std::shared_ptr is a popular smart pointer class in the C++ standard library that provides automatic memory management. By default, std::shared_ptr uses the default allocator for memory allocation and deallocation. However, sometimes you may want to use a custom allocator to manage the memory for std::shared_ptr.

In this blog post, we will explore how to use std::shared_ptr with a custom allocator in C++.

Custom Allocator for std::shared_ptr

To use a custom allocator with std::shared_ptr, you need to create a class that satisfies the allocator requirements defined by the C++ standard.

Here’s an example of a custom allocator class:

#include <memory>

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

    MyAllocator() noexcept = default;
    template<typename U>
    MyAllocator(const MyAllocator<U>&) noexcept {}

    T* allocate(std::size_t n) {
        // Your custom allocation logic here
        return static_cast<T*>(::operator new(sizeof(T) * n));
    }

    void deallocate(T* p, std::size_t n) noexcept {
        // Your custom deallocation logic here
        ::operator delete(p);
    }
};

In this example, we define a custom allocator called MyAllocator that satisfies the requirements of the allocator concept. The allocate function is responsible for allocating memory, and the deallocate function is used for deallocating memory.

Using std::shared_ptr with Custom Allocator

To use std::shared_ptr with a custom allocator, you need to specify the allocator type as a template parameter when creating the smart pointer. Here’s an example:

int main() {
    using CustomSharedPtr = std::shared_ptr<int, MyAllocator<int>>;
    CustomSharedPtr ptr = std::allocate_shared<int, MyAllocator<int>>(MyAllocator<int>(), 42);

    // Use the custom shared pointer
    // ...

    return 0;
}

In this example, we create a typedef CustomSharedPtr for std::shared_ptr that uses our custom allocator MyAllocator<int>. We then use std::allocate_shared to create a shared pointer with the provided allocator. The allocator is passed as a constructor argument along with the desired value for the shared pointer.

Conclusion

Using std::shared_ptr with a custom allocator can be useful in scenarios where you need more control over memory allocation and deallocation. By implementing a custom allocator that satisfies the requirements of the allocator concept, you can use it with std::shared_ptr to manage memory using your own allocation logic.

#C++ #stdsharedptr #customallocator