Playing around with C++ concepts.

concepts.cc

#include <string>

// Simple C++ concepts example for future reference
//  compile with `gcc -Wall -fconcepts -std=c++1z concepts.cc -c -Wall` using a
//  recent gcc (I'm using 6.2).
//
// https://accu.org/index.php/journals/2157
// https://accu.org/index.php/journals/2198
// http://en.cppreference.com/w/cpp/language/constraints

// Define a Configuration concept that has a constructor that takes a string
template<typename T>
concept bool Configuration() {
	return std::is_constructible<T, std::string>::value;

	// Note that the following would not work as expected
	//return requires(T a) {
	//     std::is_constructible<T, std::string>::value;
	//};
	// Instead, you would need:
	// return requires(T a)
	//	requires std::is_constructible<T, std::string>::value;
	//};
}

// Define a Subsystem concept with:
//   - a start method
//   - a stop method
//   - a ::Conf inner type
//        - that adheres to the Configuration concept above
template<typename T>
concept bool Subsys() {
	return requires(T a) {
		{ a.start() } -> int;
		{ a.stop() }  -> int;
		typename T::Conf;
		requires Configuration<typename T::Conf>();
	};
}

struct SubsysDummy {
	struct Conf {
		Conf() {}
		// XXX: does not match the Configuration concept
		Conf(std::string, int) {}
	};

	SubsysDummy(Conf c) {}
	int start(void) { return 0;}
	int stop(void) { return 0;}
};

// static assertion will fail, but will not explain the error (gcc 6.2)
//static_assert(Subsys<SubsysDummy>(), "SubsysDummy does not adhere to Subsys");

// This, on the other hand, explains the error
template<typename T> requires Subsys<T>()
constexpr bool SubsysTest() {return true;}
static_assert(SubsysTest<SubsysDummy>(), "SubSysTest does not adhere to SubSys");