Requires expression (since C++20)
Yields a prvalue expression of type bool that describes the constraints.
Syntax
requires { requirement-seq }
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requires ( parameter-list (optional) ) { requirement-seq }
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| parameter-list | - | a comma-separated list of parameters like in a function declaration, except that default arguments are not allowed and it cannot end with an ellipsis (other than one signifying a pack expansion). These parameters have no storage, linkage or lifetime, and are only used to assist in specifying requirements. These parameters are in scope until the closing } of the requirement-seq.
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| requirement-seq | - | sequence of requirements, each requirement is one of the following:
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Explanation
Requirements may refer to the template parameters that are in scope, to the local parameters introduced in the parameter-list, and to any other declarations that are visible from the enclosing context.
The substitution of template arguments into a requires-expression used in a declaration of a templated entity may result in the formation of invalid types or expressions in its requirements, or the violation of semantic constraints of those requirements. In such cases, the requires-expression evaluates to false and does not cause the program to be ill-formed. The substitution and semantic constraint checking proceeds in lexical order and stops when a condition that determines the result of the requires-expression is encountered. If substitution (if any) and semantic constraint checking succeed, the requires-expression evaluates to true.
If a substitution failure would occur in a requires-expression for every possible template argument, the program is ill-formed, no diagnostic required:
template<class T> concept C = requires { new int[-(int)sizeof(T)]; // invalid for every T: ill-formed, no diagnostic required };
If a requires-expression contains invalid types or expressions in its requirements, and it does not appear within the declaration of a templated entity, then the program is ill-formed.
Simple requirements
A simple requirement is an arbitrary expression statement that does not start with the keyword requires. It asserts that the expression is valid. The expression is an unevaluated operand; only language correctness is checked.
template<typename T> concept Addable = requires (T a, T b) { a + b; // "the expression a+b is a valid expression that will compile" }; template<class T, class U = T> concept Swappable = requires(T&& t, U&& u) { swap(std::forward<T>(t), std::forward<U>(u)); swap(std::forward<U>(u), std::forward<T>(t)); };
A requirement that starts with the keyword requires is always interpreted as a nested requirement. Thus a simple requirement cannot start with an unparenthesized requires-expression.
Type requirements
A type requirement is the keyword typename followed by a type name, optionally qualified. The requirement is that the named type is valid: this can be used to verify that a certain named nested type exists, or that a class template specialization names a type, or that an alias template specialization names a type. A type requirement naming a class template specialization does not require the type to be complete.
template<typename T> using Ref = T&; template<typename T> concept C = requires { typename T::inner; // required nested member name typename S<T>; // required class template specialization typename Ref<T>; // required alias template substitution }; template<class T, class U> using CommonType = std::common_type_t<T, U>; template<class T, class U> concept Common = requires (T&& t, U&& u) { typename CommonType<T, U>; // CommonType<T, U> is valid and names a type { CommonType<T, U>{std::forward<T>(t)} }; { CommonType<T, U>{std::forward<U>(u)} }; };
Compound requirements
A compound requirement has the form
{ expression } noexcept(optional) return-type-requirement (optional) ;
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| return-type-requirement | - | -> type-constraint
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and asserts properties of the named expression. Substitution and semantic constraint checking proceeds in the following order:
template<typename T> concept C2 = requires(T x) { // the expression *x must be valid // AND the type T::inner must be valid // AND the result of *x must be convertible to T::inner {*x} -> std::convertible_to<typename T::inner>; // the expression x + 1 must be valid // AND std::same_as<decltype((x + 1)), int> must be satisfied // i.e., (x + 1) must be a prvalue of type int {x + 1} -> std::same_as<int>; // the expression x * 1 must be valid // AND its result must be convertible to T {x * 1} -> std::convertible_to<T>; };
Nested requirements
A nested requirement has the form
requires constraint-expression ;
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It can be used to specify additional constraints in terms of local parameters. The constraint-expression must be satisfied by the substituted template arguments, if any. Substitution of template arguments into a nested requirement causes substitution into the constraint-expression only to the extent needed to determine whether the constraint-expression is satisfied.
template<class T> concept Semiregular = DefaultConstructible<T> && CopyConstructible<T> && CopyAssignable<T> && Destructible<T> && requires(T a, std::size_t n) { requires Same<T*, decltype(&a)>; // nested: "Same<...> evaluates to true" { a.~T() } noexcept; // compound: "a.~T()" is a valid expression that doesn't throw requires Same<T*, decltype(new T)>; // nested: "Same<...> evaluates to true" requires Same<T*, decltype(new T[n])>; // nested { delete new T }; // compound { delete new T[n] }; // compound };
Note
The keyword requires is also used to introduce requires clauses.
template<typename T> concept Addable = requires (T x) { x + x; }; // requires-expression template<typename T> requires Addable<T> // requires-clause, not requires-expression T add(T a, T b) { return a + b; } template<typename T> requires requires (T x) { x + x; } // ad-hoc constraint, note keyword used twice T add(T a, T b) { return a + b; }
Keywords
See also
| Constraints and concepts(C++20) | specifies the requirements on template arguments |