how enable if use

c++-enable_if-tester.cpp

#include <iostream>

class A
{
public:
	static constexpr bool dujin = true;
};

class B
{
public:
	static constexpr bool dujin = false;
};

class EnableIfTester
{
public:

	template<class T>
	typename std::enable_if<T::dujin, bool>::type check()
	{
		return true;
	}

	template<class T>
	typename std::enable_if<!T::dujin, bool>::type check()
	{
		return false;
	}

	template <
		typename T,
		std::enable_if_t<T::dujin, bool> = true >
		void check2()
	{
		std::cout << "true\n";
	}

	template <
		typename T,
		std::enable_if_t<!T::dujin, bool> = true >
		void check2()
	{
		std::cout << "false\n";
	}

	template<
		typename T >
		void check3()
	{
		if constexpr ( T::dujin )
			std::cout << "true\n";
		else
			std::cout << "false\n";
	}
};

int main()
{
	EnableIfTester tester;

	std::cout << tester.check<A>() << '\n';
	std::cout << tester.check<B>() << '\n';

	tester.check2<A>();
	tester.check2<B>();

	tester.check3<A>();
	tester.check3<B>();
}