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Patterns.md

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SerenityOS patterns

Introduction

Over time numerous reoccurring patterns have emerged from or were adopted by the serenity code base. This document aims to track and describe them so they can be propagated further and keep the code base consistent.

Intrusive Lists

Intrusive lists are common in the Kernel and in some specific cases are used in the SerenityOS userland. A data structure is said to be "intrusive" when each element holds the metadata that tracks the element's membership in the data structure. In the case of a list, this means that every element in an intrusive linked list has a node embedded inside of it. The main advantage of intrusive data structures is you don't need to worry about handling out of memory (OOM) on insertion into the data structure. This means error handling code is much simpler than say, using a Vector in environments that need to be durable to OOM.

The common pattern for declaring an intrusive list is to add the storage for the intrusive list node as a private member. A public type alias is then used to expose the list type to anyone who might need to create it. Here is an example from the Region class in the Kernel:

class Region final
    : public Weakable<Region> {

public:

... snip ...

private:
    bool m_syscall_region : 1 { false };

    IntrusiveListNode<Region> m_memory_manager_list_node;
    IntrusiveListNode<Region> m_vmobject_list_node;

public:
    using ListInMemoryManager = IntrusiveList<Region, RawPtr<Region>, &Region::m_memory_manager_list_node>;
    using ListInVMObject = IntrusiveList<Region, RawPtr<Region>, &Region::m_vmobject_list_node>;
};

You can then use the list by referencing the public type alias like so:

class MemoryManager {

... snip ...

    Region::ListInMemoryManager m_kernel_regions;
    Vector<UsedMemoryRange> m_used_memory_ranges;
    Vector<PhysicalMemoryRange> m_physical_memory_ranges;
    Vector<ContiguousReservedMemoryRange> m_reserved_memory_ranges;
};

Static Assertions of the size of a type

It's a universal pattern to use static_assert to validate the size of a type matches the author's expectations. Unfortunately when these assertions fail they don't give you the values that actually caused the failure. This forces one to go investigate by printing out the size, or checking it in a debugger, etc.

For this reason AK::AssertSize was added. It exploits the fact that the compiler will emit template argument values for compiler errors to provide debugging information. Instead of getting no information you'll get the actual type sizes in your compiler error output.

Example Usage:

#include <AK/StdLibExtras.h>

struct Empty { };

static_assert(AssertSize<Empty, 1>());