22796e972f
Currently we can't use gdb::optional<T> as function return type,
because gdb::optional's copy ctor is deleted. For example, with:
gdb::optional<int> function ()
{
gdb::optional<int> opt;
....
return opt;
we get:
src/gdb/foo.c: In function ‘gdb::optional<int> foo()’:
src/gdb/foo.c:75:10: error: use of deleted function ‘gdb::optional<T>::optional(const gdb::optional<T>&) [with T = int]’
return opt;
^
In file included from src/gdb/foo.c:68:0:
src/gdb/common/gdb_optional.h:53:3: note: declared here
optional (const optional &other) = delete;
^
I started by fixing that, and then ran into another missing feature,
also fixed by this patch.
The next feature I'm missing most from gdb::optional<T> compared to
std::optional<T> is construction/move/assignment from a T, instead of
having to default construct an gdb::optional and then use
optional::emplace(....).
For example:
gdb::optional<std::string> function ()
{
gdb::optional<std::string> opt;
std::string str;
...
opt.emplace (std::move (str));
return opt;
vs
gdb::optional<std::string> function ()
{
std::string str;
...
return str;
The copy/move ctor/assign methods weren't initialy implemented because
std::optional supports construction from a type U if U is convertible
to T too, and has rules to decide whether the ctors are
explicit/implicit based on that, and rules for whether the ctor should
be trivial or not, etc., which leads to a much more complicated
implementation.
If we stick to supporting copy/move construction/assignment of/to an
optional<T> from exactly only optional<T> and T, then all that
conversion-related complication disappears, and we still gain
convenience in most use cases.
The patch also makes emplace return a reference to the constructor
object, per C++17 std::optional, and adds a reset method, againt
because std::optional has one and it's trivial to support it. These
two changes are a requirement of the gdb::optional unit testing patch
that will follow.
gdb/ChangeLog:
2017-04-18 Pedro Alves <palves@redhat.com>
* common/gdb_optional.h: Include common/traits.h.
(in_place_t): New type.
(in_place): New constexpr variable.
(optional::optional): Remove member initialization of
m_instantiated.
(optional::optional(in_place_t...)): New constructor.
(optional::~optional): Use reset.
(optional::optional(const optional&)): New.
(optional::optional(const optional&&)): New.
(optional::optional(T &)): New.
(optional::optional(T &&)): New.
(operator::operator=(const optional &)): New.
(operator::operator=(optional &&)): New.
(operator::operator= (const T &))
(operator::operator= (T &&))
(operator::emplace (Args &&... args)): Return a T&. Use reset.
(operator::reset): New.
(operator::m_instantiated):: Add in-class initializer.
* common/traits.h: Include <type_traits>.
(struct And): New types.
60 lines
1.6 KiB
C++
60 lines
1.6 KiB
C++
/* Copyright (C) 2017 Free Software Foundation, Inc.
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This file is part of GDB.
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This program is free software; you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation; either version 3 of the License, or
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(at your option) any later version.
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This program is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with this program. If not, see <http://www.gnu.org/licenses/>. */
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#ifndef COMMON_TRAITS_H
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#define COMMON_TRAITS_H
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#include <type_traits>
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namespace gdb {
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/* Pre C++14-safe (CWG 1558) version of C++17's std::void_t. See
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<http://en.cppreference.com/w/cpp/types/void_t>. */
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template<typename... Ts>
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struct make_void { typedef void type; };
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template<typename... Ts>
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using void_t = typename make_void<Ts...>::type;
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/* A few trait helpers, mainly stolen from libstdc++. Uppercase
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because "and" is a keyword. */
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template<typename...>
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struct And;
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template<>
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struct And<> : public std::true_type
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{};
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template<typename B1>
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struct And<B1> : public B1
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{};
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template<typename B1, typename B2>
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struct And<B1, B2>
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: public std::conditional<B1::value, B2, B1>::type
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{};
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template<typename B1, typename B2, typename B3, typename... Bn>
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struct And<B1, B2, B3, Bn...>
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: public std::conditional<B1::value, And<B2, B3, Bn...>, B1>::type
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{};
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}
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#endif /* COMMON_TRAITS_H */
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