c++-gtk-utils
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This is a smart pointer for managing the lifetime of objects allocated on freestore, with a thread safe reference count. More...
#include <c++-gtk-utils/shared_ptr.h>
Public Member Functions | |
SharedLockPtr (T *ptr=0) | |
SharedLockPtr (T *ptr, Cgu::SharedPtrAllocFail::Leave tag) | |
void | reset (T *ptr=0) |
void | reset (T *ptr, Cgu::SharedPtrAllocFail::Leave tag) |
SharedLockPtr (const SharedLockPtr &sh_ptr) | |
SharedLockPtr (SharedLockPtr &&sh_ptr) | |
template<class U > | |
SharedLockPtr (const SharedLockPtr< U > &sh_ptr) | |
template<class U > | |
SharedLockPtr (SharedLockPtr< U > &&sh_ptr) | |
SharedLockPtr & | operator= (SharedLockPtr sh_ptr) |
template<class U > | |
SharedLockPtr & | operator= (const SharedLockPtr< U > &sh_ptr) |
template<class U > | |
SharedLockPtr & | operator= (SharedLockPtr< U > &&sh_ptr) |
T * | get () const |
T & | operator* () const |
T * | operator-> () const |
unsigned int | get_refcount () const |
~SharedLockPtr () |
Friends | |
class | SharedLockPtr |
This is a smart pointer for managing the lifetime of objects allocated on freestore, with a thread safe reference count.
Class SharedLockPtr is a version of the shared pointer class which includes locking so that it can be accessed in multiple threads (although the word Lock is in the title, by default it uses glib atomic functions to access the reference count rather than a mutex, so the overhead should be very small). Note that only the reference count is protected, so this is thread safe in the sense in which a raw pointer is thread safe. A shared pointer accessed in one thread referencing a particular object is thread safe as against another shared pointer accessing the same object in a different thread. It is thus suitable for use in different Std C++ containers which exist in different threads but which contain shared objects by reference. But:
As mentioned, by default glib atomic functions are used to provide thread-safe manipulation of the reference count. However, a library user can define the symbol CGU_SHARED_LOCK_PTR_USE_MUTEX before shared_ptr.h is parsed so as to use mutexes instead, which might be useful for some debugging purposes.
Comparison with std::shared_ptr
Most of the things that can be done by this class can be done by using std::shared_ptr in C++11, but this class is retained in the c++-gtk-utils library not only to retain compatibility with series 1.2 of the library, but also to cater for some cases not met (or not so easily met) by std::shared_ptr:
(i) Glib memory slices provide an efficient small object allocator (they are likely to be significantly more efficient than global operator new()/new[](), which generally hand off to malloc(), and whilst malloc() is good for large block allocations it is generally poor as a small object allocator). Internal Cgu::SharedLockPtr allocation using glib memory slices can be achieved by compiling the library with the –with-glib-memory-slices-no-compat configuration option.
(ii) If glib memory slices are not used (which do not throw), constructing a shared pointer for a new managed object (or calling reset() for a new managed object) might throw if internal allocation fails. Although by default the Cgu::SharedLockPtr implementation will delete the new managed object in such a case, it also provides an alternative constructor and reset() method which instead enable the new object to be accessed via the thrown exception object so that user code can decide what to do; std::shared_ptr deletes the new object in every case.
(iii) A user can explicitly state whether the shared pointer object is to have atomic increment and decrement-and-test with respect to the reference count so that the reference count is thread safe ('no' in the case of Cgu::SharedPtr, and 'yes' in the case of Cgu::SharedLockPtr). Using atomic functions is unnecessary if the managed object concerned is only addressed in one thread (and might cause unwanted cache flushing in certain circumstances). std::shared_ptr will generally always use atomic functions with respect to its reference count in a multi-threaded program.
In favour of C++11's std::shared_ptr, it has an associated std::make_shared() factory function which will construct both the referenced object and the shared pointer's reference count within a single memory block when the first shared pointer managing a particular object is constructed. Cgu::SharedPtr and Cgu::SharedLockPtr always allocate these separately, but this is partly mitigated by the use of glib memory slices to allocate the reference count where the –with-glib-memory-slices-no-compat configuration option is chosen.
In addition, std::shared_ptr has an associated std::weak_ptr class, which Cgu::SharedLockPtr does not (there is a Cgu::GobjWeakHandle class, but that is cognate with Cgu::GobjHandle and is only usable with GObjects), and shared_ptr objects also have some atomic store, load and exchange functions provided for them which enable concurrent modifications of the same instance of shared_ptr in different threads to have defined results.
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inlineexplicit |
Constructor taking an unmanaged object.
ptr | The object which the SharedLockPtr is to manage (if any). |
std::bad_alloc | This constructor will not throw if the 'ptr' argument has a NULL value (the default), otherwise it might throw std::bad_alloc if memory is exhausted and the system throws in that case. If such an exception is thrown, this constructor is exception safe (it does not leak resources), but as well as cleaning itself up this constructor will also delete the managed object passed to it to avoid a memory leak. If such automatic deletion is not wanted in that case, use the version of this constructor taking a Cgu::SharedPtrAllocFail::Leave tag argument. |
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inline |
Constructor taking an unmanaged object.
ptr | The object which the SharedLockPtr is to manage. |
tag | Passing the tag emumerator Cgu::SharedPtrAllocFail::leave causes this constructor not to delete the new managed object passed as the 'ptr' argument in the event of internal allocation in this method failing because of memory exhaustion (in that event, Cgu::SharedPtrError will be thrown). |
Cgu::SharedPtrError | This constructor might throw Cgu::SharedPtrError if memory is exhausted and the system would otherwise throw std::bad_alloc in that case. This constructor is exception safe (it does not leak resources), and if such an exception is thrown it will clean itself up, but it will not attempt to delete the new managed object passed to it. Access to the object passed to the 'ptr' argument can be obtained via the thrown Cgu::SharedPtrError object. |
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This copy constructor does not throw.
sh_ptr | The shared pointer to be copied. |
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inline |
The move constructor does not throw. It has move semantics.
sh_ptr | The shared pointer to be moved. |
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A version of the copy constructor which enables pointer type conversion (assuming the type passed is implicitly type convertible to the managed type, such as a derived type). This copy constructor does not throw.
sh_ptr | The shared pointer to be copied. |
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A version of the move constructor which enables pointer type conversion (assuming the type passed is implicitly type convertible to the managed type, such as a derived type). This move constructor does not throw.
sh_ptr | The shared pointer to be moved. |
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The destructor does not throw unless the destructor of a managed object throws - that should never happen.
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inline |
This method does not throw.
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inline |
This method does not throw.
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inline |
This method does not throw.
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inline |
This method does not throw.
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This method (and so copy or move assignment) does not throw unless the destructor of a managed object throws.
sh_ptr | the assignor. |
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A version of the assignment operator which enables pointer type conversion (assuming the type passed is implicitly type convertible to the managed type, such as a derived type). This method does not throw unless the destructor of a managed object throws.
sh_ptr | the assignor. |
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inline |
A version of the operator for move assignment which enables pointer type conversion (assuming the type passed is implicitly type convertible to the managed type, such as a derived type). This method does not throw unless the destructor of a managed object throws.
sh_ptr | the shared pointer to be moved. |
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inline |
Causes the SharedLockPtr to cease to manage its managed object (if any), deleting it if this is the last SharedLockPtr object managing it. If the argument passed is not NULL, the SharedLockPtr object will manage the new object passed (which must not be managed by any other SharedLockPtr object). This method is exception safe, but see the comments below on std::bad_alloc.
ptr | NULL (the default), or a new unmanaged object to manage. |
std::bad_alloc | This method will not throw if the 'ptr' argument has a NULL value (the default) and the destructor of a managed object does not throw, otherwise it might throw std::bad_alloc if memory is exhausted and the system throws in that case. Note that if such an exception is thrown then this method will do nothing (it is strongly exception safe and will continue to manage the object it was managing prior to the call), except that it will delete the new managed object passed to it to avoid a memory leak. If such automatic deletion in the event of such an exception is not wanted, use the reset() method taking a Cgu::SharedPtrAllocFail::Leave tag type as its second argument. |
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inline |
Causes the SharedLockPtr to cease to manage its managed object (if any), deleting it if this is the last SharedLockPtr object managing it. The SharedLockPtr object will manage the new object passed (which must not be managed by any other SharedLockPtr object). This method is exception safe, but see the comments below on Cgu::SharedPtrError.
ptr | A new unmanaged object to manage (if no new object is to be managed, use the version of reset() taking a default value of NULL). |
tag | Passing the tag emumerator Cgu::SharedPtrAllocFail::leave causes this method not to delete the new managed object passed as the 'ptr' argument in the event of internal allocation in this method failing because of memory exhaustion (in that event, Cgu::SharedPtrError will be thrown). |
Cgu::SharedPtrError | This method might throw Cgu::SharedPtrError if memory is exhausted and the system would otherwise throw std::bad_alloc in that case. Note that if such an exception is thrown then this method will do nothing (it is strongly exception safe and will continue to manage the object it was managing prior to the call), and it will not attempt to delete the new managed object passed to it. Access to the object passed to the 'ptr' argument can be obtained via the thrown Cgu::SharedPtrError object. |
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