Custom Deleters in Smart Pointers

A custom deleter is a powerful feature of std::unique_ptr that allows you to define a specific way to release a resource. This is particularly useful for resources that require custom cleanup.

Example of Custom Deleter

Let’s create an example where we manage a resource that requires a custom cleanup process:

1#include <iostream>
2#include <memory>
3
4// Custom deleter function
5void CustomDeleter(int* p) {
6  std::cout << "Custom delete called\n";
7  delete p;
8}
9
10void CustomDeleterExample() {
11  std::unique_ptr<int, decltype(&CustomDeleter)>
12    ptr(new int(42), CustomDeleter);
13  std::cout << *ptr;  // Output: 42
14}

Explanation

Custom Deleter Function: CustomDeleter is a function that prints a message and then deletes the pointer. It takes a raw pointer as an argument.

1void CustomDeleter(int* p) {
2  std::cout << "Custom delete called\n";
3  delete p;
4}

Using with std::unique_ptr: The std::unique_ptr is constructed with the raw pointer and the custom deleter. When the unique_ptr goes out of scope, it calls CustomDeleter to release the resource.

1std::unique_ptr<int, decltype(&CustomDeleter)>
2  ptr(new int(42), CustomDeleter);

Output: When the unique_ptr is destroyed, it outputs "Custom delete called" before deallocating the memory.

When to Use Custom Deleters

Custom deleters are useful in several scenarios:

• Non-Standard Resource Management: When dealing with resources that do not use new/delete for allocation and deallocation, such as file handles or network connections.
• Integration with Legacy Code: When interfacing with C libraries that require specific cleanup functions.
• Additional Logging: For debugging purposes, custom deleters can provide additional information when resources are released.

Conclusion

Custom deleters in std::unique_ptr offer flexibility in managing resources by allowing you to define specific cleanup actions. This can be particularly valuable for non-standard resources or when integrating with existing codebases.