c++-concepts

来源： https://stackoverflow.com/questions/59952704/c20-concepts-which-template-specialization-gets-chosen-when-the-template-arg

来源： https://stackoverflow.com/questions/63380249/how-can-i-overload-a-function-for-a-concept

来源： https://stackoverflow.com/questions/55198202/unable-to-deduce-placeholder-type-in-concept

问题 Currently, I have implemented the Allocator concept (which refers to the Boost proposal) using C++20 constraints and concepts: #include <concepts> #include <iterator> template <class A> concept allocator = std::copy_constructible<A> && std::equality_comparable<A> && requires(A a) { { a.allocate(0) } -> std::regular; { a.allocate(0) } -> std::constructible_from<std::nullptr_t>; { a.allocate(0) } -> std::equality_comparable_with<std::nullptr_t>; { a.allocate(0) } -> std::random_access_iterator;

问题 Currently, I have implemented the Allocator concept (which refers to the Boost proposal) using C++20 constraints and concepts: #include <concepts> #include <iterator> template <class A> concept allocator = std::copy_constructible<A> && std::equality_comparable<A> && requires(A a) { { a.allocate(0) } -> std::regular; { a.allocate(0) } -> std::constructible_from<std::nullptr_t>; { a.allocate(0) } -> std::equality_comparable_with<std::nullptr_t>; { a.allocate(0) } -> std::random_access_iterator;

问题 I'm trying to write a C++20 concept to express the requirement that a type have a certain method, which takes an argument, but for the purposes of this concept I don't care what the argument type is. I've tried to write something like: template <typename T> concept HasFooMethod = requires(T t, auto x) { { t.Foo(x) } -> std::same_as<void>; }; however, both gcc and clang reject this, giving an error that 'auto' cannot be used in the parameter list of a requires expression this way. An

问题 A generic std::atomic<T> is required to have a T that is Copy Constructible and Copy Assignable : [atomics.types.generic]/1 The program is ill-formed if any of (1.1) is_­trivially_­copyable_­v<T> , (1.2) is_­copy_­constructible_­v<T> , (1.3) is_­move_­constructible_­v<T> , (1.4) is_­copy_­assignable_­v<T> , or (1.5) is_­move_­assignable_­v<T> is false . Above is not new to C++20. Compilers may use static_assert to issue an error for a non-conforming T. However, C++20 could use formal

问题 I've been looking through concepts library on C++ reference and i couldn't find a concept for arithmetic types. I couldn't also find it in p0898. I think such concept would be very helpful. Insted of doing: template <typename T> T some_function(T arg) requires std::integral<T> || std::floating_point<T> { /* functions body */ } I could just do: template <std::arithmetic T> T some_function(T arg) { /* functions body */ } I could obviously define it myself and it wouldn't be hard (ex. template

问题 I've been looking through concepts library on C++ reference and i couldn't find a concept for arithmetic types. I couldn't also find it in p0898. I think such concept would be very helpful. Insted of doing: template <typename T> T some_function(T arg) requires std::integral<T> || std::floating_point<T> { /* functions body */ } I could just do: template <std::arithmetic T> T some_function(T arg) { /* functions body */ } I could obviously define it myself and it wouldn't be hard (ex. template

问题 Let us consider following code: #include <concepts> template<typename X> struct Concrete_M { X f() const { return X{}; } }; struct Concrete_X {}; template<typename T, typename X> concept M = requires (T t) { { t.f() } -> std::convertible_to<X>; }; template<M<Concrete_X>> struct C {}; const C<Concrete_M<Concrete_X>> c{}; Can I use following template template parameter T ? template<template<typename> typename T, typename X> concept M = requires (T<X> t) { { t.f() } -> std::convertible_to<X>; };