辅助性的“比较操作符”

标准库中有四个template function，分别定义了!=,>,<=,>=四个比较操作符。它们都是利用操作符==和<完成的。

namespace rel_ops
		{	// nested namespace to hide relational operators from std
template<class _Ty> inline
	bool operator!=(const _Ty& _Left, const _Ty& _Right)
	{	// test for inequality, in terms of equality
	return (!(_Left == _Right));
	}

template<class _Ty> inline
	bool operator>(const _Ty& _Left, const _Ty& _Right)
	{	// test if _Left > _Right, in terms of operator<
	return (_Right < _Left);
	}

template<class _Ty> inline
	bool operator<=(const _Ty& _Left, const _Ty& _Right)
	{	// test if _Left <= _Right, in terms of operator<
	return (!(_Right < _Left));
	}

template<class _Ty> inline
	bool operator>=(const _Ty& _Left, const _Ty& _Right)
	{	// test if _Left >= _Right, in terms of operator<
	return (!(_Left < _Right));
	}
		}

只需定义<和==操作符，就可以使用它们。只要加上using namespace std::rel_ops，上述四个比较操作符就自动获得了定义。例如：

class X
{
private:
	int value;
public:
	X(int x) :value(x){};

	bool operator==(const X& x)const
	{
		return value == x.value;
	}
	bool operator<(const X& x)const
	{
		return value < x.value;
	}
};

int main()
{
	cout<<numeric_limits<int>::max()<<endl;
	cout << boolalpha;  //以符号形式显示bool值
	using namespace std::rel_ops;//引入命名空间使其他四个模板比较操作符有效
	X x1(1), x2(2);
	cout << "x1 == x2" << (x1 == x2) << endl;
	cout << "x1 != x2" << (x1 != x2) << endl;
	cout << "x1<x2" << (x1 < x2) << endl;
	cout << "x1>x2" << (x1 > x2) << endl;

	system("pause");
	return 0;
}