排序在STL中的顺序并设置 [英] Sort Order in STL map and set

问题描述

用户定义的对象如何在map和set中排序？
据我所知，map / set是Sorted关联容器：插入的元素根据它所包含的键排序。

How are the user defined objects sorted in map and set? As far as I know, map/set are Sorted Associative Containers: the elements being inserted are sorted based on the key that it holds.

但是map和

在SGI网站上，我有以下示例：

From the SGI site, I have the following examples:

struct ltstr
{
    bool operator()(const char* s1, const char* s2) const
    {
        return strcmp(s1, s2) < 0;
    }
};

int main()
{
    map<const char*, int, ltstr> months;

    months["january"] = 31;
    months["february"] = 28;
    months["march"] = 31;
    months["april"] = 30;
    months["may"] = 31;
    months["june"] = 30;
    months["july"] = 31;
    months["august"] = 31;
    months["september"] = 30;
    months["october"] = 31;
    months["november"] = 30;
    months["december"] = 31;

    cout << "june -> " << months["june"] << endl;

    map<const char*, int, ltstr>::iterator cur  = months.find("june");
    map<const char*, int, ltstr>::iterator prev = cur;
    map<const char*, int, ltstr>::iterator next = cur;

    ++next;
    --prev;

    cout << "Previous (in alphabetical order) is " << (*prev).first << endl;
    cout << "Next (in alphabetical order) is " << (*next).first << endl;
}

在上面的例子中，值是如何排序的？

In the above example, how are the values sorted?

编辑：从注释中移除代码：

Code moved from comment:

typedef map <string, int> Mint ;

int main() 
{
    string Name ;
    int Marks;
    Mint Grade;
    for (int i = 0; i<4; i++) 
    {
        cin>> Name ; 
        cin >> Marks; 
        Grade [Name] = Marks ; 
    } 
    Mint :: iterator iter; 
    for( iter = Grade.begin(); iter != Grade.end(); iter++)
        cout<< (*iter).first<<" \t " <<(*iter).second<<"\n" ; 
    return 0; 

} 

如何对值进行排序？

推荐答案

std :: map 使用函子对元素进行排序。默认情况下是 std :: less< Key> ，它使用运算符<。在你的示例中有一个用户定义的函数 ltstr ，这将有助于根据字母顺序的按键对元素进行排序。

std::map uses a functor to sort elements. By default is it std::less<Key> which uses operator<. In your sample there is an user defined functor ltstr which will help to sort elements according to its keys in alphabetical order.

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