map(op)这种构造函数,该怎么处理

map(op)这种构造函数

C/C++ code

map这个容器的构造函数很多，其中一个是： map(op)一直没机会体会这种构造函数，于是写了个程序测试一下： 发现无法编译，网上一查询， 有人说：map只能够按照key来排序，既然按照key排序，那么提供map(op)这种方式作甚？class YuanGong{    unsigned int m_ID;    string m_strName;public:    YuanGong(unsigned int id, string strName):m_ID(id),m_strName(strName){}public:    unsigned int GetID() const    {        return m_ID;    }    string GetName() const    {        return m_strName;    }    friend ostream& operator<<(ostream& o, YuanGong const& obj)    {        o<<obj.m_ID<<"  "<<obj.m_strName<<endl;        return o;    }};class YuanGongCompare{public:    /*bool operator()(YuanGong const& obj1,YuanGong const& obj2 )    {        return (obj1.GetName()>obj2.GetName())?true:false;    }*/    bool operator()(YuanGong const& obj1,YuanGong const& obj2 )    {        return (obj1.GetID()>obj2.GetID())?true:false;    }};int main(){    map<unsigned int,YuanGong,YuanGongCompare> myMap;    unsigned int const n=3;    YuanGong obj[n]={YuanGong(23,"csdn"),YuanGong(623,"c34sdn"),YuanGong(334,"cbhhsdn")};    unsigned int pos;    for(pos=0; pos<n; pos++)    {        myMap.insert(make_pair(obj[pos].GetID(), obj[pos].GetName()));    }    //copy(myMap.begin(),myMap.end(), ostream_iterator<YuanGong>(cout," "));    return 0;}

还有种写法：map<unsigned int,YuanGong> myMap(YuanGongCompare)，都是错的啊



[解决办法]
map(iterator begin, iterator end, const traits& _compare) //带比较谓词的构造函数

map(iterator begin, iterator end, const traits& _compare, const allocator& all) //带分配器

map的构造函数是这样的
[解决办法]

C/C++ code

class YuanGongCompare{public:    /*bool operator()(YuanGong const& obj1,YuanGong const& obj2 )    {        return (obj1.GetName()>obj2.GetName())?true:false;    }*/    bool operator()(const int &obj1,const int &obj2 )   //注意这里 函数形参应该与map键类型一致 因为 在map内部 是使用的键的值作为形参传给比较器    {        return obj1>obj2;   //这样写就可以了    }};
[解决办法]
template < class Key, class T, class Compare = less<Key>,
          class Allocator = allocator<pair<const Key,T> > > class map;
这是关于比较器的描述：
Compare: Comparison class: A class that takes two arguments of the key type and returns a bool. The expression comp(a,b), where comp is an object of this comparison class and a and b are key values, shall return true if a is to be placed at an earlier position than b in a strict weak ordering operation. This can either be a class implementing a function call operator or a pointer to a function (see constructor for an example). This defaults to less<Key>, which returns the same as applying the less-than operator (a<b)
以下是使用例子
C/C++ code
#include <iostream>#include <map>using namespace std;bool fncomp (char lhs, char rhs) {return lhs<rhs;}struct classcomp {  bool operator() (const char& lhs, const char& rhs) const  {return lhs<rhs;}};int main (){  map<char,int> first;  first['a']=10;  first['b']=30;  first['c']=50;  first['d']=70;  map<char,int> second (first.begin(),first.end());  map<char,int> third (second);  map<char,int,classcomp> fourth;                 // class as Compare  bool(*fn_pt)(char,char) = fncomp;  map<char,int,bool(*)(char,char)> fifth (fn_pt); // function pointer as Compare  return 0;}
------解决方案-------------------- 
 
C/C++ code
1, key是unsigned int, 你的operator()传入YuanGong? 2, operator()末尾加上const.3, 你的map到底是存uint->string还是uint->YuanGong? 

 

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