Final step for full polymorphic handling of Partition objects is to
implement a virtual copy constructor. C++ doesn't directly support
virtual copy constructors, so instead use the virtual copy constructor
idiom [1]. (Just a virtual method called clone() which is implemented
in every polymorphic class and creates a clone of the current object and
returns a pointer to it).
Then replace all calls to the (monomorphic) Partition object copy
constructor throughout the code, except in the clone() implementation
itself, with calls to the new virtual clone() method "virtual copy
constructor".
Also have to make the Partition destructor virtual too [2][3] so that
the derived class destructor is called when deleting using a base class
pointer. C++ supports this directly.
[1] Wikibooks: More C++ Idioms / Virtual Constructor
https://en.wikibooks.org/wiki/More_C%2B%2B_Idioms/Virtual_Constructor
[2] When to use virtual destructors?
http://stackoverflow.com/questions/461203/when-to-use-virtual-destructors
[3] Virtuality
Guideline #4: A base class destructor should be either public and
virtual, or protected and nonvirtual
http://www.gotw.ca/publications/mill18.htm
Bug 759726 - Implement Partition object polymorphism
SQUASH: When first using pointers to Partition and calling delete
Operation classes now internally use pointers to Partition objects and
take on management of their lifetimes. As before, with the
PartitionVector class, when storing pointers in a class the Big 3 of
destructor, copy constructor and copy assignment operator also have to
be considered.
First, all the Partition objects are allocated in the derived Operation*
class parameterised constructors and freed in the associated
destructors. However the Operation classes are never copy constructed
or copy assigned; they are only ever created and destroyed. Only
pointers to the derived Operations are copied into the vector of pending
operations. Therefore the copy construtor and copy assignment operator
aren't needed. To enforce this provide inaccessible private
declarations without any implementation so that the compiler will
enforce this [1][2].
This example code fragment:
1 OperationCheck o1( device, partition );
2 OperationCheck o2 = o1;
3 o2 = o1;
Does these OperationCheck calls:
1 Implemented parameterised construtor,
2 Disallowed copy constructor,
3 Disallowed copy assignment
Trying to compile the above code would fail with errors like these:
../include/OperationCheck.h: In member function 'void GParted::Win_GParted::activate_check()':
../include/OperationCheck.h:36:2: error: 'GParted::OperationCheck::OperationCheck(const GParted::OperationCheck&)' is private
OperationCheck( const OperationCheck & src ); // Not implemented copy constructor
^
test.cc:2:21: error: within this context
OperationCheck o2 = o1;
^
../include/OperationCheck.h:37:19: error: 'GParted::OperationCheck& GParted::OperationCheck::operator=(const GParted::OperationCheck&)' is private
OperationCheck & operator=( const OperationCheck & rhs ); // Not implemented copy assignment operator
^
test.cc:3:4: error: within this context
o2 = o1;
^
[1] Disable copy constructor
http://stackoverflow.com/questions/6077143/disable-copy-constructor
[2] Disable compiler-generated copy-assignment operator [duplicate]
http://stackoverflow.com/questions/7823845/disable-compiler-generated-copy-assignment-operator
Bug 759726 - Implement Partition object polymorphism
The Operation classes contain partition objects which are copied by
value. Need to replace these with pointers to Partition objects instead
and manage their lifetimes so that they can be used polymorphically.
First step is to protect the partition members partition_new,
partition_original, and for OperationCopy class only, partition_copied
within the Operation classes and provide accessor methods.
get_partition_new() and get_partition_original() accessors are
implemented in the Operation base class so all derived classes get an
implementation. get_partition_new() is also virtual so that
OperationCheck and OperationDelete can override the implementation and
assert that they don't use partition_new. get_partition_copied() is
provided for the OperationCopy class only so can only be accessed via an
OperationCopy type variable.
Bug 759726 - Implement Partition object polymorphism
The apply_to_visual() method for the change UUID, format, label file
system and name partition operations duplicated identical code. This
code was just substituting the partition in the disk graphic vector with
the new partition recorded in the operation, as none of these operations
change the partition boundaries. Move this duplicate code into the
parent class in new method Operation::substitute_new().
Bug 755214 - Refactor operation merging
Win_GParted::Merge_Operations() method was modifying the internals of
Operation* objects; in particular the partition_new member variable.
This is breaking data hiding and encapsulation tenant of object oriented
programming.
Implement exactly the same operation merge semantics, but hide the
manipulation of the internals of the Operation* objects within the
Operation* classes themselves.
Bug 755214 - Refactor operation merging
When Operation objects are created they take a copy of the Device object
to which the operation is to be applied. The Device object includes a
vector of all the contained Partition objects currently on the device,
so these get copied too.
These additional deep copied Partition objects in the Operation object
are never accessed. Therefore don't copy the contained Partition
objects when copying the Device object into the Operation object.
Bug 750168 - Reduce the amount of copying of partition objects
Mike Fleetwood