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More gdb::optional features

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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.
This commit is contained in:
Pedro Alves 2017-04-18 21:39:24 +01:00
parent a7fc9b6193
commit 22796e972f
3 changed files with 172 additions and 21 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

23
gdb/ChangeLog
View File

@ -1,3 +1,26 @@
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.
2017-04-18 Pedro Alves <palves@redhat.com>
* xml-support.c: Include <vector>.

144
gdb/common/gdb_optional.h
View File

@ -20,46 +20,141 @@
#ifndef GDB_OPTIONAL_H
#define GDB_OPTIONAL_H
#include "common/traits.h"
namespace gdb
{
struct in_place_t
{
explicit in_place_t () = default;
};
constexpr gdb::in_place_t in_place {};
/* This class attempts to be a compatible subset of std::optional,
which is slated to be available in C++17. This class optionally
holds an object of some type -- by default it is constructed not
holding an object, but later the object can be "emplaced". This is
similar to using std::unique_ptr, but in-object allocation is
guaranteed. */
guaranteed.
Unlike std::optional, we currently only support copy/move
construction/assignment of an optional<T> from either exactly
optional<T> or T. I.e., we don't support copy/move
construction/assignment from optional<U> or U, when U is a type
convertible to T. Making that work depending on the definitions of
T and U is somewhat complicated, and currently the users of this
class don't need it. */
template<typename T>
class optional
{
public:
constexpr optional ()
: m_dummy (),
m_instantiated (false)
{
}
~optional ()
{
if (m_instantiated)
destroy ();
}
/* These aren't deleted in std::optional, but it was simpler to
delete them here, because currently the users of this class don't
need them, and making them depend on the definition of T is
somewhat complicated. */
optional (const optional &other) = delete;
optional<T> &operator= (const optional &other) = delete;
: m_dummy ()
{}
template<typename... Args>
void emplace (Args &&... args)
constexpr optional (in_place_t, Args &&... args)
: m_item (std::forward<Args> (args)...),
m_instantiated (true)
{}
~optional ()
{ this->reset (); }
/* Copy and move constructors. */
optional (const optional &other)
{
if (other.m_instantiated)
this->emplace (other.get ());
}
optional (optional &&other)
noexcept(std::is_nothrow_move_constructible<T> ())
{
if (other.m_instantiated)
this->emplace (std::move (other.get ()));
}
constexpr optional (const T &other)
: m_item (other),
m_instantiated (true)
{}
constexpr optional (T &&other)
noexcept (std::is_nothrow_move_constructible<T> ())
: m_item (std::move (other)),
m_instantiated (true)
{}
/* Assignment operators. */
optional &
operator= (const optional &other)
{
if (m_instantiated && other.m_instantiated)
this->get () = other.get ();
else
{
if (other.m_instantiated)
this->emplace (other.get ());
else
this->reset ();
}
return *this;
}
optional &
operator= (optional &&other)
noexcept (And<std::is_nothrow_move_constructible<T>,
std::is_nothrow_move_assignable<T>> ())
{
if (m_instantiated && other.m_instantiated)
this->get () = std::move (other.get ());
else
{
if (other.m_instantiated)
this->emplace (std::move (other.get ()));
else
this->reset ();
}
return *this;
}
optional &
operator= (const T &other)
{
if (m_instantiated)
destroy ();
this->get () = other;
else
this->emplace (other);
return *this;
}
optional &
operator= (T &&other)
noexcept (And<std::is_nothrow_move_constructible<T>,
std::is_nothrow_move_assignable<T>> ())
{
if (m_instantiated)
this->get () = std::move (other);
else
this->emplace (std::move (other));
return *this;
}
template<typename... Args>
T &emplace (Args &&... args)
{
this->reset ();
new (&m_item) T (std::forward<Args>(args)...);
m_instantiated = true;
return this->get ();
}
/* Observers. */
@ -87,6 +182,13 @@ public:
constexpr bool has_value () const noexcept
{ return m_instantiated; }
/* 'reset' is a 'safe' operation with no precondition. */
void reset () noexcept
{
if (m_instantiated)
this->destroy ();
}
private:
/* Destroy the object. */
@ -109,7 +211,7 @@ private:
};
/* True if the object was ever emplaced. */
bool m_instantiated;
bool m_instantiated = false;
};
}

26
gdb/common/traits.h
View File

@ -18,6 +18,8 @@
#ifndef COMMON_TRAITS_H
#define COMMON_TRAITS_H
#include <type_traits>
namespace gdb {
/* Pre C++14-safe (CWG 1558) version of C++17's std::void_t. See
@ -29,6 +31,30 @@ struct make_void { typedef void type; };
template<typename... Ts>
using void_t = typename make_void<Ts...>::type;
/* A few trait helpers, mainly stolen from libstdc++. Uppercase
because "and" is a keyword. */
template<typename...>
struct And;
template<>
struct And<> : public std::true_type
{};
template<typename B1>
struct And<B1> : public B1
{};
template<typename B1, typename B2>
struct And<B1, B2>
: public std::conditional<B1::value, B2, B1>::type
{};
template<typename B1, typename B2, typename B3, typename... Bn>
struct And<B1, B2, B3, Bn...>
: public std::conditional<B1::value, And<B2, B3, Bn...>, B1>::type
{};
}
#endif /* COMMON_TRAITS_H */