This is a result of The Equality Operator You Are Looking For (P2468). The comparison changes in C++20 – which allow a == b and a != b to find rewritten and synthesized candidates – can break a lot of C++17 code. You can see some examples in the paper. In an effort to mitigate … Read more
The fundamental difficulty that would arise is that concepts could become dependent: template<class T> struct A { template<T::Q X> void f(); }; Is X a non-type template parameter of (dependent) type T::Q (which does not require typename in C++20), or is it a type template parameter constrained by the concept T::Q? The rule is that … Read more
std::is_convertible<From, To> (the type trait) checks is type From is implicitly convertible to type To. std::convertible_to<From, To> (the concept) checks that From is both implicitly and explicitly convertible to To. It’s rare that this is not the case, such types are ridiculous, but it’s nice in generic code to just not have to worry about … Read more
Concepts are named boolean predicates on template parameters, evaluated at compile time. In a constexpr if statement, the value of the condition must be a contextually converted constant expression of type bool. So in this case, usage is simple: if constexpr ( integral<T> )
Concepts (as defined by the Concepts TS) and type classes are related only in the sense that they restrict the sets of types that can be used with a generic function. Beyond that, I can only think of ways in which the two features differ. I should note that I am not a Haskell expert. … Read more
As of Fri, 7 Aug 2015 01:44:49 -0400 (05:44 +0000) concepts support has been merged into gcc’s trunk. Using a build from after that point, you can enable concepts support with the -std=c++1z flag. As of gcc 6.1 (27-04-2016), concepts are enabled by -fconcepts as they are unlikely to be included in C++17, so they … Read more
Here the concept D is the same as the concept C They are not. Constraints (and concept-ids) are normalized when checked for satisfaction and broken down to atomic constraints. [temp.names] 8 A concept-id is a simple-template-id where the template-name is a concept-name. A concept-id is a prvalue of type bool, and does not name a … Read more
TL;DR: std::equality_comparable_with<T, U> requires that both T and U are convertible to the common reference of T and U. For the case of std::unique_ptr<T> and std::nullptr_t, this requires that std::unique_ptr<T> is copy-constructible, which it is not. Buckle in. This is quite the ride. Consider me nerd-sniped. Why don’t we satisfy the concept? std::equality_comparable_with requires: template … Read more
These are not new things, hence the term “legacy”. This is simply how the cppreference site chooses to reconcile the fact that C++20 will have two different things that are both “concepts” called “RandomAccessIterator” (well, until C++20 decided to rename their version random_access_iterator). Pre-C++20, a “concept” was just a set of requirements in the standard … Read more
Disclaimer: I have not yet used concepts, all I know about them was gleaned from the various proposals and cppreference, so take this answer with a grain of salt. Run-Time Polymorphism Rust Traits are used both for Compile-Time Polymorphism and, sometimes, Run-Time Polymorphism; Concepts are only about Compile-Time Polymorphism. Structural vs Nominal. The greatest difference … Read more