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Doc: write up the docs for Q_GLOBAL_STATIC 

Change-Id: I5bf4d0d027dc8f960c94b4be3ebf7381e9ef4be1
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
bb10
Thiago Macieira 2013-01-22 18:26:25 -08:00 committed by The Qt Project
parent 314e5ce5ee
commit c0860d26a1
3 changed files with 523 additions and 13 deletions
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1
src/corelib/global/global.pri
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@ -20,6 +20,7 @@ HEADERS += \
SOURCES += \
global/qglobal.cpp \
global/qglobalstatic.cpp \
global/qlibraryinfo.cpp \
global/qmalloc.cpp \
global/qnumeric.cpp \

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src/corelib/global/qglobalstatic.cpp Normal file
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@ -0,0 +1,522 @@
/****************************************************************************
**
** Copyright (C) 2013 Intel Corporation.
** Contact: http://www.qt-project.org/legal
**
** This file is part of the QtCore module of the Qt Toolkit.
**
** $QT_BEGIN_LICENSE:LGPL$
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** use the contact form at http://qt.digia.com/contact-us.
**
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** Alternatively, this file may be used under the terms of the GNU Lesser
** General Public License version 2.1 as published by the Free Software
** Foundation and appearing in the file LICENSE.LGPL included in the
** packaging of this file. Please review the following information to
** ensure the GNU Lesser General Public License version 2.1 requirements
** will be met: http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html.
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** In addition, as a special exception, Digia gives you certain additional
** rights. These rights are described in the Digia Qt LGPL Exception
** version 1.1, included in the file LGPL_EXCEPTION.txt in this package.
**
** GNU General Public License Usage
** Alternatively, this file may be used under the terms of the GNU
** General Public License version 3.0 as published by the Free Software
** Foundation and appearing in the file LICENSE.GPL included in the
** packaging of this file. Please review the following information to
** ensure the GNU General Public License version 3.0 requirements will be
** met: http://www.gnu.org/copyleft/gpl.html.
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** $QT_END_LICENSE$
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****************************************************************************/
#include "qglobalstatic.h"
/*!
\macro Q_GLOBAL_STATIC(Type, VariableName)
\since 5.1
\relates QGlobalStatic
Creates a global and static object of type \l QGlobalStatic, of name \a
VariableName and that behaves as a pointer to \a Type. The object created
by Q_GLOBAL_STATIC initializes itself on the first use, which means that it
will not increase the application or the library's load time. Additionally,
the object is initialized in a thread-safe manner on all platforms.
The typical use of this macro is as follows, in a global context (that is,
outside of any function bodies):
\code
Q_GLOBAL_STATIC(MyType, staticType)
\endcode
This macro is intended to replace global static objects that are not POD
(Plain Old Data, or in C++11 terms, not made of a trivial type), hence the
name. For example, the following C++ code creates a global static:
\code
static MyType staticType;
\endcode
Compared to Q_GLOBAL_STATIC, and assuming that \c MyType is a class or
struct that has a constructor, a destructor, or is otherwise non-POD, the
above has the following drawbacks:
\list
\li it requires load-time initialization of \c MyType (that is, the
default constructor for \c MyType is called when the library or
application is loaded);
\li the type will be initialized even if it is never used;
\li the order of initialization and destruction among different
translation units is not determined, leading to possible uses before
initialization or after destruction;
\li if it is found inside a function (that is, not global), it will be
initialized on first use, but many current compilers (as of 2013) do
not guarantee that the initialization will be thread-safe;
\endlist
The Q_GLOBAL_STATIC macro solves all of the above problems by guaranteeing
thread-safe initialization on first use and allowing the user to query for
whether the type has already been destroyed, to avoid the
use-after-destruction problem (see QGlobalStatic::isDestroyed()).
\section1 Constructor and destructor
For Q_GLOBAL_STATIC, the type \c Type must be publicly
default-constructible and publicly destructible. For
Q_GLOBAL_STATIC_WITH_ARGS(), there must be a public constructor that
matches the arguments passed.
It is not possible to use Q_GLOBAL_STATIC with types that have protected or
private default constructors or destructors (for Q_GLOBAL_STATIC_WITH_ARGS(),
a protected or private constructor matching the arguments). If the type in
question has those members as protected, it is possible to overcome the
issue by deriving from the type and creating public a constructor and
destructor. If the type has them as private, a friend declaration is
necessary before deriving.
For example, the following is enough to create \c MyType based on a
previously-defined \c MyOtherType which has a protected default constructor
and/or a protected destructor (or has them as private, but that defines \c
MyType as a friend).
\code
class MyType : public MyOtherType { };
Q_GLOBAL_STATIC(MyType, staticType)
\endcode
No body for \c MyType is required since the destructor is an implicit
member and so is the default constructor if no other constructors are
defined. For use with Q_GLOBAL_STATIC_WITH_ARGS(), however, a suitable
constructor body is necessary:
\code
class MyType : public MyOtherType
{
public:
MyType(int i) : MyOtherType(i) {}
};
Q_GLOBAL_STATIC_WITH_ARGS(MyType, staticType, (42))
\endcode
Alternatively, if the compiler supports C++11 inheriting constructors, one could write:
\code
class MyType : public MyOtherType
{
public:
using MyOtherType::MyOtherType;
};
Q_GLOBAL_STATIC_WITH_ARGS(MyType, staticType, (42))
\endcode
\section1 Placement
The Q_GLOBAL_STATIC macro creates a type that is necessarily static, at the
global scope. It is not possible to place the Q_GLOBAL_STATIC macro inside
a function (doing so will result in compilation errors).
More importantly, this macro should be placed in source files, never in
headers. Since the resulting object is has static linkage, if the macro is
placed in a header and included by multiple source files, the object will
be defined multiple times and will not cause linking errors. Instead, each
translation unit will refer to a different object, which could lead to
subtle and hard-to-track errors.
\section1 Non-recommended uses
Note that the macro is not recommended for use with types that are POD or
that have C++11 constexpr constructors (trivially constructible and
destructible). For those types, it is still recommended to use regular
static, whether global or function-local.
This macro will work, but it will add unnecessary overhead.
\section1 Reentrancy, thread-safety, deadlocks, and exception-safety on construction
The Q_GLOBAL_STATIC macro creates an object that initializes itself on
first use in a thread-safe manner: if multiple threads attempt to
initialize the object at the same time, only one thread will proceed to
initialize, while all other threads wait for completion.
If the initialization process throws an exception, the initialization is
deemed not complete and will be attempted again when control reaches any
use of the object. If there are any threads waiting for initialization, one
of them will be woken up to attempt to initialize.
The macro makes no guarantee about reentrancy from the same thread. If the
global static object is accessed directly or indirectly from inside the
constructor, a deadlock will surely happen.
In addition, if two Q_GLOBAL_STATIC objects are being initialized on two
different threads and each one's initialization sequence accesses the
other, a deadlock might happen. For that reason, it is recommended to keep
global static constructors simple or, failing that, to ensure that there's
no cross-dependency of uses of global static during construction.
\section1 Destruction
If the object is never used during the lifetime of the program, aside from
the QGlobalStatic::exists() and QGlobalStatic::isDestroyed() functions, the
contents of type \a Type will not be created and there will not be any
exit-time operation.
If the object is created, it will be destroyed at exit-time, similar to the
C \c atexit function. On most systems, in fact, the destructor will also be
called if the library or plugin is unloaded from memory before exit.
Since the destruction is meant to happen at program exit, no thread-safety
is provided. This includes the case of plugin or library unload. In
addition, since destructors are not supposed to throw exceptions, no
exception safety is provided either.
However, reentrancy is permitted: during destruction, it is possible to
access the global static object and the pointer returned will be the same
as it was before destruction began. After the destruction has completed,
accessing the global static object is not permitted, except as noted in the
\l QGlobalStatic API.
\omit
\section1 Compatibility with Qt 4 and Qt 5.0
This macro, in its current form and behavior, was introduced in Qt 5.1.
Prior to that version, Qt had another macro with the same name that was
private API. This section is not meant to document how to use
Q_GLOBAL_STATIC in those versions, but instead to serve as a porting guide
for Qt code that used those macros.
The Qt 4 Q_GLOBAL_STATIC macro differed in behavior in the following ways:
\list
\li the object created was not of type \l QGlobalStatic, but instead
it was a function that returned a pointer to \a Type; that means the
\l QGlobalStatic API was not present;
\li the initialization was thread-safe, but not guaranteed to be
unique: instead, if N threads tried to initialize the object at the
same time, N objects would be created on the heap and N-1 would be
destroyed;
\li the object was always created on the heap.
\endlist
\section1 Implementation details
Q_GLOBAL_STATIC is implemented by creating a QBasicAtomicInt called the \c
guard and a free, inline function called \c innerFunction. The guard
variable is initialized to value 0 (chosen so that the guard can be placed
in the .bss section of the binary file), which denotes that construction
has not yet taken place (uninitialized). The inner function is implemented
by the helper macro Q_GLOBAL_STATIC_INTERNAL.
Both the guard variable and the inner function are passed as template
parameters to QGlobalStatic, along with the type \a Type. Both should also
have static linkage or be placed inside an anonymous namespace, so that the
visibility of Q_GLOBAL_STATIC is that of a global static. To permit
multiple Q_GLOBAL_STATIC per translation unit, the guard variable and the
inner function must have unique names, which can be accomplished by
concatenating with \a VariableName or by placing them in a namespace that
has \a VariableName as part of the name. To simplify and improve
readability on Q_GLOBAL_STATIC_INTERNAL, we chose the namespace solution.
It's also required for C++98 builds, since static symbols cannot be used as
template parameters.
The guard variable can assume the following values:
\list
\li -2: object was once initialized but has been destroyed already;
\li -1: object was initialized and is still valid;
\li 0: object was not initialized yet;
\li +1: object is being initialized and any threads encountering this
value must wait for completion (not used in the current implementation).
\endlist
Collectively, all positive values indicate that the initialization is
progressing and must be waited on, whereas all negative values indicate
that the initialization has terminated and must not be attempted again.
Positive values are not used in the current implementation, but were in
earlier versions. They could be used again in the future.
The QGlobalStatic::exists() and QGlobalStatic::isDestroyed() functions
operate solely on the guard variable: the former returns true if the guard
is negative, whereas the latter returns true only if it is -2.
The Q_GLOBAL_STATIC_INTERNAL macro implements the actual construction and
destruction. There are two implementations of it: one for compilers that
support thread-safe initialization of function-local statics and one for
compilers that don't. Thread-safe initialization is required by C++11 in
[stmt.decl], but as of the time of this writing, only compilers based on
the IA-64 C++ ABI implemented it properly. The implementation requiring
thread-safe initialization is also used on the Qt bootstrapped tools, which
define QT_NO_THREAD.
The implementation requiring thread-safe initialization from the compiler
is the simplest: it creates the \a Type object as a function-local static
and returns its address. The actual object is actually inside a holder
structure so holder's destructor can set the guard variable to the value -2
(destroyed) when the type has finished destruction. Since we need to set
the guard \b after the destruction has finished, this code needs to be in a
base struct's destructor. A holder structure is used to avoid creating two
statics, which the ABI might require duplicating the thread-safe control
structures for.
The other implementation is similar to Qt 4's Q_GLOBAL_STATIC, but unlike
that one, it uses a \l QBasicMutex to provide locking. It is also more
efficient memory-wise. It use a simple double-checked locking of the mutex
and then creates the contents on the heap. After that, it creates a
function-local structure called "Cleanup", whose destructor will be run at
program exit and will actually destroy the contents.
\endomit
\sa Q_GLOBAL_STATIC_WITH_ARGS(), QGlobalStatic
*/
/*!
\macro Q_GLOBAL_STATIC_WITH_ARGS(Type, VariableName, Arguments)
\since 5.1
\relates QGlobalStatic
Creates a global and static object of type \l QGlobalStatic, of name \a
VariableName, initialized by the arguments \a Arguments and that behaves as
a pointer to \a Type. The object created by Q_GLOBAL_STATIC_WITH_ARGS
initializes itself on the first use, which means that it will not increase
the application or the library's load time. Additionally, the object is
initialized in a thread-safe manner on all platforms.
The typical use of this macro is as follows, in a global context (that is,
outside of any function bodies):
\code
Q_GLOBAL_STATIC_WITH_ARGS(MyType, staticType, (42, "Hello", "World"))
\endcode
The \a Arguments macro parameter must always include the parentheses or, if
C++11 uniform initialization is allowed, the braces.
Aside from the actual initialization of the contents with the supplied
arguments, this macro behaves identically to Q_GLOBAL_STATIC(). Please
see that macro's documentation for more information.
\sa Q_GLOBAL_STATIC(), QGlobalStatic
*/
/*!
\class QGlobalStatic
\threadsafe
\inmodule QtCore
\since 5.1
\brief The QGlobalStatic class is used to implement a global static object
The QGlobalStatic class is the front-end API exported when
Q_GLOBAL_STATIC() is used. See the documentation for the macro for a
discussion on when to use it and its requirements.
Normally, you will never use this class directly, but instead you will use
the Q_GLOBAL_STATIC() or Q_GLOBAL_STATIC_WITH_ARGS() macros, as
follows:
\code
Q_GLOBAL_STATIC(MyType, staticType)
\endcode
The above example creates an object of type QGlobalStatic called \c
staticType. After the above declaration, the \c staticType object may be
used as if it were a pointer, guaranteed to be initialized exactly once. In
addition to the use as a pointer, the object offers two methods to
determine the current status of the global: exists() and isDestroyed().
\sa Q_GLOBAL_STATIC(), Q_GLOBAL_STATIC_WITH_ARGS()
*/
/*!
\typedef QGlobalStatic::Type
This type is equivalent to the \c Type parameter passed to the
Q_GLOBAL_STATIC() or Q_GLOBAL_STATIC_WITH_ARGS() macros. It is used in the
return types of some functions.
*/
/*!
\fn bool QGlobalStatic::isDestroyed() const
This function returns true if the global static object has already
completed destruction (that is, if the destructor for the type has already
returned). In specific, note that this function returns false if
the destruction is still in progress.
Once this function has returned true once, it will never return
false again until either the program is restarted or the plugin or library
containing the global static is unloaded and reloaded.
This function is safe to call at any point in the program execution: it
cannot fail and cannot cause a deadlock. Additionally, it will not cause
the contents to be created if they have not yet been created.
This function is useful in code that may be executed at program shutdown,
to determine whether the contents may still be accessed or not.
\omit
Due to the non-atomic nature of destruction, it's possible that
QGlobalStatic::isDestroyed might return false for a short time after the
destructor has finished. However, since the destructor is only run in an
environment where concurrent multithreaded access is impossible, no one can
see that state. (omitted because it's useless information)
\endomit
\sa exists()
*/
/*!
\fn bool QGlobalStatic::exists() const
This function returns true if the global static object has already
completed initialization (that is, if the constructor for the type has
already returned). In specific, note that this function returns false if
the initialization is still in progress.
Once this function has returned true once, it will never return false again
until either the program is restarted or the plugin or library containing
the global static is unloaded and reloaded.
This function is safe to call at any point in the program execution: it
cannot fail and cannot cause a deadlock. Additionally, it will not cause
the contents to be created if they have not yet been created.
This function is useful if one can determine the initial conditions of the
global static object and would prefer to avoid a possibly expensive
construction operation.
For example, in the following code sample, this function is used to
short-circuit the creation of the global static called \c globalState and
returns a default value:
\code
Q_GLOBAL_STATIC(MyType, globalState)
QString someState()
{
if (globalState.exists())
return globalState->someState;
return QString();
}
\endcode
\b{Thread-safety notice:} this function is thread-safe in the sense that it
may be called from any thread at any time and will always return a valid
reply. But due to the non-atomic nature of construction, this function may
return false for a short time after the construction has completed.
\b{Memory ordering notice:} this function does not impose any memory
ordering guarantees. That is instead provided by the accessor functions
that return the pointer or reference to the contents. If you bypass the
accessor functions and attempt to access some global state set by the
constructor, be sure to use the correct memory ordering semantics provided
by \l QAtomicInt or \l QAtomicPointer.
\sa isDestroyed()
*/
/*!
\fn QGlobalStatic::operator Type*()
This function returns the address of the contents of this global static. If
the contents have not yet been created, they will be created thread-safely
by this function. If the contents have already been destroyed, this
function will return a null pointer.
This function can be used, for example, to store the pointer to the
contents of the global static in a local variable, thus avoiding multiple
calls to the function. The implementation of Q_GLOBAL_STATIC() is quite
efficient already, but in performance-critical sections it might be useful
to help the compiler a little. For example:
\code
Q_GLOBAL_STATIC(MyType, globalState)
QString someState()
{
MyType *state = globalState;
if (!state) {
// we're in a post-destruction state
return QString();
}
if (state->condition)
return state->value1;
else
return state->value2;
}
\endcode
\sa operator->(), operator*()
*/
/*!
\fn Type *QGlobalStatic::operator()()
\deprecated
This function returns the address of the contents of this global static. If
the contents have not yet been created, they will be created thread-safely
by this function. If the contents have already been destroyed, this
function will return a null pointer.
This function is equivalent to \l {operator Type *()}. It is provided for
compatibility with the private Q_GLOBAL_STATIC implementation that existed
in Qt 4.x and 5.0. New code should avoid using it and should instead treat
the object as a smart pointer.
*/
/*!
\fn Type *QGlobalStatic::operator->()
This function returns the address of the contents of this global static. If
the contents have not yet been created, they will be created thread-safely
by this function.
This function does not check if the contents have already been destroyed and
will never return null. If this function is called after the object has
been destroyed, it will return a dangling pointer that should not be
dereferenced.
*/
/*!
\fn Type &QGlobalStatic::operator*()
This function returns a reference to the contents of this global static. If
the contents have not yet been created, they will be created thread-safely
by this function.
This function does not check if the contents have already been destroyed.
If this function is called after the object has been destroyed, it will
return an invalid reference that must not be used.
*/

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src/corelib/global/qglobalstatic.h
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@ -49,19 +49,6 @@
QT_BEGIN_HEADER
QT_BEGIN_NAMESPACE
/*
* QGlobalStatic internals:
*
* The pointer is initialized to 0.
* The guard is initialized to 0.
* The guard can assume the following values:
* -2: object initialized and already destroyed
* -1: object initialized and is still valid
* 0: not initialized, the value of the pointer should be null
* +1: initializing, must wait until a state change happens
* (not used in the current implementation)
*/
namespace QtGlobalStatic {
enum GuardValues {
Destroyed = -2,