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In their implementation in the C++ Standard Template Library, map containers take four template parameters:

template < class Key, class T, class Compare = less<Key>,class Allocator = allocator<pair<const Key,T> > > class map;

Where the template parameters have the following meanings:

• Key: Type of the key values. Each element in a map is uniquely identified by its key value.
• T: Type of the mapped value. Each element in a map is used to store some data as its mapped value.
• 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).
  The map object uses this expression to determine the position of the elements in the container. All elements in a map container are ordered following this rule at all times.
• Allocator: Type of the allocator object used to define the storage allocation model. By default, theallocator class template is used, which defines the simplest memory allocation model and is value-independent.

// constructing maps 

#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; 

}

ref : http://www.cplusplus.com/reference/stl/map/

ref : http://www.cplusplus.com/reference/stl/map/map/