ladybird/Documentation/Patterns.md

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::m_memory_manager_list_node>;
    using ListInVMObject = IntrusiveList<&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>());

String View Literals

AK::StringView support for operator"" sv which is a special string literal operator that was added as of C++17 to enable std::string_view literals.

[[nodiscard]] ALWAYS_INLINE constexpr AK::StringView operator"" sv(const char* cstring, size_t length)
{
    return AK::StringView(cstring, length);
}

This allows AK::StringView to be constructed from string literals with no runtime cost to find the string length, and the data the AK::StringView points to will reside in the data section of the binary.

Example Usage:

#include <AK/String.h>
#include <AK/StringView.h>
#include <LibTest/TestCase.h>

TEST_CASE(string_view_literal_operator)
{
    StringView literal_view = "foo"sv;
    String test_string = "foo";

    EXPECT_EQ(literal_view.length(), test_string.length());
    EXPECT_EQ(literal_view, test_string);
}

Source Location

C++20 added std::source_location, which lets you capture the callers FILE / LINE / FUNCTION etc as a default argument to functions. See: https://en.cppreference.com/w/cpp/utility/source_location

AK::SourceLocation is the implementation of this feature in SerenityOS. It's become the idiomatic way to capture the location when adding extra debugging instrumentation, without resorting to litering the code with preprocessor macros.

To use it, you can add the AK::SourceLocation as a default argument to any function, using AK::SourceLocatin::current() to initialize the default argument.

Example Usage:

#include <AK/SourceLocation.h>
#include <AK/StringView.h>

static StringView example_fn(const SourceLocation& loc = SourceLocation::current())
{
    return loc.function_name();
}

int main(int, char**)
{
    return example_fn().length();
}

If you only want to only capture AK::SourceLocation data with a certain debug macro enabled, avoid adding #ifdef's to all functions which have the AK::SourceLocation argument. Since SourceLocation is just a simple struct, you can just declare an empty class which can be optimized away by the compiler, and alias both to the same name.

Example Usage:

#if LOCK_DEBUG
#    include <AK/SourceLocation.h>
#endif

#if LOCK_DEBUG
using LockLocation = SourceLocation;
#else
struct LockLocation {
    static constexpr LockLocation current() { return {}; }

private:
    constexpr LockLocation() = default;
};
#endif