C++ map operator=() function

There are following three uses of operator=() in a map:

Operator=() is used to assign new contents to the map container by replacing its old content (or copy the contents) and modifies size if necessary.
Operator=() is used to move the contents of one map container into another and modifies size if necessary.
Operator= is used to copy the elements from the initializer list to map container.

Syntax

copy(1) map& operator= (const map& x); //until C++ 11
copy (1) map& operator= (const map& x); //since C++ 11
move (2) map& operator= (map&& x); //since C++ 11
initializer list (3) map& operator= (initializer_list<value_type> il); //since C++ 11

copy (1):- Copies all the elements from x into the map container.

move (2):- Moves the contents of x into the map container.

initializer_list (3):- Copies the elements of il into the map container.

Parameter

x: A map object with the same type.

il: An initializer list object.

Return value

this pointer.

Example 1

Let's see a simple example to copy the content of one map to another.

#include <iostream>
#include <map>

using namespace std;

int main(void) {

   map<char, int> m1 = {
            {'a', 10},
            {'b', 20},
            {'c', 30} };

   cout << "Map m1 contains following elements" << endl;
    for (auto it = m1.begin(); it != m1.end(); ++it)
      cout << it->first << " = " << it->second << endl;
      
    map<char, int> m2 = m1;  
    cout<<"\nAfter Copying the elements from m1 to m2... \n";  
    
    cout << "\nMap m2 contains following elements" << endl;
    for (auto it = m2.begin(); it != m2.end(); ++it)
      cout << it->first << " = " << it->second << endl;

   return 0;
}

Output:

Map m1 contains following elements
a = 10
b = 20
c = 30

After copying the elements from m1 to m2... 

Map m2 contains following elements
a = 10
b = 20
c = 30

In the above example, operator=() function is used to copy the content of one map m1 to another map m2.

Example 2

Let's see a simple example to move the elements of one map to another.

#include <iostream>
#include <map>

using namespace std;

int main(void) {
   
   map<char, int> m1 = {
            {'a', 1},
            {'b', 2},
            {'c', 3} };


      cout << "Map m1 contains following elements" << endl;
    for (auto it = m1.begin(); it != m1.end(); ++it)
      cout << it->first << " = " << it->second << endl;
      
    map<char, int> m2 = move(m1); 
    cout<<"\nAfter moving the elements from m1 to m2... \n";  
    
    cout << "\nMap m2 contains following elements" << endl;
    for (auto it = m2.begin(); it != m2.end(); ++it)
      cout << it->first << " = " << it->second << endl;

   return 0;
}

Output:

Map m1 contains following elements
a = 1
b = 2
c = 3

After moving the elements from m1 to m2... 

Map m2 contains following elements
a = 1
b = 2
c = 3

In the above example, operator=() function is used to move the content of one map m1 to another map m2.

Example 3

Let's see a simple example to copy the content from the initializer list to map.

#include <iostream>
#include <map>

using namespace std;

int main(void) {
   map<char, int> m;

   m =  {
      {'a', 100},
      {'b', 200},
      {'c', 300},
      {'d', 400}  };

   cout << "Map contains the following elements" << endl;

   for (auto it = m.begin(); it != m.end(); ++it)
      cout << it->first << " = " << it->second << endl;

   return 0;
}

Output:

Map contains the following elements
a = 100
b = 200
c = 300
d = 400

In the above example, operator=() is used to copy the content from initializer list to map m.

Example 4

Let's see a simple example.

#include <iostream>
#include <map>

using namespace std;

int main ()
{
  map<char,int> first;
  map<char,int> second;

  first['x']=8;
  first['y']=16;
  first['z']=32;
 
  second=first;                // second now contains 3 ints
  first=map<char,int>();       // and first is now empty

  cout << "Size of first: " << first.size() << '\n';
  cout << "Size of second: " << second.size() << '\n';
  return 0;
}

Output: