C++ Core Guidelines: Regular and SemiRegular Types
The topic for today is quite important when you create your type: Regular and SemiRegular types.
Here is the exact rule for today.
T.46: Require template arguments to be at least Regular
or SemiRegular
Okay, the first question I have to answer is quite obvious. What is a Regular or a SemiRegular type? My answer is based on the proposal p0898. I assume you may already guess it. Regular and SemiRegular are concepts that are defined by concepts.
Regular
- DefaultConstructible
- CopyConstructible, CopyAssignable
- MoveConstructible, MoveAssignable
- Destructible
- Swappable
- EqualityComparable
SemiRegular
- Regular – EqualityComparable
The term Regular goes back to the father of the Standard Template Library Alexander Stepanov. He introduced the term in his book Fundamentals of Generic Programming. Here is a short excerpt. It’s quite easy to remember the eight concepts used to define a regular type. There is a well-known rule of six:
- Default constructor:
X()
- Copy constructor:
X(const X&)
- Copy assignment:
operator=(const X&)
- Move constructor:
X(X&&)
- Move assignment:
operator=(X&&)
- Destructor:
~X()
Just add the Swappable and EqualityComparable concepts to it. There is a more informal way to say that a type T is regular: T behaves like an int.
To get SemiRegular, you have to subtract EqualityComparable from Regular.
I hear your next question: Why should our template arguments at least be Regular or SemiRegular or do as the ints do? The STL containers and algorithms, in particular, assume Regular data types.
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What is commonly used but not a Regular type? Right: a reference.
References are not Regular
Thanks to the type-traits library, the following program checks at compile time if int& is a SemiRegular type.
// semiRegular.cpp #include <iostream> #include <type_traits> int main(){ std::cout << std::boolalpha << std::endl; std::cout << "std::is_default_constructible<int&>::value: " << std::is_default_constructible<int&>::value << std::endl; std::cout << "std::is_copy_constructible<int&>::value: " << std::is_copy_constructible<int&>::value << std::endl; std::cout << "std::is_copy_assignable<int&>::value: " << std::is_copy_assignable<int&>::value << std::endl; std::cout << "std::is_move_constructible<int&>::value: " << std::is_move_constructible<int&>::value << std::endl; std::cout << "std::is_move_assignable<int&>::value: " << std::is_move_assignable<int&>::value << std::endl; std::cout << "std::is_destructible<int&>::value: " << std::is_destructible<int&>::value << std::endl; std::cout << std::endl; std::cout << "std::is_swappable<int&>::value: " << std::is_swappable<int&>::value << std::endl; // requires C++17 std::cout << std::endl; }
First of all. The function std::is_swappable requires C++17. Second, here is the output.
You see a reference such as int& is not default constructible. The output shows that a reference is not SemiRegular and, therefore, not Regular. To check if a type is Regular at compile-time, I need a function isEqualityComparable which is not part of the type-traits library. Let’s do it by myself.
isEqualityComparable
In C++20, we might get the detection idiom which is part of the library fundamental TS v2. Now, it’s a piece of cake to implement isEqualityComparable.
// equalityComparable.cpp #include <experimental/type_traits> // (1) #include <iostream> template<typename T> using equal_comparable_t = decltype(std::declval<T&>() == std::declval<T&>()); // (2) template<typename T> struct isEqualityComparable: std::experimental::is_detected<equal_comparable_t, T>{}; // (3) struct EqualityComparable { }; // (4) bool operator == (EqualityComparable const&, EqualityComparable const&) { return true; } struct NotEqualityComparable { }; // (5) int main() { std::cout << std::boolalpha << std::endl; std::cout << "isEqualityComparable<EqualityComparable>::value: " << isEqualityComparable<EqualityComparable>::value << std::endl; std::cout << "isEqualityComparable<NotEqualityComparable>::value: " << isEqualityComparable<NotEqualityComparable>::value << std::endl; std::cout << std::endl; }
The new feature is in the experimental namespace (1). Line (3) is the crucial one. It detects if the expression (2) is valid for type T. The type-trait isEqualityComparable works for an EqualityComparable (4) and a NotEqualityComparable (5) type. Only EqualityCompable returns true because I overloaded the Equal-Comparison Operator.
To compile the program, you need a new C++ compiler, such as GCC 8.2.
Until C++20, comparison operators are automatically generated for arithmetic types, enumerations, and with restrictions for pointers. This may change with C++20 due to the spaceship operator: <=>. With C++20, when a class defines operator <=>, automatically the operators ==, !=, <, <=, >, and >= are generated. It’s possible to define operator <=> as defaulted, such as for the type Point.
class Point { int x; int y; public: auto operator<=>(const Point&) const = default; .... }; // compiler generates all six relational operators
In this case, the compiler will generate the implementation. The default operator<=> performs a lexicographical comparison on its bases (left-to-right, depth-first) and continues with its non-static member in declaration order. This comparison applies to short-circuit evaluation. This means evaluating a logical expression ends if the result is known.
Now, I have all the ingredients to define Regular and SemiRegular. Here are my new type traits.
// isRegular.cpp #include <experimental/type_traits> #include <iostream> template<typename T> using equal_comparable_t = decltype(std::declval<T&>() == std::declval<T&>()); template<typename T> struct isEqualityComparable: std::experimental::is_detected<equal_comparable_t, T> {}; template<typename T> struct isSemiRegular: std::integral_constant<bool, std::is_default_constructible<T>::value && std::is_copy_constructible<T>::value && std::is_copy_assignable<T>::value && std::is_move_constructible<T>::value && std::is_move_assignable<T>::value && std::is_destructible<T>::value && std::is_swappable<T>::value >{}; template<typename T> struct isRegular: std::integral_constant<bool, isSemiRegular<T>::value && isEqualityComparable<T>::value >{}; int main(){ std::cout << std::boolalpha << std::endl; std::cout << "isSemiRegular<int>::value: " << isSemiRegular<int>::value << std::endl; std::cout << "isRegular<int>::value: " << isRegular<int>::value << std::endl; std::cout << std::endl; std::cout << "isSemiRegular<int&>::value: " << isSemiRegular<int&>::value << std::endl; std::cout << "isRegular<int&>::value: " << isRegular<int&>::value << std::endl; std::cout << std::endl; }
The usage of the new type-traits isSemiRegular and isRegular makes the main program quite readable.
What’s next?
With my next post, I jump directly to the template definition.
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