CS计算机代考程序代写 data structure compiler Java c++ algorithm Memory Management and Copy Control

Memory Management and Copy Control
Overview
• Types of memory
• Copy constructor, assignment operator, and destructor • Reference counting with smart pointers
References
• Stanley B. Lippman, Josée Lajoie, and Barbara E. Moo: C++ Primer. 4th Edition. Addison-Wesley (2006)
• Bruno R. Preiss: Data Structures and Algorithms with Object-Oriented Design Patterns in C++. John Wiley & Sons, Inc. (1999)
• Andrew W. Appel with Jens Palsberg: Modern Compiler Implementation in Java. 2nd Edition, Cambridge University Press (2002).
• Alfred V. Aho, Ravi Sethi, and Jeffrey D. Ullman: Compilers – Principles, Techniques, and Tools. Addison-Wesley (1988)
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Static Read-Write Memory
• C++ allows for two forms of global variables: • Static non-class variables,
• Static class variables.
• Static variables are mapped to the global memory. Access to them depends on the visibility specifiers.
• We can find a program’s global memory in the so- called read-write .data segment.
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The Keyword static
• The keyword static can be used to
mark the linkage of a variable or function internal, retain the value of a local variable between
declare a class instance variable, define a class method.
•
•
function calls,
• •
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Read-Write Static Variables
Static class variables must be initialized outside the class.
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Static Read-Only Memory
• In combination with the const specifier we can also define read-only global variables or class variables:
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Const variables are often stored in the program’s read-only .text segment.

Program Memory: Stack
• All value-based objects are stored in the program’s stack.
• The program stack is automatically allocated and freed.
•References to stack locations are only valid when passed to a callee. References to stack locations cannot be returned from a function.
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Stack Frames (C)
temporaries save registers
arguments
return address
temporaries save registers
arguments
incoming arguments stack pointer
outgoing arguments
higher addresses
previous frame
current frame next frame
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Program Memory: Heap
•Every program maintains a heap for dynamically allocated objects.
• Each heap object is accessed through a pointer.
• Heap objects are not automatically freed when pointer variables become inaccessible (i.e., go out of scope).
•Memory management becomes essential in C++ to reclaim memory and to prevent the occurrences of so- called memory leaks.
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List::~List()
Release memory associated with list node object on the heap.
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The Dominion Over Objects
• Alias control is one of the most difficult problems to master in object-oriented programming.
• Aliases are the default in reference-based object models used, for example, in Java and C#.
• To guarantee uniqueness of value-based objects in C++, we are required to define copy constructors.
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The Copy Constructor
• Whenever one defines a new type, one needs to specify implicitly or explicitly what has to happen when objects of that that type are copied, assigned, and destroyed.
• The copy constructor is a special member, taking just a single parameter that is a const reference to an object of the class itself.
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A simple String class
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SimpleString
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SimpleString: Constructor & Destructor
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SimpleString: The Operators
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Implicit Copy Constructor
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On Windows you may not see anything.

What Has Happened?
Shallow copy:
s2.fCharacters == s1.fCharacters
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Double free:
delete s2.fCharacters, which was called in s2 + ‘B’.

We need an explicit copy constructor!
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The Explicit Copy Constructor
• When a copy constructor is called, then all instance variables are uninitialized in the beginning.
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Explicit Copy Constructor in Use
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What Has Happened?
Deep copy:
s2.fCharacters != s1.fCharacters
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That’s it. No more problems, or?
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A Simple Assignment
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What Has Happened?
Shallow copy:
s2.fCharacters == s1.fCharacters
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Double free:
delete s2.fCharacters, which is the same as s1.fCharacters.

Rule Of Thumb
• Copy control in C++ requires three elements: • a copy constructor
• an assignment operator • a destructor
• Whenever one defines a copy constructor, one must also define an assignment operator and a destructor.
• C++ also supports move constructor and move assignment operator. They work similarly, but steal the memory for its r-value source. Moreover, while the compiler still synthesizes missing l-value copy constructors and assignment operators, their r-value counterparts are not synthesized if the programmer does specify either but not both.
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We need an explicit assignment operator!
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The Explicit Assignment Operator
protection against accidental suicide
• When the assignment operator is invoked, then all instance variables are initialized in the beginning. We
need to release the memory first!
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Explicit Assignment Operator in Use
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What Has Happened?
Deep copy:
s2.fCharacters != s1.fCharacters
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Cloning: Alias Control for References
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Copying Pointers
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What Has Happened?
Shallow copy:
ps2. == ps1
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Double free:
delete ps2, which is the same as ps1.

Solution: A clone() Method
Destructor must be virtual!
• It is best to define the destructor of a class virtual
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Problems With Cloning
• The member function clone() must be defined virtual to allow for proper redefinition in subtypes.
• Whenever a class contains a virtual function, then its destructor is required to be defined virtual as well.
• The member function clone() can only return one type. When a subtype redefines clone(), only the super type can be returned.
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Non-virtual Cloning Does Not Work!
• One could define clone() non-virtual and use overloading. But this does not work as method selection starts at the static type of the pointer.
SimpleString* pToString = new SubtypeOfSimpleString(); SimpleString* c1 = pToString->clone(); // SimpleString::clone()
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Reference Counting
• A simple technique to record the number of active uses of an object is reference counting.
• Each time a heap-based object is assigned to a variable the object’s reference count is incremented and the reference count of what the variable previously pointed to is decremented.
• Some compilers emit the necessary code, but in case of C++ reference counting must be defined (semi-)manually.
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The Use of Handle
• The template class Handle provides a pointer-like behavior: • Copying a Handle will create a shared alias of the underlying
object.
• To create a Handle, the user will be expected to pass a fresh,
dynamically allocated object of the type managed by the Handle.
• The Handle will own the underlying object. In particular, the Handle assumes responsibility for deleting the owned object once there are no longer any Handles attached to it.
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Handle: addRef & releaseRef
Increment reference count
Decrement reference count and delete object if it is no longer referenced anywhere.
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Reference Counting Limits
Reference Count == 1 Reference Count == 1
• Reference counting fails on circular data structures like double-linked lists.
• Circular data structures require extra effort to reclaim allocated memory. Know solution: Mark-and-Sweep
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