C++ compile time detection of member variable with SFINAE

c++_has_member.cpp

#include <iostream>

struct Foo
{
	int one;
};

struct Bar
{
	char one;
};

struct Boo : Foo
{
	int two;
};

struct Baz
{
	int two;
};

template <class T>
struct HasOne
{
	typedef char Yes[1];
	typedef char No[2];

	template <typename C, C>
	struct Check;

	struct FakeBase
	{
		int one;
	};

	struct TestSubject : public FakeBase, public T {};

	// This would SFINAE because if C really has `one`, it would be 
	// ambiguous
	template <typename C>
	static No & Test(Check<int FakeBase::*, &C::one> *);

	template <typename>
	static Yes & Test(...);

	const static bool value = sizeof(Test<TestSubject>(0)) == sizeof(Yes);
};

int main()
{
	std::cout << HasOne<Foo>::value << std::endl; // 1
	std::cout << HasOne<Bar>::value << std::endl; // 1
	std::cout << HasOne<Boo>::value << std::endl; // 1
	std::cout << HasOne<Baz>::value << std::endl; // 0

	return 0;
}

gistfile1.cpp

#include <iostream>

struct Foo       { int one; };
struct Bar       { char one; };
struct Boo : Foo { int two; };
struct Baz       { int two; };

template <class T>
struct HasOne
{
	typedef char Yes[1];
	typedef char No[2];

	template <typename C, C>
	struct Check;

	struct FakeBase { int one; };

	struct TestSubject : public FakeBase, public T {};

	// This would SFINAE because if C really has `one`, it would be 
	// ambiguous
	template <typename C>
	static No & Test(Check<int FakeBase::*, &C::one> *);

	template <typename>
	static Yes & Test(...);

	const static bool value = sizeof(Test<TestSubject>(0)) == sizeof(Yes);
};

int main()
{
	std::cout << HasOne<Foo>::value << std::endl; // 1
	std::cout << HasOne<Bar>::value << std::endl; // 1
	std::cout << HasOne<Boo>::value << std::endl; // 1
	std::cout << HasOne<Baz>::value << std::endl; // 0

	return 0;
}