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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