Using Concepts

main.cpp

// a blank struct to give an example of a failing concept
struct Something {};

// a concept called Addable wich to satisfy requires that instances of T can use the + operator
template<typename T>
concept Addable = requires(T x) { x + x; };

// a struct called Thing, which takes a type that is constrained by the 3 concepts after the 'requires' statement.
template<typename T> requires Addable<T> && std::copyable<T> && std::movable<T>
struct Thing
{
	T i {};
};

int wmain(int argc, wchar_t* args[])
{
	Thing<int> i;		// OK! int is Addable, Copyable and moveable.

	Thing<Something> j; // error: 
						//message : see declaration of 'Thing'
						//message: the concept 'Addable<Something>' evaluated to false
						//message: binary '+' : 'Something' does not define this operator or a conversion to a type acceptable to the predefined operator
}

// requires statement: https://en.cppreference.com/w/cpp/language/constraints#Requires_clauses
// constraints and convepts: https://en.cppreference.com/w/cpp/language/constraints