std::semiregular

 

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核心語言概念 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)

比較概念 equality_comparableequality_comparable_with (C++20)(C++20) totally_orderedtotally_ordered_with (C++20)(C++20) 物件概念 movable (C++20) copyable (C++20) semiregular (C++20) regular (C++20) 可呼叫概念 invocableregular_invocable (C++20)(C++20) predicate (C++20) relation (C++20) equivalence_relation (C++20) strict_weak_order (C++20)

僅用於闡釋的概念 boolean-testable  (C++20)

 

定義於標頭檔案 <concepts>
template< class T > concept semiregular = std::copyable<T> && std::default_initializable<T>;		(C++20 起)

semiregular 概念指定一個型別既是可複製的 (copyable) 又是可預設構造的 (default constructible)。它由行為類似於內建型別（如 int）的型別滿足，只是它們不需要支援與 == 的比較。

示例

執行此程式碼

#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 << '\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

輸出

4
4
4

參考

• C++23 標準 (ISO/IEC 14882:2024)

• 18.6 物件概念 [concepts.object]

• C++20 標準 (ISO/IEC 14882:2020)

• 18.6 物件概念 [concepts.object]

參閱

regular (C++20)

指定一個型別是常規的 (regular)，也就是說，它既是 semiregular 又是 equality_comparable (概念)