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AK: Add FixedMemoryStream methods for reading values "in place"

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When working with FixedMemoryStreams, and especially MappedFiles, you
may don't want to copy the underlying data when you read from the
stream. Pointing into that data is perfectly fine as long as you know
the lifetime of it is long enough.

This commit adds a couple of methods for reading either a single value,
or a span of them, in this way. As noted, for single values you sadly
get a raw pointer instead of a reference, but that's the only option
right now.
This commit is contained in:
Sam Atkins
2023-10-06 17:03:54 +01:00
committed by Tim Schumacher
parent 8e51c7112c
commit 253a96277e
Notes: sideshowbarker 2024-07-17 05:23:40 +09:00 
Author: https://github.com/AtkinsSJ
Commit: https://github.com/SerenityOS/serenity/commit/253a96277e
Pull-request: https://github.com/SerenityOS/serenity/pull/21298
Reviewed-by: https://github.com/timschumi ✅
2 changed files with 62 additions and 0 deletions
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35
AK/MemoryStream.h
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@@ -1,11 +1,13 @@
/*
* Copyright (c) 2021, kleines Filmröllchen <filmroellchen@serenityos.org>.
* Copyright (c) 2023, Sam Atkins <atkinssj@serenityos.org>.
*
* SPDX-License-Identifier: BSD-2-Clause
*/
#pragma once
#include <AK/Error.h>
#include <AK/OwnPtr.h>
#include <AK/Stream.h>
#include <AK/Vector.h>
@@ -34,6 +36,39 @@ public:
size_t offset() const;
size_t remaining() const;
/// Read a value, but referring to the stream's underlying data instead of copying it.
/// Of course, only use this if you know the lifetime of the data will exceed the value's.
// FIXME: Would be nicer to be able to return T& but Variant (and thus ErrorOr) can't hold references.
template<typename T>
requires(Traits<T>::is_trivially_serializable())
ErrorOr<T*> read_in_place()
{
if constexpr (!IsConst<T>) {
if (!m_writing_enabled)
return Error::from_string_view_or_print_error_and_return_errno("Tried to obtain a non-const reference from a read-only FixedMemoryStream"sv, EINVAL);
}
T* value = reinterpret_cast<T*>(m_bytes.offset_pointer(m_offset));
TRY(discard(sizeof(T)));
return value;
}
/// Read a span of values, referring to the stream's underlying data instead of copying it.
/// Of course, only use this if you know the lifetime of the data will exceed the span's.
template<typename T>
requires(Traits<T>::is_trivially_serializable())
ErrorOr<Span<T>> read_in_place(size_t count)
{
if constexpr (!IsConst<T>) {
if (!m_writing_enabled)
return Error::from_string_view_or_print_error_and_return_errno("Tried to obtain a non-const span from a read-only FixedMemoryStream"sv, EINVAL);
}
Span<T> span { m_bytes.offset_pointer(m_offset), count };
TRY(discard(sizeof(T) * count));
return span;
}
private:
Bytes m_bytes;
size_t m_offset { 0 };