards, a constexpr static data member may be redundantly redeclared outside the class with no initializer. This usage is deprecated. [Example: struct … Read more
Here’s another solution, which is more generic (applicable to any expression, without defining a separate template each time). This solution leverages that (1) lambda expressions can be constexpr as of C++17 (2) the type of a captureless lambda is default constructible as of C++20. The idea is, the overload that returns true is selected when … Read more
A constexpr function can also be invoked with arguments evaluated at run-time (in that case, it just gets executed just like any regular function). See, for instance, this live example. A static_assert(), on the other hand, strictly requires its condition to be a constant expression that can be evaluated at compile time.
int vec[s_maxSize]; is actually illegal in the second example, so that is not possible to do in C++. But your first example is perfectly legal C++0x. So there’s your answer. You can’t actually do what you propose in C++. It can only be done in C++0x with constexpr. I would also like to point out, … Read more
At runtime the C++ language has the concept of Undefined Behavior. Under certain (well specified) conditions, the program has Undefined Behavior, that means that it can exhibit any behavior: it can crash, it can hang forever, it can print gibberish, it can appear to work, or it can do anything. A simplified explanation of why … Read more
This behaviour is vexing me time and again. The cause of the trouble is that your A(int n): v(n, A::kDefaultValue) {} odr-uses the static constexpr member, since the constructor of v takes a constant reference second argument. Odr-usage requires a definition somewhere, i.e. const int A::kDefaultValue; in some compilation unit (which is compiled and linked … Read more
Yes, it is ill-formed. Here’s why: A constexpr function needs to be defined (not just declared) before being used in a constant expression. So for example: constexpr int f(); // declare f constexpr int x = f(); // use f – ILLEGAL, f not defined constexpr int f() { return 5; } // define f, … Read more
constexpr variables are not compile-time values A value is immutable and does not occupy storage (it has no address), however objects declared as constexpr can be mutable and do occupy storage (under the as-if rule). Mutability Most objects declared as constexpr are immutable, but it is possible to define a constexpr object that is (partially) … Read more
I don’t think this is a bug. If you change the constexpr to const, it still fails, with the exact same error. You’ve declared S::X, but not defined it anywhere, so there’s no storage for it. If you do anything with it that needs to know the address of it then you’ll need to define … Read more
Does it mean that a static constexpr variable is not odr-used (and therefore can be in-class initialized) if it is returned from a function? Yes. Essentially, as long as you treat it as a value, rather than an object, then it is not odr-used. Consider that if you pasted in the value, the code would … Read more