Constraints and concepts (since C++20)
 This page describes the core language feature adopted for C++20. For named type requirements used in the specification of the standard library, see named requirements. For the Concepts TS version of this feature, see here.
Class templates, function templates, and nontemplate functions (typically members of class templates) may be associated with a constraint, which specifies the requirements on template arguments, which can be used to select the most appropriate function overloads and template specializations.
Named sets of such requirements are called concepts. Each concept is a predicate, evaluated at compile time, and becomes a part of the interface of a template where it is used as a constraint:
#include <string> #include <cstddef> using namespace std::literals; // Declaration of the concept "Hashable", which is satisfied by // any type T such that for values a of type T, // the expression std::hash<T>{}(a) compiles and its result is convertible to std::size_t template<typename T> concept Hashable = requires(T a) { { std::hash<T>{}(a) } > std::size_t; }; struct meow {}; template<Hashable T> void f(T); // constrained C++20 function template // Alternative ways to apply the same constraint: // template<typename T> // requires Hashable<T> // void f(T); // // template<typename T> // void f(T) requires Hashable<T>; int main() { f("abc"s); // OK, std::string satisfies Hashable f(meow{}); // Error: meow does not satisfy Hashable }
Violations of constraints are detected at compile time, early in the template instantiation process, which leads to easy to follow error messages.
std::list<int> l = {3,1,10}; std::sort(l.begin(), l.end()); //Typical compiler diagnostic without concepts: // invalid operands to binary expression ('std::_List_iterator<int>' and // 'std::_List_iterator<int>') // std::__lg(__last  __first) * 2); // ~~~~~~ ^ ~~~~~~~ // ... 50 lines of output ... // //Typical compiler diagnostic with concepts: // error: cannot call std::sort with std::_List_iterator<int> // note: concept RandomAccessIterator<std::_List_iterator<int>> was not satisfied
The intent of concepts is to model semantic categories (Number, Range, RegularFunction) rather than syntactic restrictions (HasPlus, Array). According to ISO C++ core guideline T.20, "The ability to specify a meaningful semantics is a defining characteristic of a true concept, as opposed to a syntactic constraint."
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[edit] Concepts
A concept is a named set of requirements. The definition of a concept must appear at namespace scope.
The definition of a concept has the form
template < templateparameterlist >


// concept template <class T, class U> concept Derived = std::is_base_of<U, T>::value;
Concepts cannot recursively refer to themselves and cannot be constrained:
template<typename T> concept V = V<T*>; // error: recursive concept template<class T> concept C1 = true; template<C1 T> concept Error1 = true; // Error: C1 T attempts to constrain a concept definition template<class T> requires C1<T> concept Error2 = true; // Error: the requiresclause attempts to constrain a concept
Explicit instantiations, explicit specializations, or partial specializations of concepts are not allowed (the meaning of the original definition of a constraint cannot be changed).
[edit] Constraints
A constraint is a sequence of logical operations and operands that specifies requirements on template arguments. They can appear within requiresexpressions (see below) and directly as bodies of concepts.
There are three types of constraints:
The constraint associated with a declaration are determined by normalizing a logical AND expression whose operands are in the following order:
 the constraint expression introduced for each constrained template parameter, in order of appearance;
 the constraint expression in the requires clause after the template parameter list;
 the constraint expression in the trailing requires clause.
This order determines the order in which constraints are instantiated when checking for satisfaction.
A constrained declaration may only be redeclared using the same syntactic form. No diagnostic is required.
template<Incrementable T> void f(T) requires Decrementable<T>; template<typename T> requires Incrementable<T> && Decrementable<T> void f(T); // Illformed, no diagnostic required // the following two declarations have different constraints: // the first declaration has Incrementable<T> && Decrementable<T> // the second declaration has Decrementable<T> && Incrementable<T> // Even though they are logically equivalent. template<Incrementable T> void g() requires Decrementable<T>; template<Decrementable T> void g() requires Incrementable<T>; // illformed, no diagnostic required
[edit] Conjunctions
The conjunction of two constraints is formed by using the &&
operator in the constraint expression:
template <class T> concept Integral = std::is_integral<T>::value; template <class T> concept SignedIntegral = Integral<T> && std::is_signed<T>::value; template <class T> concept UnsignedIntegral = Integral<T> && !SignedIntegral<T>;
A conjunction of two constraints is satisfied only if both constraints are satisfied. Conjunctions are evaluated left to right and shortcircuited (if the left constraint is not satisfied, template argument substitution into the right constraint is not attempted: this prevents failures due to substitution outside of immediate context).
template<typename T> constexpr bool get_value() { return T::value; } template<typename T> requires (sizeof(T) > 1) && get_value<T>() void f(T); // #1 void f(int); // #2 void g() { f('A'); // OK, calls #2. When checking the constraints of #1, // 'sizeof(char) > 1' is not satisfied, so get_value<T>() is not checked }
[edit] Disjunctions
The disjunction of two constraints is formed by using the 
operator in the constraint expression.
A disjunction of two constraints is satisfied if either constraint is satisfied. Disjunctions are evaluated left to right and shortcircuited (if the left constraint is satisfied, template argument substitution into the right constraint is not attempted).
template <class T = void> requires EqualityComparable<T>()  Same<T, void> struct equal_to;
[edit] Atomic constraints
An atomic constraint consists of an expression E
and a mapping from the template parameters that appear within E
to template arguments involving the template parameters of the constrained entity, called its parameter mapping.
Atomic constraints are formed during constraint normalization. E
is never a logical AND or logical OR expression (those form conjunctions and disjunctions, respectively).
Satisfaction of an atomic constraint is checked by substituting the parameter mapping and template arguments into the expression E
. If the substitution results in an invalid type of expression, the constraint is not satisfied. Otherwise, E
, after any lvaluetorvalue conversion, shall be a prvalue constant expression of type bool , and the constraint is satisfied if and only if it evaluates to true
.
The type of E
after substitution must be exactly bool. No conversion is permitted:
template<typename T> struct S { constexpr operator bool() const { return true; } }; template<typename T> requires (S<T>{}) void f(T); // #1 void f(int); // #2 void g() { f(0); // error: S<int>{} does not have type bool when checking #1, // even though #2 is a better match }
Two atomic constraints are considered identical if they are formed from the same expression at the source level and their parameter mappings are equivalent.
template<class T> constexpr bool is_meowable = true; template<class T> constexpr bool is_cat = true; template<class T> concept Meowable = is_meowable<T>; template<class T> concept BadMeowableCat = is_meowable<T> && is_cat<T>; template<class T> concept GoodMeowableCat = Meowable<T> && is_cat<T>; template<Meowable T> void f1(T); // #1 template<BadMeowableCat T> void f1(T); // #2 template<Meowable T> void f2(T); // #3 template<GoodMeowableCat T> void f2(T); // #4 void g(){ f1(0); // error, ambiguous: // the is_meowable<T> in Meowable and BadMeowableCat forms distinct // atomic constraints that are not identical (and so do not subsume each other) f2(0); // OK, calls #4, more constrained than #3 // GoodMeowableCat got its is_meowable<T> from Meowable }
[edit] Constraint normalization
Constraint normalization is the process that transforms a constraint expression into a sequence of conjunctions and disjunctions of atomic constraints. The normal form of an expression is defined as follows:
 The normal form of an expression (E) is the normal form of E;
 The normal form of an expression E1 && E2 is the conjunction of the normal forms of E1 and E2.
 The normal form of an expression E1  E2 is the disjunction of the normal forms of E1 and E2.
 The normal form of an expression C<A1, A2, ... , AN>, where
C
names a concept, is the normal form of the constraint expression ofC
, after substituting A1, A2, ... , AN forC
's respective template parameters in the parameter mappings of each atomic constraint of C. If any such substitution into the parameter mappings results in an invalid type or expression, the program is illformed, no diagnostic required.
template<typename T> concept A = T::value  true; template<typename U> concept B = A<U*>; // OK: normalized to the disjunction of //  T::value (with mapping T > U*) and //  true (with an empty mapping). // No invalid type in mapping even though // T::value is illformed for all pointer types template<typename V> concept C = B<V&>; // Normalizes to the disjunction of //  T::value (with mapping T> V&*) and //  true (with an empty mapping. // Invalid type V&* formed in mapping => illformed NDR
 The normal form of any other expression E is the atomic constraint whose expression is E and whose parameter mapping is the identity mapping.
Userdefined overloads of &&
or 
have no effect on constraint normalization.
[edit] Requires clauses
The keyword requires is used to introduce a requiresclause, which specifies constraints on template arguments or on a function declaration.
template<typename T> void f(T&&) requires Eq<T>; // can appear as the last element of a function declarator template<typename T> requires Addable<T> // or right after a template parameter list T add(T a, T b) { return a + b; }
In this case, the keyword requires must be followed by some constant expression (so it's possible to write requires true), but the intent is that a named concept (as in the example above) or a conjunction/disjunction of named concepts or a requiresexpression is used.
The expression must have one of the following forms:
 a primary expression, e.g. Swappable<T>, std::is_integral<T>::value, or (std::is_object_v<Args> && ...)
 a sequence of primary expressions joined with the operator
&&
 a sequence of aforementioned expressions joined with the operator

template<class T> constexpr bool is_meowable = true; template<class T> constexpr bool is_purrable() { return true; } template<class T> void f(T) requires is_meowable<T>; // OK template<class T> void g(T) requires is_purrable<T>(); // error, is_purrable<T>() is not a primary expression template<class T> void h(T) requires (is_purrable<T>()); // OK
[edit] Requires expressions
The keyword requires is also used to begin a requiresexpression, which is a prvalue expression of type bool that describes the constraints on some template arguments. Such an expression is true
if the constraints are satisfied, and false
otherwise:
template<typename T> concept Addable = requires (T x) { x + x; }; // requiresexpression template<typename T> requires Addable<T> // requiresclause, not requiresexpression T add(T a, T b) { return a + b; } template<typename T> requires requires (T x) { x + x; } // adhoc constraint, note keyword used twice T add(T a, T b) { return a + b; }
The syntax of requiresexpression is as follows:
requires { requirementseq }


requires ( parameterlist(optional) ) { requirementseq }


parameterlist    a commaseparated 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 requirementseq.

requirementseq    sequence of requirements, described below (each requirement ends with a semicolon). 
Each requirement in the requirementsseq is one of the following:
 simple requirement
 type requirements
 compound requirements
 nested requirements
Requirements may refer to the template parameters that are in scope, to the local parameters introduced in the parameterlist, and to any other declarations that are visible from the enclosing context.
The substitution of template arguments into a requiresexpression 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 requiresexpression evaluates to false and does not cause the program to be illformed. The substitution and semantic constraint checking proceeds in lexical order and stops when a condition that determines the result of the requiresexpression is encountered. If substitution (if any) and semantic constraint checking succeed, the requiresexpression evaluates to true.
If a substitution failure would occur in a requiresexpression for every possible template argument, the program is illformed, no diagnostic required:
template<class T> concept C = requires { new int[(int)sizeof(T)]; // invalid for every T: illformed, no diagnostic required };
If a requiresexpression contains invalid types or expressions in its requirements, and it does not appear within a templated entity declaration, then the program is illformed.
[edit] Simple requirements
A simple requirement is an arbitrary expression statement. 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)); };
[edit] 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 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)} }; };
[edit] Compound Requirements
A compound requirement has the form
{ expression } noexcept (optional) returntyperequirement(optional) ;


returntyperequirement    either a trailingreturntype, or of the form > cv1(optional) qualifiedconceptname cv2(optional) abstractdeclarator(optional)

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) { {*x} > typename T::inner; // 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 };
[edit] Nested requirements
A nested requirement has the form
requires constraintexpression ;


It can be used to specify additional constraints in terms of local parameters. The constraintexpression must be satisfied by the substituted template arguments, if any. Substitution of template arguments into a nested requirement causes substitution into the constraintexpression only to the extent needed to determine whether the constraintexpression is satisfied.
template <class T> concept Semiregular = DefaultConstructible<T> && CopyConstructible<T> && Destructible<T> && CopyAssignable<T> && requires(T a, 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 };
[edit] Partial ordering of constraints
Before any further analysis, constraints are normalized by substituting the body of every name concept and every requires expression until what is left is a sequence of conjunctions and disjunctions on atomic constraints.
A constraint P
is said to subsume constraint Q
if it can be proven that P
implies Q
up to the identity of atomic constraints in P and Q. (Types and expressions are not analyzed for equivalence: N >= 0
does not subsume N > 0
).
Specifically, first P
is converted to disjunctive normal form and Q
is converted to conjunctive normal form. P
subsumes Q
if and only if:
 every disjunctive clause in the disjunctive normal form of
P
subsumes every conjunctive clause in the conjunctive normal form ofQ
, where  a disjunctive clause subsumes a conjunctive clause if and only if there is an atomic constraint
U
in the disjunctive clause and an atomic constraintV
in the conjunctive clause such thatU
subsumesV
;  an atomic constraint
A
subsumes an atomic constraintB
if and only if they are identical using the rules described above.
Subsumption relationship defines partial order of constraints, which is used to determine:
 the best viable candidate for a nontemplate function in overload resolution
 the address of a nontemplate function in an overload set
 the best match for a template template argument
 partial ordering of class template specializations
 partial ordering of function templates
This section is incomplete Reason: backlinks from the above to here 
If declarations D1
and D2
are constrained and D1's associated constraints subsume D2's associated constraints (or if D2 is unconstrained), then D1 is said to be at least as constrained as D2. If D1 is at least as constrained as D2 and D2 is not at least as constrained as D1, then D1 is more constrained than D2.
template<typename T> concept Decrementable = requires(T t) { t; }; template<typename T> concept RevIterator = Decrementable<T> && requires(T t) { *t; }; // RevIterator subsumes Decrementable, but not the other way around template<Decrementable T> void f(T); // #1 template<RevIterator T> void f(T); // #2, more constrained than #1 f(0); // int only satisfies Decrementable, selects #1 f((int*)0); // int* satisfies both constraints, selects #2 as more constrained template<class T> void g(T); // #3 (unconstrained) template<Decrementable T> void g(T); // #4 g(true); // bool does not satisfy Decrementable, selects #3 g(0); // int satisfies Decrementable, selects #4 because it is more constrained template<typename T> concept RevIterator2 = requires(T t) { t; *t; }; template<Decrementable T> void h(T); // #5 template<RevIterator2 T> void h(T); // #6 h((int*)0); //ambiguous