std::semiregular(3)

NAME
std::semiregular - std::semiregular

Synopsis
   Defined in header <concepts>
   template <class T>
   concept semiregular = std::copyable<T> && (since C++20)
   std::default_initializable<T>;
   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; => assignment
 struct 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::semiregular auto x)
 {
     std::cout << x.value << ' ';
 }
 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

See also
   regular specifies that a type is regular, that is, it is both semiregular and
   (C++20) equality_comparable
           (concept)