c++ - User Defined Class as a Template Parameter

Question

I' m re-implementing std::map. I need to make sure that any data type (basic or user defined) key will work with it. I declared the Map class as a template which has two parameters for the key and the value. My question is if I need to use a string as the key type, how can I overload the < and > operators for string type keys only?? In template specialization we have to specialize the whole class with the type we need as I understand it.
Is there any way I can do this in a better way?? What if I add a separate Key class and use it as the template type for Key?

Answer 1

您应该将比较作为一种类型考虑在内，就像正常std::map情况一样。也就是说，有一个实用程序类less_compare：

template <typename T>
struct less_compare
{
    bool operator()(const T& pLhs, const T& pRhs) const
    {
        return pLhs < pRhs;
    }
};

接着：

template <typename Key, typename Value, typename Compare = less_compare<Key> >
class map
{
    // ...

private:
    Compare mCompare;
};

并且要比较两个值，请执行：if (mCompare(someThing, someOtherThing))，这将是真实的someThingis "less than" someOtherThing。请注意，此因式分解还允许用户定义的比较（这就是引用“小于”的原因）。这称为基于策略的设计。

现在你可以只专门化less_compareC 字符串类。（并且还提供greater_compare和亲属。）

---

请记住，除非这是为了学习，否则您不应该实现自己的地图。另请注意，std::string已经operator<超载。

Answer 2

你也可以使用类型特征。它将为您提供一个框架来解决类型之间可能的未来差异。