What is the role of dividers in smart pointers?

Deleters in smart pointers allow defining custom logic for releasing the resource pointed to by the smart pointer. By default, std::unique_ptr uses delete, and std::shared_ptr uses delete or delete[]. A deleter can be a function, a functor, or a lambda expression.

Examples:

std::unique_ptr with a function deleter:

#include <cstdio> // For fclose

void file_deleter(FILE* file) {
    if (file) {
        fclose(file);
    }
}

// ...
// std::unique_ptr<FILE, decltype(&file_deleter)> file_ptr(fopen("example.txt", "r"), &file_deleter);
// ...

std::unique_ptr with a lambda deleter:

#include <iostream>
#include <memory>

// ...
// auto resource_deleter = [](int* p){ std::cout << "Deleting resource\n"; delete p; };
// std::unique_ptr<int, decltype(resource_deleter)> ptr(new int(10), resource_deleter);
// ...

std::shared_ptr with a lambda deleter:

#include <iostream>
#include <memory>

// ...
// std::shared_ptr<int> shared_ptr_with_deleter(new int(20), [](int* p){ std::cout << "Deleting shared resource\n"; delete p; });
// ...

The role of deleters is important in the following scenarios:

• Managing resources other than memory: files, network sockets, database handlers, etc.
• Customizing the memory release process: for example, using delete[] for arrays or object pools.
• Additional logic during release: logging, cleaning up related structures.

For std::unique_ptr, the deleter type is part of the pointer's type, which can complicate polymorphism. For std::shared_ptr, the deleter type is not part of the pointer's type, providing greater flexibility.