Commit 54b65c9b authored by Pedro Alves's avatar Pedro Alves
Browse files

Introduce scope_exit

This add a new template class scope_exit.  scope_exit is a
general-purpose scope guard that calls its exit function at the end of
the current scope.  A scope_exit may be canceled by calling the
"release" method.  The API is modeled on P0052R5 - Generic Scope Guard
and RAII Wrapper for the Standard Library, which is itself based on
Andrej Alexandrescu's ScopeGuard/SCOPE_EXIT.

The main advantage of scope_exit is avoiding writing single-use RAII
classes and its boilerplate.  Following patches will remove a few of
such classes.

There are two forms available:

 - The "make_scope_exit" form allows canceling the scope guard.  Use
   it like this:

     auto cleanup = make_scope_exit ( <function, function object, lambda> );
     ...
     cleanup.release (); // cancel

 - If you don't need to cancel the guard, you can use the SCOPE_EXIT
   macro, like this:

     SCOPE_EXIT { /* any code you like here. */ }

Note: scope_exit instances do not allocate anything on the heap.

gdb/ChangeLog:
2019-01-23  Pedro Alves  <palves@redhat.com>
	    Andrew Burgess  <andrew.burgess@embecosm.com>
	    Tom Tromey  <tom@tromey.com>

	* common/scope-exit.h: New file.
parent cf08fb29
2019-01-23 Pedro Alves <palves@redhat.com>
Andrew Burgess <andrew.burgess@embecosm.com>
Tom Tromey <tom@tromey.com>
* common/scope-exit.h: New file.
2019-01-23 Pedro Alves <palves@redhat.com>
* common/preprocessor.h (ESC): Rename to ...
......
/* Copyright (C) 2019 Free Software Foundation, Inc.
This file is part of GDB.
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 3 of the License, or
(at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program. If not, see <http://www.gnu.org/licenses/>. */
#ifndef COMMON_SCOPE_EXIT_H
#define COMMON_SCOPE_EXIT_H
#include <functional>
#include <type_traits>
#include "common/preprocessor.h"
/* scope_exit is a general-purpose scope guard that calls its exit
function at the end of the current scope. A scope_exit may be
canceled by calling the "release" method. The API is modeled on
P0052R5 - Generic Scope Guard and RAII Wrapper for the Standard
Library, which is itself based on Andrej Alexandrescu's
ScopeGuard/SCOPE_EXIT.
There are two forms available:
- The "make_scope_exit" form allows canceling the scope guard. Use
it like this:
auto cleanup = make_scope_exit ( <function, function object, lambda> );
...
cleanup.release (); // cancel
- If you don't need to cancel the guard, you can use the SCOPE_EXIT
macro, like this:
SCOPE_EXIT
{
// any code you like here.
}
See also forward_scope_exit.
*/
/* CRTP base class for cancelable scope_exit-like classes. Implements
the common call-custom-function-from-dtor functionality. Classes
that inherit this implement the on_exit() method, which is called
from scope_exit_base's dtor. */
template <typename CRTP>
class scope_exit_base
{
public:
scope_exit_base () = default;
~scope_exit_base ()
{
if (!m_released)
{
auto *self = static_cast<CRTP *> (this);
self->on_exit ();
}
}
/* This is needed for make_scope_exit because copy elision isn't
guaranteed until C++17. An optimizing compiler will usually skip
calling this, but it must exist. */
scope_exit_base (const scope_exit_base &other)
: m_released (other.m_released)
{
other.m_released = true;
}
void operator= (const scope_exit_base &) = delete;
/* If this is called, then the wrapped function will not be called
on destruction. */
void release () noexcept
{
m_released = true;
}
private:
/* True if released. Mutable because of the copy ctor hack
above. */
mutable bool m_released = false;
};
/* The scope_exit class. */
template<typename EF>
class scope_exit : public scope_exit_base<scope_exit<EF>>
{
/* For access to on_exit(). */
friend scope_exit_base<scope_exit<EF>>;
public:
template<typename EFP,
typename = gdb::Requires<std::is_constructible<EF, EFP>>>
scope_exit (EFP &&f)
try : m_exit_function ((!std::is_lvalue_reference<EFP>::value
&& std::is_nothrow_constructible<EF, EFP>::value)
? std::move (f)
: f)
{
}
catch (...)
{
/* "If the initialization of exit_function throws an exception,
calls f()." */
f ();
}
template<typename EFP,
typename = gdb::Requires<std::is_constructible<EF, EFP>>>
scope_exit (scope_exit &&rhs)
noexcept (std::is_nothrow_move_constructible<EF>::value
|| std::is_nothrow_copy_constructible<EF>::value)
: m_exit_function (std::is_nothrow_constructible<EFP>::value
? std::move (rhs)
: rhs)
{
rhs.release ();
}
/* This is needed for make_scope_exit because copy elision isn't
guaranteed until C++17. An optimizing compiler will usually skip
calling this, but it must exist. */
scope_exit (const scope_exit &other)
: scope_exit_base<scope_exit<EF>> (other),
m_exit_function (other.m_exit_function)
{
}
void operator= (const scope_exit &) = delete;
void operator= (scope_exit &&) = delete;
private:
void on_exit ()
{
m_exit_function ();
}
/* The function to call on scope exit. */
EF m_exit_function;
};
template <typename EF>
scope_exit<typename std::decay<EF>::type>
make_scope_exit (EF &&f)
{
return scope_exit<typename std::decay<EF>::type> (std::forward<EF> (f));
}
namespace detail
{
enum class scope_exit_lhs {};
template<typename EF>
scope_exit<typename std::decay<EF>::type>
operator+ (scope_exit_lhs, EF &&rhs)
{
return scope_exit<typename std::decay<EF>::type> (std::forward<EF> (rhs));
}
}
/* Register a block of code to run on scope exit. Note that the local
context is captured by reference, which means you should be careful
to avoid inadvertently changing a captured local's value before the
scope exit runs. */
#define SCOPE_EXIT \
auto CONCAT(scope_exit_, __LINE__) = ::detail::scope_exit_lhs () + [&] ()
#endif /* COMMON_SCOPE_EXIT_H */
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