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)