Hash Sets using std::unordered_set

Using a Custom Allocator with std::unordered_set

How can I use a custom allocator with std::unordered_set?

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std::unordered_set allows you to specify a custom allocator type as the fourth template parameter. This can be useful for specialized memory management or for integration with custom memory allocation libraries.

Here's an example of using a custom allocator with std::unordered_set:

#include <iostream>
#include <unordered_set>

template <typename T>
struct CustomAllocator {
  using value_type = T;

  CustomAllocator() = default;

  template <typename U>
  CustomAllocator(const CustomAllocator<U>&) {}

  T* allocate(std::size_t n) {
    std::cout << "Allocating " << n
      << " elements\n";
    return static_cast<T*>(
      ::operator new(n * sizeof(T)));

  void deallocate(T* p, std::size_t n) {
    std::cout << "Deallocating " << n 
      << " elements\n";
    ::operator delete(p); 

int main() {
    int, std::hash<int>,
      Set{1, 2, 3};
Allocating 16 elements
Allocating 1 elements
Allocating 1 elements
Allocating 1 elements
Deallocating 16 elements
Deallocating 1 elements
Deallocating 1 elements
Deallocating 1 elements

In this example:

  • We define a¬†CustomAllocator¬†struct that implements the necessary functions for an allocator:¬†allocate()¬†and¬†deallocate()
  • Inside¬†allocate(), we print a message and then allocate memory using the global¬†::operator new
  • Inside¬†deallocate(), we print a message and then free the memory using¬†::operator delete
  • We specify¬†CustomAllocator<int>¬†as the fourth template argument when creating the¬†std::unordered_set

When we insert or erase elements from the set, our custom allocator's allocate() and deallocate() functions are called, as evident from the output.

Using a custom allocator allows you to have fine-grained control over how memory is allocated and freed for the elements stored in the std::unordered_set.

Answers to questions are automatically generated and may not have been reviewed.

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