pair map multimap说明

Sparse array example: (why hold space for thousands of elements when all we have is five)

#include <string.h>
#include <iostream>
#include <map>
#include <utility>

using namespace std;

int main()
{
   map<int, string> Employees;

   // 1) Assignment using array index notation
   Employees[5234] = "Mike C.";
   Employees[3374] = "Charlie M.";
   Employees[1923] = "David D.";
   Employees[7582] = "John A.";
   Employees[5328] = "Peter Q.";

   cout << "Employees[3374]=" << Employees[3374] << endl << endl;

   cout << "Map size: " << Employees.size() << endl;

   for( map<int,string>::iterator ii=Employees.begin(); ii!=Employees.end(); ++ii)
   {
       cout << (*ii).first << ": " << (*ii).second << endl;
   }
}

Compile: g++ testMap.cpp 

Run: ./a.out

Employees[3374]=Charlie M.

Map size: 5
1923: David D.
3374: Charlie M.
5234: Mike C.
5328: Peter Q.
7582: John A.

Example using a "string" as an array index:

#include <string.h>
#include <iostream>
#include <map>
#include <utility>

using namespace std;

int main()
{
   map<string, int> Employees;

   // Examples of assigning Map container contents

   // 1) Assignment using array index notation
   Employees["Mike C."] = 5234;
   Employees["Charlie M."] = 3374;

   // 2) Assignment using member function insert() and STL pair
   Employees.insert(std::pair<string,int>("David D.",1923));
 
   // 3) Assignment using member function insert() and "value_type()"
   Employees.insert(map<string,int>::value_type("John A.",7582));

   // 4) Assignment using member function insert() and "make_pair()"
   Employees.insert(std::make_pair("Peter Q.",5328));

   cout << "Map size: " << Employees.size() << endl;

   for( map<string, int>::iterator ii=Employees.begin(); ii!=Employees.end(); ++ii)
   {
       cout << (*ii).first << ": " << (*ii).second << endl;
   }
}

Compile: g++ testMap.cpp 
Run: ./a.out

Map size: 5
Charlie M.: 3374
David D.: 1923
John A.: 7582
Mike C.: 5234
Peter Q.: 5328

Note: The fully defined STL map defines a comparison operator in the map declaration so that the indexes can be ordered. This is used to provide a default comparison operator for the data type. In the above example, the key type is an integer and the C++ "equals" (=) and "less than" operator (<) can operate on an integer so the example works. The use of the STL algorithm std::less<> can be specified explicitly as in the following declaration:

std::map<int, string, std::less< int > >

If defining your own class as the index (first value), C++ will not know how to perform the comparison, thus you will have to provide an operator to perform this function.

The first element in a map can be a class or even another STL container such as a pair. If using a class, the class must provide an overloaded "equals" (=) and "less than" operator (<).

Thus using "char" instead of "string" requires the use of a comparison function:

#include <string.h>
#include <iostream>
#include <map>
#include <utility>

using namespace std;

struct cmp_str 
{
   bool operator()(char const *a, char const *b) 
   {
      return std::strcmp(a, b) < 0;
   }
};

int main()
{
   map<char *, int, cmp_str> Employees;

   // Examples of assigning Map container contents

   // 1) Assignment using array index notation
   Employees["Mike C."] = 5234;
   Employees["Charlie M."] = 3374;

   // 2) Assignment using member function insert() and STL pair
   Employees.insert(std::pair<char *,int>("David D.",1923));
 
   // 3) Assignment using member function insert() and "value_type()"
   Employees.insert(map<char *,int>::value_type("John A.",7582));

   // 4) Assignment using member function insert() and "make_pair()"
   Employees.insert(std::make_pair((char *)"Peter Q.",5328));

   cout << "Map size: " << Employees.size() << endl;

   for( map<char *, int, cmp_str>::iterator ii=Employees.begin(); ii!=Employees.end(); ++ii)
   {
       cout << (*ii).first << ": " << (*ii).second << endl;
   }
}

Compile: g++ testMap.cpp 
Run: ./a.out

Map size: 5
Charlie M.: 3374
David D.: 1923
John A.: 7582
Mike C.: 5234
Peter Q.: 5328

SGI Map Documentation:

• map

---

Using STL MAP to store class objects:

This example uses a string as the MAP index and an object as the value pair. The overloaded = and < operators are required to satisfy MAP container use.

#include <iostream>
#include <map>
using namespace std;

class AAA
{
   friend ostream &operator<<(ostream &, const AAA &);

   public:
      int x;
      int y;
      float z;

      AAA();
      AAA(const AAA &);
      ~AAA(){};
      AAA &operator=(const AAA &rhs);
      int operator==(const AAA &rhs) const;
      int operator<(const AAA &rhs) const;
};

AAA::AAA()   // Constructor
{
   x = 0;
   y = 0;
   z = 0;
}

AAA::AAA(const AAA &copyin)   // Copy constructor to handle pass by value.
{                             
   x = copyin.x;
   y = copyin.y;
   z = copyin.z;
}

ostream &operator<<(ostream &output, const AAA &aaa)
{
   output << aaa.x << ' ' << aaa.y << ' ' << aaa.z << endl;
   return output;
}

AAA& AAA::operator=(const AAA &rhs)
{
   this->x = rhs.x;
   this->y = rhs.y;
   this->z = rhs.z;
   return *this;
}

int AAA::operator==(const AAA &rhs) const
{
   if( this->x != rhs.x) return 0;
   if( this->y != rhs.y) return 0;
   if( this->z != rhs.z) return 0;
   return 1;
}

int AAA::operator<(const AAA &rhs) const
{
   if( this->x == rhs.x && this->y == rhs.y && this->z < rhs.z) return 1;
   if( this->x == rhs.x && this->y < rhs.y) return 1;
   if( this->x < rhs.x ) return 1;
   return 0;
}

main()
{
   map<string, AAA> M;
   AAA Ablob ;

   Ablob.x=7;
   Ablob.y=2;
   Ablob.z=4.2355;
   M["A"] = Ablob;

   Ablob.x=5;
   M["B"] = Ablob;

   Ablob.z=3.2355;
   M["C"] = Ablob;

   Ablob.x=3;
   Ablob.y=7;
   Ablob.z=7.2355;
   M["D"] = Ablob;

   for( map<string, AAA>::iterator ii=M.begin(); ii!=M.end(); ++ii)
   {
       cout << (*ii).first << ": " << (*ii).second << endl;
   }

   return 0;
}

Compile: g++ testMap.cpp 

Run:  ./a.out  
Output:

A: 7 2 4.2355

B: 5 2 4.2355

C: 5 2 3.2355

D: 3 7 7.2355

[Potential Pitfall] : The following error occurs when the < operator is omitted from the class AAA.

/tmp/cca5a9G9.o: In function `main': 
testMap.cpp:(.text+0x3c6): undefined reference to `operator<<(std::basic_ostream<char, std::char_traits<char> >&, AAA const&)' 
collect2: ld returned 1 exit status

[Potential Pitfall] : The following error occurs when the = operator is omitted from the class AAA.

/tmp/cciGBZgS.o: In function `main':
testMap.cpp:(.text+0x263): undefined reference to `AAA::operator=(AAA const&)'
testMap.cpp:(.text+0x2c8): undefined reference to `AAA::operator=(AAA const&)'
testMap.cpp:(.text+0x32e): undefined reference to `AAA::operator=(AAA const&)'
testMap.cpp:(.text+0x3a2): undefined reference to `AAA::operator=(AAA const&)'
collect2: ld returned 1 exit status

---

Example: STL multimap:

The STL multipmap allows duplicate keys.

 
#include <string.h>
#include <iostream>
#include <map>
#include <utility>

using namespace std;

int main()
{
  // Compare (<) function not required since it is built into string class.
  // else declaration would comparison function in multimap definition.
  // i.e. multimap<string, int, compare> m;

  multimap<string, int> m;

  m.insert(pair<string, int>("a", 1));
  m.insert(pair<string, int>("c", 2));
  m.insert(pair<string, int>("b", 3));
  m.insert(pair<string, int>("b", 4));
  m.insert(pair<string, int>("a", 5));
  m.insert(pair<string, int>("b", 6));

  cout << "Number of elements with key a: " << m.count("a") << endl;
  cout << "Number of elements with key b: " << m.count("b") << endl;
  cout << "Number of elements with key c: " << m.count("c") << endl;

  cout << "Elements in m: " << endl;
  for (multimap<string, int>::iterator it = m.begin();
       it != m.end();
       ++it)
   {
       cout << "  [" << (*it).first << ", " << (*it).second << "]" << endl;
   }

   pair<multimap<string, int>::iterator, multimap<string, int>::iterator> ppp;

   // equal_range(b) returns pair<iterator,iterator> representing the range
   // of element with key b
   ppp = m.equal_range("b");

   // Loop through range of maps of key "b"
   cout << endl << "Range of \"b\" elements:" << endl;
   for (multimap<string, int>::iterator it2 = ppp.first;
       it2 != ppp.second;
       ++it2)
   {
       cout << "  [" << (*it2).first << ", " << (*it2).second << "]" << endl;
   }

// Can't do this (??)
//   cout << ppp.first << endl;
//   cout << ppp.second << endl;

   m.clear();
}

Compile: g++ testMultimap.cpp 
Run: ./a.out

Number of elements with key a: 2
Number of elements with key b: 3
Number of elements with key c: 1
Elements in m:
  [a, 1]
  [a, 5]
  [b, 3]
  [b, 4]
  [b, 6]
  [c, 2]

Range of "b" elements:
  [b, 3]
  [b, 4]
  [b, 6]

原文：

http://www.yolinux.com/TUTORIALS/CppStlMultiMap.html

#include <string.h>
#include <iostream>
#include <map>
#include <utility>

using namespace std;

int main()
{
   map<int, string> Employees;

   // 1) Assignment using array index notation
   Employees[5234] = "Mike C.";
   Employees[3374] = "Charlie M.";
   Employees[1923] = "David D.";
   Employees[7582] = "John A.";
   Employees[5328] = "Peter Q.";

   cout << "Employees[3374]=" << Employees[3374] << endl << endl;

   cout << "Map size: " << Employees.size() << endl;

   for( map<int,string>::iterator ii=Employees.begin(); ii!=Employees.end(); ++ii)
   {
       cout << (*ii).first << ": " << (*ii).second << endl;
   }
}