C++ List::get_allocator() Function

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The C++ std::list::get_allocator() function is used to retrieve an allocator associated with the list.

In C++, allocators are used in the standard library to handle the allocation and deallocation of elements stored in containers. Allocation is a process of requesting access to a data set. If you allocate a data set that exists, the system allows you to open the data set, and if you allocate a data set that does not exist, the system creates space for it on an available device and allows you to open that space.

Syntax

Following is the syntax of the C++ std::list::get_allocator() −

allocator_type get_allocator() const;

Parameters

• It does not accept any parameter.

Return Value

This function returns an allocator associated with list.

Example 1

In the following program, we are using the C++ std::list::get_allocator() function to retrieve an allocator that is associated with the current list {10, 20, 30, 40, 50}.

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

int main(void) {
   //create a list
   list<int> num_list = {10, 20, 30, 40, 50};
   cout<<"List elements are: ";
   for(int i : num_list){
      cout<<i<<" ";
   }
   int *p = NULL;
   p = num_list.get_allocator().allocate(5);
   for (int i = 0; i<5; ++i)
      p[i] = i + 1;
   cout<<"\nList contains following elements"<<endl;
   for (int i = 0; i<5; ++i)
      cout<<p[i]<<" ";
   return 0;
}

Output

Following is the output of the above program −

List elements are: 10 20 30 40 50 
List contains following elements
1 2 3 4 5

Example 2

Following is another example of the C++ ,std::list::get_allocator() function. Here, we are creating a list(type int) named num_list. Then, using the get_allocator() function, we are trying to retrieve an allocator associated with the current list.

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

int main(void) {
   //create a list
   list<int> num_list;
   //creating array using this list get_allocator
   int *arr;
   arr = num_list.get_allocator().allocate(5);
   arr[0] = 10;
   arr[1] = 20;
   arr[2] = 30;
   arr[3] = 40;
   arr[4] = 50;
   cout<<"List contains the following elements: ";
   for(int i = 0; i<5; i++) {
      cout<<arr[i]<<" ";
   }
}

Output

This will generate the following output −

List contains the following elements: 10 20 30 40 50 

Example 3

In this example, we create a list(type char) named char_list with an empty value. Then, using the get_allocator() function, we are trying to get the allocator associated with this list. We use the allocate() function to specify the number of objects 5 to allocate storage for the pointer to a nearby memory location.

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

int main(void) {
   //create a list
   list<char> char_list = {};
   cout<<"Size of list: "<<char_list.size()<<endl;
   //creating array using this list get_allocator
   char *char_arr;
   char_arr = char_list.get_allocator().allocate(3);
   char_arr[0] = 'a';
   char_arr[1] = 'b';
   char_arr[2] = 'c';
   cout<<"List contains the following elements: ";
   for(int i = 0; i<5; i++) {
      cout<<char_arr[i]<<" ";
   }
}

Output

On executing the above program, it will produce the following output −

Size of list: 0
List contains the following elements: a b c ..