ladybird/AK/Try.h
Timothy Flynn 604d5f5bca AK+Everywhere: Do not implicitly copy variables in TRY macros
For example, consider cases where we want to propagate errors only in
specific instances:

    auto result = read_data(); // something like ErrorOr<ByteBuffer>
    if (result.is_error() && result.error().code() != EINTR)
        continue;
    auto bytes = TRY(result);

The TRY invocation will currently copy the byte buffer when the
expression (in this case, just a local variable) is stored into
_temporary_result.

This patch binds the expression to a reference to prevent such copies.
In less trival invocations (such as TRY(some_function()), this will
incur only temporary lifetime extensions, i.e. no functional change.
2023-02-10 09:08:52 +00:00

46 lines
2.7 KiB
C

/*
* Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
*
* SPDX-License-Identifier: BSD-2-Clause
*/
#pragma once
#include <AK/Diagnostics.h>
#include <AK/StdLibExtras.h>
// NOTE: This macro works with any result type that has the expected APIs.
// It's designed with AK::Result and AK::Error in mind.
//
// It depends on a non-standard C++ extension, specifically
// on statement expressions [1]. This is known to be implemented
// by at least clang and gcc.
// [1] https://gcc.gnu.org/onlinedocs/gcc/Statement-Exprs.html
//
// If the static_assert below is triggered, it means you tried to return a reference
// from a fallible expression. This will not do what you want; the statement expression
// will create a copy regardless, so it is explicitly disallowed.
#define TRY(expression) \
({ \
/* Ignore -Wshadow to allow nesting the macro. */ \
AK_IGNORE_DIAGNOSTIC("-Wshadow", \
auto&& _temporary_result = (expression)); \
static_assert(!::AK::Detail::IsLvalueReference<decltype(_temporary_result.release_value())>, \
"Do not return a reference from a fallible expression"); \
if (_temporary_result.is_error()) [[unlikely]] \
return _temporary_result.release_error(); \
_temporary_result.release_value(); \
})
#define MUST(expression) \
({ \
/* Ignore -Wshadow to allow nesting the macro. */ \
AK_IGNORE_DIAGNOSTIC("-Wshadow", \
auto&& _temporary_result = (expression)); \
static_assert(!::AK::Detail::IsLvalueReference<decltype(_temporary_result.release_value())>, \
"Do not return a reference from a fallible expression"); \
VERIFY(!_temporary_result.is_error()); \
_temporary_result.release_value(); \
})