Table 41: Fundamental concepts library summary [tab:concepts.summary]

Subclause | Header | ||

Equality preservation | |||

Language-related concepts | <concepts> | ||

Comparison concepts | |||

Object concepts | |||

Callable concepts |

An expression is *equality-preserving* if,
given equal inputs, the expression results in equal outputs.

The inputs to an
expression are the set of the expression's operands.

The output of an expression
is the expression's result and all operands modified by the expression.

For the purposes of this subclause,
the operands of an expression are the largest subexpressions that include only:

- an
*id-expression*([expr.prim.id]), and - invocations of the library function templates std::move, std::forward, and std::declval ([forward], [declval]).

Not all input values need be valid for a given expression.

Expressions required to be equality-preserving are further
required to be stable: two evaluations of such an expression with the same input
objects are required to have equal outputs absent any explicit intervening
modification of those input objects.

[*Note 1*: *end note*]

This requirement allows generic code to reason about the current values of
objects based on knowledge of the prior values as observed via
equality-preserving expressions.

It effectively forbids spontaneous changes to
an object, changes to an object from another thread of execution, changes to an
object as side effects of non-modifying expressions, and changes to an object as
side effects of modifying a distinct object if those changes could be observable
to a library function via an equality-preserving expression that is required to
be valid for that object.

— Expressions declared in a *requires-expression* in the library clauses are
required to be equality-preserving, except for those annotated with the comment
“not required to be equality-preserving.”

An expression so annotated
may be equality-preserving, but is not required to be so.

An expression that may alter the value of one or more of its inputs in a manner
observable to equality-preserving expressions is said to modify those inputs.

The library clauses use a notational convention to specify which expressions declared
in a *requires-expression* modify which inputs: except where
otherwise specified, an expression operand that is a non-constant lvalue or
rvalue may be modified.

Operands that are constant lvalues or rvalues are
required to not be modified.

For the purposes of this subclause,
the cv-qualification and value category of each operand
are determined by assuming
that each template type parameter
denotes a cv-unqualified complete non-array object type.

Where a *requires-expression* declares an expression that is
non-modifying for some constant lvalue operand, additional variations of that
expression that accept a non-constant lvalue or (possibly constant) rvalue for
the given operand are also required except where such an expression variation is
explicitly required with differing semantics.

These
*implicit expression variations* are required to meet the semantic
requirements of the declared expression.

The extent to which an implementation
validates the syntax of the variations is unspecified.

[*Example 3*: template<class T> concept C = requires(T a, T b, const T c, const T d) {
c == d; // #1
a = std::move(b); // #2
a = c; // #3
};

For the above example:

— - Expression #1 does not modify either of its operands, #2 modifies both of its operands, and #3 modifies only its first operand a.
- Expression #1 implicitly requires additional expression variations that meet the requirements for c == d (including non-modification), as if the expressions c == b; c == std::move(d); c == std::move(b); std::move(c) == d; std::move(c) == b; std::move(c) == std::move(d); std::move(c) == std::move(b); a == d; a == b; a == std::move(d); a == std::move(b); std::move(a) == d; std::move(a) == b; std::move(a) == std::move(d); std::move(a) == std::move(b); had been declared as well.
- Expression #3 implicitly requires additional expression variations that meet the requirements for a = c (including non-modification of the second operand), as if the expressions a = b and a = std::move(c) had been declared.Expression #3 does not implicitly require an expression variation with a non-constant rvalue second operand, since expression #2 already specifies exactly such an expression explicitly.

[*Example 4*:

The following type T meets the explicitly stated syntactic requirements
of concept C above but does not meet the additional implicit
requirements:
struct T {
bool operator==(const T&) const { return true; }
bool operator==(T&) = delete;
};

// all freestanding
namespace std {
// [concepts.lang], language-related concepts
// [concept.same], concept same_as
template<class T, class U>
concept same_as = *see below*;
// [concept.derived], concept derived_from
template<class Derived, class Base>
concept derived_from = *see below*;
// [concept.convertible], concept convertible_to
template<class From, class To>
concept convertible_to = *see below*;
// [concept.commonref], concept common_reference_with
template<class T, class U>
concept common_reference_with = *see below*;
// [concept.common], concept common_with
template<class T, class U>
concept common_with = *see below*;
// [concepts.arithmetic], arithmetic concepts
template<class T>
concept integral = *see below*;
template<class T>
concept signed_integral = *see below*;
template<class T>
concept unsigned_integral = *see below*;
template<class T>
concept floating_point = *see below*;
// [concept.assignable], concept assignable_from
template<class LHS, class RHS>
concept assignable_from = *see below*;
// [concept.swappable], concept swappable
namespace ranges {
inline namespace *unspecified* {
inline constexpr *unspecified* swap = *unspecified*;
}
}
template<class T>
concept swappable = *see below*;
template<class T, class U>
concept swappable_with = *see below*;
// [concept.destructible], concept destructible
template<class T>
concept destructible = *see below*;
// [concept.constructible], concept constructible_from
template<class T, class... Args>
concept constructible_from = *see below*;
// [concept.default.init], concept default_initializable
template<class T>
concept default_initializable = *see below*;
// [concept.moveconstructible], concept move_constructible
template<class T>
concept move_constructible = *see below*;
// [concept.copyconstructible], concept copy_constructible
template<class T>
concept copy_constructible = *see below*;
// [concepts.compare], comparison concepts
// [concept.equalitycomparable], concept equality_comparable
template<class T>
concept equality_comparable = *see below*;
template<class T, class U>
concept equality_comparable_with = *see below*;
// [concept.totallyordered], concept totally_ordered
template<class T>
concept totally_ordered = *see below*;
template<class T, class U>
concept totally_ordered_with = *see below*;
// [concepts.object], object concepts
template<class T>
concept movable = *see below*;
template<class T>
concept copyable = *see below*;
template<class T>
concept semiregular = *see below*;
template<class T>
concept regular = *see below*;
// [concepts.callable], callable concepts
// [concept.invocable], concept invocable
template<class F, class... Args>
concept invocable = *see below*;
// [concept.regularinvocable], concept regular_invocable
template<class F, class... Args>
concept regular_invocable = *see below*;
// [concept.predicate], concept predicate
template<class F, class... Args>
concept predicate = *see below*;
// [concept.relation], concept relation
template<class R, class T, class U>
concept relation = *see below*;
// [concept.equiv], concept equivalence_relation
template<class R, class T, class U>
concept equivalence_relation = *see below*;
// [concept.strictweakorder], concept strict_weak_order
template<class R, class T, class U>
concept strict_weak_order = *see below*;
}

Subclause [concepts.lang] contains the definition of concepts corresponding to language
features.

These concepts express relationships between types, type
classifications, and fundamental type properties.

```
template<class T, class U>
concept
```*same-as-impl* = is_same_v<T, U>; // *exposition only*
template<class T, class U>
concept same_as = *same-as-impl*<T, U> && *same-as-impl*<U, T>;

```
template<class Derived, class Base>
concept derived_from =
is_base_of_v<Base, Derived> &&
is_convertible_v<const volatile Derived*, const volatile Base*>;
```

[*Note 1*: *end note*]

derived_from<Derived, Base> is satisfied if and only if
Derived is publicly and unambiguously derived from Base, or
Derived and Base are the same class type ignoring cv-qualifiers.

— Given types From and To and
an expression E
whose type and value category are the same as those of declval<From>(),
convertible_to<From, To> requires E
to be both implicitly and explicitly convertible to type To.

The implicit and explicit conversions are required to produce equal
results.

```
template<class From, class To>
concept convertible_to =
is_convertible_v<From, To> &&
requires {
static_cast<To>(declval<From>());
};
```

Let FromR be add_rvalue_reference_t<From> and
test be the invented function:
To test(FromR (&f)()) {
return f();
}
and let f be a function with no arguments and return type FromR
such that f() is equality-preserving.

Types From and To model convertible_to<From, To>
only if:

- FromR is not a reference-to-object type, or
- If FromR is an rvalue reference to a non const-qualified type, the resulting state of the object referenced by f() after either above expression is valid but unspecified ([lib.types.movedfrom]).
- Otherwise, the object referred to by f() is not modified by either above expression.

For two types T and U, if common_reference_t<T, U>
is well-formed and denotes a type C such that both
convertible_to<T, C>
and
convertible_to<U, C>
are modeled, then T and U share a
*common reference type*, C.

```
template<class T, class U>
concept common_reference_with =
same_as<common_reference_t<T, U>, common_reference_t<U, T>> &&
convertible_to<T, common_reference_t<T, U>> &&
convertible_to<U, common_reference_t<T, U>>;
```

Let t1 and t2 be equality-preserving
expressions ([concepts.equality]) such that
decltype((t1)) and decltype((t2)) are each T, and
let u1 and u2 be equality-preserving expressions such that
decltype((u1)) and decltype((u2)) are each U.

T and U model common_reference_with<T, U>
only if:

- C(t1) equals C(t2) if and only if t1 equals t2, and
- C(u1) equals C(u2) if and only if u1 equals u2.

[*Note 2*: *end note*]

Users can customize the behavior of common_reference_with by specializing
the basic_common_reference class template ([meta.trans.other]).

— If T and U can both be explicitly converted to some third type,
C, then T and U share a *common type*,
C.

```
template<class T, class U>
concept common_with =
same_as<common_type_t<T, U>, common_type_t<U, T>> &&
requires {
static_cast<common_type_t<T, U>>(declval<T>());
static_cast<common_type_t<T, U>>(declval<U>());
} &&
common_reference_with<
add_lvalue_reference_t<const T>,
add_lvalue_reference_t<const U>> &&
common_reference_with<
add_lvalue_reference_t<common_type_t<T, U>>,
common_reference_t<
add_lvalue_reference_t<const T>,
add_lvalue_reference_t<const U>>>;
```

Let t1 and t2 be
equality-preserving expressions ([concepts.equality]) such that
decltype((t1)) and decltype((t2)) are each T, and
let u1 and u2 be equality-preserving expressions such that
decltype((u1)) and decltype((u2)) are each U.

T and U model common_with<T, U>
only if:

- C(t1) equals C(t2) if and only if t1 equals t2, and
- C(u1) equals C(u2) if and only if u1 equals u2.

[*Note 2*: *end note*]

Users can customize the behavior of common_with by specializing the
common_type class template ([meta.trans.other]).

— ```
template<class T>
concept integral = is_integral_v<T>;
template<class T>
concept signed_integral = integral<T> && is_signed_v<T>;
template<class T>
concept unsigned_integral = integral<T> && !signed_integral<T>;
template<class T>
concept floating_point = is_floating_point_v<T>;
```

[*Note 1*: *end note*]

signed_integral can be modeled even by types that are
not signed integer types ([basic.fundamental]); for example, char.

— [*Note 2*: *end note*]

unsigned_integral can be modeled even by types that are
not unsigned integer types ([basic.fundamental]); for example, bool.

— ```
template<class LHS, class RHS>
concept assignable_from =
is_lvalue_reference_v<LHS> &&
common_reference_with<const remove_reference_t<LHS>&, const remove_reference_t<RHS>&> &&
requires(LHS lhs, RHS&& rhs) {
{ lhs = std::forward<RHS>(rhs) } -> same_as<LHS>;
};
```

Let:

- lhs be an lvalue that refers to an object lcopy such that decltype((lhs)) is LHS,
- rhs be an expression such that decltype((rhs)) is RHS, and
- rcopy be a distinct object that is equal to rhs.

LHS and RHS model
assignable_from<LHS, RHS> only if

- addressof(lhs = rhs) == addressof(lcopy).
- After evaluating lhs = rhs:
- If rhs is a non-const xvalue, the resulting state of the object to which it refers is valid but unspecified ([lib.types.movedfrom]).
- Otherwise, if rhs is a glvalue, the object to which it refers is not modified.

[*Note 1*: *end note*]

Assignment need not be a total function ([structure.requirements]);
in particular, if assignment to an object x can result in a modification
of some other object y, then x = y is likely not in the domain
of =.

— Let t1 and t2 be equality-preserving expressions that denote
distinct equal objects of type T, and let u1 and u2
similarly denote distinct equal objects of type U.

An operation
*exchanges the values* denoted by t1 and u1 if and only
if the operation modifies neither t2 nor u2 and:

- If T and U are different types and common_reference_with<decltype((t1)), decltype((u1))> is modeled, the result of the operation is that C(t1) equals C(u2) and C(u1) equals C(t2) where C is common_reference_t<decltype((t1)), decltype((u1))>.

The expression
ranges::swap(E1, E2) for subexpressions E1
and E2 is expression-equivalent to an expression
S determined as follows:

- S is (void)swap(E1, E2)201 if E1 or E2 has class or enumeration type ([basic.compound]) and that expression is valid, with overload resolution performed in a context that includes the declaration template<class T> void swap(T&, T&) = delete; and does not include a declaration of ranges::swap.If the function selected by overload resolution does not exchange the values denoted by E1 and E2, the program is ill-formed, no diagnostic required.[
*Note 2*:This precludes calling unconstrained program-defined overloads of swap.When the deleted overload is viable, program-defined overloads need to be more specialized ([temp.func.order]) to be selected.—*end note*] - Otherwise, if E1 and E2 are lvalues of array types ([basic.compound]) with equal extent and ranges::swap(*E1, *E2) is a valid expression, S is (void)ranges::swap_ranges(E1, E2), except that noexcept(S) is equal to noexcept(ranges::swap(*E1, *E2)).
- Otherwise, if E1 and E2 are lvalues of the same type T that models move_constructible<T> and assignable_from<T&, T>, S is an expression that exchanges the denoted values.S is a constant expression if
- T is a literal type ([basic.types.general]),
- both E1 = std::move(E2) and E2 = std::move(E1) are constant subexpressions ([defns.const.subexpr]), and
- the full-expressions of the initializers in the declarations T t1(std::move(E1)); T t2(std::move(E2)); are constant subexpressions.

```
template<class T, class U>
concept swappable_with =
common_reference_with<T, U> &&
requires(T&& t, U&& u) {
ranges::swap(std::forward<T>(t), std::forward<T>(t));
ranges::swap(std::forward<U>(u), std::forward<U>(u));
ranges::swap(std::forward<T>(t), std::forward<U>(u));
ranges::swap(std::forward<U>(u), std::forward<T>(t));
};
```

[*Note 5*: *end note*]

The semantics of the swappable and swappable_with
concepts are fully defined by the ranges::swap customization point object.

— [*Example 1*: *end example*]

User code can ensure that the evaluation of swap calls
is performed in an appropriate context under the various conditions as follows:
#include <cassert>
#include <concepts>
#include <utility>
namespace ranges = std::ranges;
template<class T, std::swappable_with<T> U>
void value_swap(T&& t, U&& u) {
ranges::swap(std::forward<T>(t), std::forward<U>(u));
}
template<std::swappable T>
void lv_swap(T& t1, T& t2) {
ranges::swap(t1, t2);
}
namespace N {
struct A { int m; };
struct Proxy {
A* a;
Proxy(A& a) : a{&a} {}
friend void swap(Proxy x, Proxy y) {
ranges::swap(*x.a, *y.a);
}
};
Proxy proxy(A& a) { return Proxy{ a }; }
}
int main() {
int i = 1, j = 2;
lv_swap(i, j);
assert(i == 2 && j == 1);
N::A a1 = { 5 }, a2 = { -5 };
value_swap(a1, proxy(a2));
assert(a1.m == -5 && a2.m == 5);
}

— The destructible concept specifies properties of all types,
instances of which can be destroyed at the end of their lifetime, or reference
types.

```
template<class T>
concept destructible = is_nothrow_destructible_v<T>;
```

The constructible_from concept constrains the initialization of a
variable of a given type with a particular set of argument types.

```
template<class T, class... Args>
concept constructible_from = destructible<T> && is_constructible_v<T, Args...>;
```

```
template<class T>
constexpr bool
```*is-default-initializable* = *see below*; // *exposition only*
template<class T>
concept default_initializable = constructible_from<T> &&
requires { T{}; } &&
*is-default-initializable*<T>;

For a type T, *is-default-initializable*<T> is true
if and only if the variable definition
T t;
is well-formed for some invented variable t;
otherwise it is false.

Access checking is performed as if in a context unrelated to T.

Only the validity of the immediate context of the variable initialization is considered.

```
template<class T>
concept move_constructible = constructible_from<T, T> && convertible_to<T, T>;
```

If T is an object type, then let rv be an rvalue of type
T and u2 a distinct object of type T equal to
rv.

T models move_constructible only if

- If T is not const, rv's resulting state is valid but unspecified ([lib.types.movedfrom]); otherwise, it is unchanged.

```
template<class T>
concept copy_constructible =
move_constructible<T> &&
constructible_from<T, T&> && convertible_to<T&, T> &&
constructible_from<T, const T&> && convertible_to<const T&, T> &&
constructible_from<T, const T> && convertible_to<const T, T>;
```

Subclause [concepts.compare] describes concepts that establish relationships and orderings
on values of possibly differing object types.

The exposition-only *boolean-testable* concept
specifies the requirements on expressions
that are convertible to bool and
for which the logical operators ([expr.log.and], [expr.log.or], [expr.unary.op])
have the conventional semantics.

```
template<class T>
concept
```*boolean-testable-impl* = convertible_to<T, bool>; // *exposition only*

T models *boolean-testable-impl* only if:

- either remove_cvref_t<T> is not a class type, or a search for the names operator&& and operator|| in the scope of remove_cvref_t<T> finds nothing; and
- argument-dependent lookup ([basic.lookup.argdep]) for the names operator&& and operator|| with T as the only argument type finds no disqualifying declaration (defined below).

A *disqualifying parameter*
is a function parameter whose declared type P

- is not dependent on a template parameter, and there exists an implicit conversion sequence ([over.best.ics]) from e to P; or
- is dependent on one or more template parameters, and either
- P contains no template parameter that participates in template argument deduction ([temp.deduct.type]), or
- template argument deduction using the rules for deducing template arguments in a function call ([temp.deduct.call]) and e as the argument succeeds.

A *key parameter* of a function template D
is a function parameter of type cv X or reference thereto,
where X names a specialization of a class template that
has the same innermost enclosing non-inline namespace as D, and
X contains at least one template parameter that
participates in template argument deduction.

[*Example 1*: *end example*]

In
namespace Z {
template<class> struct C {};
template<class T>
void operator&&(C<T> x, T y);
template<class T>
void operator||(C<type_identity_t<T>> x, T y);
}
the declaration of Z::operator&&
contains one key parameter, C<T> x, and
the declaration of Z::operator||
contains no key parameters.

— A *disqualifying declaration* is

- a (non-template) function declaration that contains at least one disqualifying parameter; or
- a function template declaration that
contains at least one disqualifying parameter, where
- at least one disqualifying parameter is a key parameter; or
- the declaration contains no key parameters; or
- the declaration declares a function template to which no name is bound ([dcl.meaning]).

[*Note 1*: *end note*]

The intention is to ensure that
given two types T1 and T2
that each model *boolean-testable-impl*,
the && and || operators within the expressions
declval<T1>() && declval<T2>() and
declval<T1>() || declval<T2>()
resolve to the corresponding built-in operators.

— ```
template<class T>
concept
```*boolean-testable* = // *exposition only*
*boolean-testable-impl*<T> && requires(T&& t) {
{ !std::forward<T>(t) } -> *boolean-testable-impl*;
};

[*Example 2*: *end example*]

The types
bool,
true_type ([meta.type.synop]),
int*, and
bitset<N>::reference ([template.bitset])
model *boolean-testable*.

— ```
template<class T, class U, class C = common_reference_t<const T&, const U&>>
concept
```*comparison-common-type-with-impl* = // *exposition only*
same_as<common_reference_t<const T&, const U&>,
common_reference_t<const U&, const T&>> &&
requires {
requires convertible_to<const T&, const C&> || convertible_to<T, const C&>;
requires convertible_to<const U&, const C&> || convertible_to<U, const C&>;
};
template<class T, class U>
concept *comparison-common-type-with* = // *exposition only*
*comparison-common-type-with-impl*<remove_cvref_t<T>, remove_cvref_t<U>>;

Let t1 and t2 be equality-preserving expressions
that are lvalues of type remove_cvref_t<T>, and
let u1 and u2 be equality-preserving expressions
that are lvalues of type remove_cvref_t<U>.

T and U model
*comparison-common-type-with*<T, U> only if:

*CONVERT_TO_LVALUE*<C>(t1) equals*CONVERT_TO_LVALUE*<C>(t2) if and only if t1 equals t2, and*CONVERT_TO_LVALUE*<C>(u1) equals*CONVERT_TO_LVALUE*<C>(u2) if and only if u1 equals u2

```
template<class T, class U>
concept
```*weakly-equality-comparable-with* = // *exposition only*
requires(const remove_reference_t<T>& t,
const remove_reference_t<U>& u) {
{ t == u } -> *boolean-testable*;
{ t != u } -> *boolean-testable*;
{ u == t } -> *boolean-testable*;
{ u != t } -> *boolean-testable*;
};

Given types T and U,
let t and u be lvalues of types
const remove_reference_t<T> and
const remove_reference_t<U> respectively.

```
template<class T>
concept equality_comparable =
```*weakly-equality-comparable-with*<T, T>;

T models equality_comparable only if
bool(a == b) is true when a is equal to
b ([concepts.equality]), and false otherwise.

```
template<class T, class U>
concept equality_comparable_with =
equality_comparable<T> && equality_comparable<U> &&
```*comparison-common-type-with*<T, U> &&
equality_comparable<
common_reference_t<
const remove_reference_t<T>&,
const remove_reference_t<U>&>> &&
*weakly-equality-comparable-with*<T, U>;

Given types T and U,
let t and t2 be lvalues
denoting distinct equal objects of types const remove_reference_t<T> and
remove_cvref_t<T>, respectively,
let u and u2 be lvalues
denoting distinct equal objects of types const remove_reference_t<U> and
remove_cvref_t<U>, respectively, and
let C be:
common_reference_t<const remove_reference_t<T>&, const remove_reference_t<U>&>
T and U model
equality_comparable_with<T, U> only if
bool(t == u) == bool(*CONVERT_TO_LVALUE*<C>(t2) == *CONVERT_TO_LVALUE*<C>(u2))

```
template<class T>
concept totally_ordered =
equality_comparable<T> &&
```*partially-ordered-with*<T, T>;

```
template<class T, class U>
concept totally_ordered_with =
totally_ordered<T> && totally_ordered<U> &&
equality_comparable_with<T, U> &&
totally_ordered<
common_reference_t<
const remove_reference_t<T>&,
const remove_reference_t<U>&>> &&
```*partially-ordered-with*<T, U>;

Given types T and U,
let t and t2 be lvalues
denoting distinct equal objects of types const remove_reference_t<T> and
remove_cvref_t<T>, respectively,
let u and u2 be lvalues
denoting distinct equal objects of types const remove_reference_t<U> and
remove_cvref_t<U>, respectively, and
let C be:
common_reference_t<const remove_reference_t<T>&, const remove_reference_t<U>&>
T and U model
totally_ordered_with<T, U> only if

- bool(t < u) == bool(
*CONVERT_TO_LVALUE*<C>(t2) <*CONVERT_TO_LVALUE*<C>(u2)). - bool(t > u) == bool(
*CONVERT_TO_LVALUE*<C>(t2) >*CONVERT_TO_LVALUE*<C>(u2)). - bool(t <= u) == bool(
*CONVERT_TO_LVALUE*<C>(t2) <=*CONVERT_TO_LVALUE*<C>(u2)). - bool(t >= u) == bool(
*CONVERT_TO_LVALUE*<C>(t2) >=*CONVERT_TO_LVALUE*<C>(u2)). - bool(u < t) == bool(
*CONVERT_TO_LVALUE*<C>(u2) <*CONVERT_TO_LVALUE*<C>(t2)). - bool(u > t) == bool(
*CONVERT_TO_LVALUE*<C>(u2) >*CONVERT_TO_LVALUE*<C>(t2)). - bool(u <= t) == bool(
*CONVERT_TO_LVALUE*<C>(u2) <=*CONVERT_TO_LVALUE*<C>(t2)). - bool(u >= t) == bool(
*CONVERT_TO_LVALUE*<C>(u2) >=*CONVERT_TO_LVALUE*<C>(t2)).

This subclause describes concepts that specify the basis of the
value-oriented programming style on which the library is based.

```
template<class T>
concept movable = is_object_v<T> && move_constructible<T> &&
assignable_from<T&, T> && swappable<T>;
template<class T>
concept copyable = copy_constructible<T> && movable<T> && assignable_from<T&, T&> &&
assignable_from<T&, const T&> && assignable_from<T&, const T>;
template<class T>
concept semiregular = copyable<T> && default_initializable<T>;
template<class T>
concept regular = semiregular<T> && equality_comparable<T>;
```

[*Note 1*: *end note*]

The semiregular concept is modeled by types that behave similarly
to fundamental types like int, except that they need not
be comparable with ==.

— The concepts in subclause [concepts.callable] describe the requirements on function
objects ([function.objects]) and their arguments.

The invocable concept specifies a relationship between a callable
type ([func.def]) F and a set of argument types Args... which
can be evaluated by the library function invoke ([func.invoke]).

```
template<class F, class... Args>
concept regular_invocable = invocable<F, Args...>;
```

```
template<class F, class... Args>
concept predicate =
regular_invocable<F, Args...> &&
```*boolean-testable*<invoke_result_t<F, Args...>>;

```
template<class R, class T, class U>
concept equivalence_relation = relation<R, T, U>;
```

```
template<class R, class T, class U>
concept strict_weak_order = relation<R, T, U>;
```