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;
}