std::semiregular

 

C++

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Concepts library

Core language concepts same_as (C++20) derived_from (C++20) convertible_to (C++20) common_reference_with (C++20) common_with (C++20) integral (C++20) signed_integral (C++20) unsigned_integral (C++20) floating_point (C++20) swappableswappable_with (C++20)(C++20) destructible (C++20) constructible_from (C++20) default_initializable (C++20)   move_constructible (C++20) copy_constructible (C++20) assignable_from (C++20)

Comparison concepts equality_comparableequality_comparable_with (C++20)(C++20) totally_orderedtotally_ordered_with (C++20)(C++20) Object concepts movable (C++20) copyable (C++20) semiregular (C++20) regular (C++20) Callable concepts invocableregular_invocable (C++20)(C++20) predicate (C++20) relation (C++20) equivalence_relation (C++20) strict_weak_order (C++20)

Exposition-only concepts boolean-testable  (C++20)

 

Defined in header <concepts>
template<class T > concept semiregular =std::copyable<T>&&std::default_initializable<T>;		(since C++20)

The semiregular concept specifies that a type is both copyable and default constructible. It is satisfied by types that behave similarly to built-in types like int, except that they need not support comparison with ==.

Example

Run this code

#include <concepts>#include <iostream>   template<std::semiregular T>// Credit Alexander Stepanov// concepts are requirements on T// Requirement on T: T is semiregular// T a(b); or T a = b; => copy constructor// T a; => default constructor// a = b; => assignmentstruct Single { T value;// Aggregation initialization for Single behaves like following constructor:// explicit Single(const T& x) : value(x) {}   // Implicitly declared special member functions behave like following definitions,// except that they may have additional properties:// Single(const Single& x) : value(x.value) {}// Single() {}// ~Single() {}// Single& operator=(const Single& x) { value = x.value; return *this; }// comparison operator is not defined; it is not required by `semiregular` concept// bool operator==(Single const& other) const = delete;};   void print(std::semiregularauto x){std::cout<< x.value<<'\n';}   int main(){ Single<int> myInt1{4};// aggregate initialization: myInt1.value = 4 Single<int> myInt2(myInt1);// copy constructor Single<int> myInt3;// default constructor myInt3 = myInt2;// copy assignment operator// myInt1 == myInt2; // Error: operator== is not defined   print(myInt1);// ok: Single<int> is a `semiregular` type print(myInt2); print(myInt3);   }// Single<int> variables are destroyed here

Output:

4 4 4

References

• C++23 standard (ISO/IEC 14882:2024):

• 18.6 Object concepts [concepts.object]

• C++20 standard (ISO/IEC 14882:2020):

• 18.6 Object concepts [concepts.object]

See also

regular (C++20)

specifies that a type is regular, that is, it is both semiregular and equality_comparable (concept)