get_allocator() in C++

Last Updated : 01 Jun, 2018

In STL, containers can change size dynamically. Allocator is an object that is responsible for dynamic memory allocation/deallocation. get_allocator() is used to allocate memory chunks of memory. It returns a copy of the allocator object associated with the container. It is defined in vector, map, list, set libraries. Syntax:

allocator_type get_allocator() const;

Parameter Used: This member function does not need any parameter to be passed. Return Type: It returns a copy of the allocator object associated with the vector. Errors and Exceptions: Never throws exceptions so we don't need any try-catch surrounding of it. Time-Complexity: Constant O(1).

Below programs illustrate the working of the function 1. std::vector::get_allocator() It returns a copy of the allocator object associated with the vector.

CPP

// C++ program to show working 
// of get_allocator function
#include <iostream>
#include <vector>

using namespace std;

// Function for allocating
char* Allocate(vector<char> arr, int size)
{
  // allocate space for size(s) elements
  return arr.get_allocator().allocate(size); 
}

void Construct(vector<char> arr,
                      char* point, int size)
{
    for (int iter = 0; iter < size; ++iter)
    
      // construct values in-place on the array:
      arr.get_allocator().construct(&point[iter],
                                      iter + 97); 
}

// Function for Deallocating
void deAllocate(vector<char> arr,
                       char* point, int size)
{
    for (int iter = 0; iter < size; ++iter)
        arr.get_allocator().destroy(&point[iter]);
    
    // free allocated memory    
    arr.get_allocator().deallocate(point, size); 
}

// Driver code
int main()
{
    vector<char> array;
    char* pointer;
    int size = 8;
    
    pointer = Allocate(array, size);
    Construct(array, pointer, size);

    cout << "Array elements:  ";
    for (int iter = 0; iter < size; ++iter)
        cout << pointer[iter] << " ";

    deAllocate(array, pointer, size);

    return 0;
}

Output:

Array elements:  a b c d e f g h

2. std::list::get_allocator() It returns a copy of the allocator object associated with the list.

CPP

// C++ program to show working 
// of get_allocator function
#include <iostream>
#include <list>

using namespace std;

// Function for allocating
char* Allocate(list<char> arr, int size)
{
  // allocate space for size(s) elements
  return arr.get_allocator().allocate(size); 
}

void Construct(list<char> arr,
                      char* point, int size)
{
    for (int iter = 0; iter < size; ++iter)
    
      // construct values in-place on the array:
      arr.get_allocator().construct(&point[iter],
                                      iter + 97); 
}

// Function for Deallocating
void deAllocate(list<char> arr,
                       char* point, int size)
{
    for (int iter = 0; iter < size; ++iter)
        arr.get_allocator().destroy(&point[iter]);
    
    // free allocated memory    
    arr.get_allocator().deallocate(point, size); 
}

// Driver code
int main()
{
    list<char> array;
    char* pointer;
    int size = 8;
    
    pointer = Allocate(array, size);
    Construct(array, pointer, size);

    cout << "Array elements:  ";
    for (int iter = 0; iter < size; ++iter)
        cout << pointer[iter] << " ";

    deAllocate(array, pointer, size);

    return 0;
}

Output:

Array elements:  a b c d e f g h

3. std::set::get_allocator() It returns a copy of the allocator object associated with the set.

CPP

// C++ program to show working 
// of get_allocator function
#include <iostream>
#include <set>

using namespace std;

// Function for allocating
char* Allocate(set<char> arr, int size)
{
  // allocate space for size(s) elements
  return arr.get_allocator().allocate(size); 
}

void Construct(set<char> arr,
                      char* point, int size)
{
    for (int iter = 0; iter < size; ++iter)
    
      // construct values in-place on the array:
      arr.get_allocator().construct(&point[iter],
                                      iter + 97); 
}

// Function for Deallocating
void deAllocate(set<char> arr,
                       char* point, int size)
{
    for (int iter = 0; iter < size; ++iter)
        arr.get_allocator().destroy(&point[iter]);
    
    // free allocated memory    
    arr.get_allocator().deallocate(point, size); 
}

// Driver code
int main()
{
    set<char> array;
    char* pointer;
    int size = 8;
    
    pointer = Allocate(array, size);
    Construct(array, pointer, size);

    cout << "Array elements:  ";
    for (int iter = 0; iter < size; ++iter)
        cout << pointer[iter] << " ";

    deAllocate(array, pointer, size);

    return 0;
}

Output:

Array elements:  a b c d e f g h

References: http://www.cplusplus.com/reference/vector/vector/get_allocator/

map get_allocator in C++ STL

Dibyendu Roy Chaudhuri

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