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Ch 4. Memory Management

Ch 4. Memory Management. Timothy Budd Oregon State University. Memory Management. Java : background memory management Advantage: allowing programmer to concentrate on more application specific details Disadvantage: programmer has little control over how memory management is performed.

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Ch 4. Memory Management

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  1. Ch 4. Memory Management Timothy Budd Oregon State University

  2. Memory Management • Java : background memory management • Advantage: allowing programmer to concentrate on more application specific details • Disadvantage: programmer has little control over how memory management is performed. • C++ : explicit memory management • Advantage:permitting much more efficient use of memory • Can bloat a program at run‑time, or make a program very inefficient

  3. Common Errors in C++ • Using a value before it has been initialized. • Allocating memory for a value, then not deleting it once it is no longer being used. • Using a value after it has been freed.

  4. Memory Model • C++ model is much closer to the actual machine representation than is Java memory model. • Permits an efficient implementation at the expense of needing increased diligence on the part of the programmer. • Stack‑resident - created automatically when a procedure is entered or exited. • Heap‑resident - must be explicitly created using the new operator.

  5. Stack Resident Memory Values • C++ • Both the stack and the heap are internal data structures managed by a run‑time system. • Values on a stack are strongly tied to procedure entry and exit. • When a procedure begins execution, a new section of the stack is created. • Java • only primitive values are truly stack resident. • objects and arrays are always allocated on the heap.

  6. void test () // Java { int i; int a[ ] = new int [10]; anObject ao = new anObject(); ….. } void test () // C++ { int i, int a [10]; anOb j e ct ao; ….. } Program Example: test()

  7. Drawbacks Using Stack • Lifetimeof stack resident memory values is tied to procedure entry & exit. Stack resident values cease to exist when a procedure returns. An attempt to use resident value after deletion will typically result in error. • Sizeof stack resident memory values must be known at compile time, which is when the structure of the activation record is laid out.

  8. Lifetime Errors • Once a procedure returns from execution, any stack resident values are deleted & no longer accessible. • A referenceto such a value will no longer be valid, although it may for a time appear to work.

  9. Program Example: readALine() Char * readALine () { char buffer[1000]; // declare a buffer for the line gets(buffer); gets(buffer); // read the line return buffer; return buffer; // return text of line } String readALine (BufferedInput inp) throws IOException { // create a buffer for the line String line = inp.readLine(); return line; } char * lineBuffer; // global declaration of pointer to buffer void readALine () { char buffer[tOOO]; // declare a buffer for the line gets(buffer); // read the line lineBuffer = buffer; // set pointer to reference buffer }

  10. Size Errors ‑ Slicing Problem • Most severe restrictions of stack‑based memory management: positions of values within the activation record are determined at compile time. • For primitive values & pointers this is a small concern. • For arrays: the array bound must be known at compile time. • For objects: a limitation on the degree to which values can be polymorphic.

  11. Array Allocations • Stack resident arrays must have a size that is known at compile time. • Often programmers avoid this problem by allocating an array with a size that is purposely too large.   • Rule: Never assume that just because an array is big, it will always be “big enough”

  12. char buffer[200]; // making array global avoids deletion error char * readALine () { gets(buffer); // read the line return buffer; } char * buffer; int newSize; // newSize is given some value …. buffer = new ChartnewSize]; // create an array of the given size …. delete [ ] buffer; // delete buffier when no longer Being used class A { public: // constructor A () : dataOne(2) { } // identification method virtual void whoAmI () {printf("class A"); } private: int dataOne; }; class B : public A { public: // constructor B () : dataTwo(4) { } // identification method virtual void whoAmI () { printf("class B"); } private: int dataTwo; }; Program Example

  13. Slicing Problem • Java: polymorphic -a variable declared as maintaining a value of one class can be holding a value derived from a child class. • In both Java and C++, it is legal to assign a value derived from class B to a variable declared as holding an instance of class A. A instanceOfA; // declare instances of B instanceOfB; // class A and B instanceOfA = instanceOfB;  instaceOfA.whoAmI();

  14. Slicing Problem • Rule: Static variables are never polymorphic. • Note carefully that slicing does not occur with references or with pointers: A & referenceToA = instanceOfB; referenceToA.whoAmI(); // will print class B   B * pointerToB = new B(); A * pointerToA = pointerToB(); pointerToA -> whoAmI(); // will print class B • Slicing only occurs with objects that are stack resident : C++ programs make the majority of their objects heap resident.

  15. Heap Resident Memory Values • Heap resident values are created using the new operator. • Memory for such values resides on the heap, or free store, which is a separate part of memory from the stack. • Typically accessed through a pointer, which will often reside on the stack. • Java hides the use of this pointer value, don’t need be concerned with it. • In C++, pointer declaration is explicitly stated.

  16. Heap Resident Memory Values void test () // Java { A anA = new A ( ); ……. } void test () // C++ { A * anA = new A; // note pointer declaration if (anA == 0) ... // handle no memory situation delete anA; }

  17. Recovery of Heap Based Memory • Java incorporates garbage collectioninto its run‑time library: the garbage collection system monitors the use of dynamically allocated variables, and will automatically recover and reuse memory that is no longer being accessed. • In C++, leaves this task to the programmer: dynamically allocated memory must be handed back to the heap manager using the delete operator. The deletion is performed by simply naming the pointer variable.

  18. Common Error in C++ • Forgetting to allocate a heap‑resident value, and using a pointer as if it were referencing a legitimate value. • Forgetting to hand unused memory back to the heap manager. • Attempting to use memory values after they have been handed back to the heap manager. • Invoking the delete statement on the same value more than once, thereby passing the same memory value back to the heap manager.

  19. Match memory allocations and deletions • Null pointer exception. • Memory leak: an allocation of memory that is never recovered- cause the memory requirements for the program to increase over time. Eventually the heap manager will be unable to service a request for further memory, and the program will halt. • Result of an over‑zealous attempt to avoid the second error. • Inconsistent state by the heap manager. • Rule: Always match memory allocations and deletions

  20. Simple Techniques to manage heap • Hide the allocation and release of dynamic memory values inside an object. • Maintaining a reference count that indicates the number of pointers to the value. When this count is decremented to zero, the value can be recovered.

  21. Encapsulating Memory Management • String literals in C++ are very low level abstractions. • A string literal in C++ is treated as an array of character values, and the only permitted operations are those common to all arrays. • The String data type in Java is designed to provide a higher level of abstraction. • A version of this data structure is provided in the new Standard Template Library.

  22. void String::resize(int size){ if (buffer == O); // no previous allocation buffer = new char[1+ size]; else if (size > strlen(buffer)) {   class String { Public: String () : buffer(0) { } // constructors String (const char * right) : buffer(0) { resize(strlen(right)); strcpy(buffer, right); } String (const String & right) : buffer(0) { resize(strlen(right.buffer)); strcpy(buffer, right.buffer); } String () { delete [ ] buffer; } // destructor  void operator = (const String & right) // assignment { resize(strlen(right.buffer)); strcpy(buffer, right.buffer); } private: void resize (int size); char * buffer; }; delete [ ] buffer; // recover old value buffer = new char[1 + size];} } Program Example: resize()

  23. Destructor in C++ • Destructor: a preocedure that performs whatever housekeeping is necessary before a variable is deleted. • Destructor class is a non‑argument procedure with a name formed by prepending a tilde before the class name. • Can match all allocations and deletes, ensuring no memory leaks will occur. • delete operator is the function used to actually return memory to the heap manger

  24. finalize in Java • A finalize method invoked when the value holding the method is about to be recovered by the garbage collector. • Cannot make assumptions concerning behavior based on the execution of code in a finalize method.

  25. Execution Tracer • The class Trace takes as argument a string value. The constructor prints a message using the strings and the destructor prints a different message using the same string: • To trace the flow of function invocations, the programmer simply creates a declaration for a dummy variable of type Trace in each procedure to be traced

  26. Program Example: class Trace class Trace { public: Trace (string); // constructor and destructor ~ trace (); private: string name; }; Trace: :Trace (string t) : name(t){ cout << "Entering " << name << end1; } Trace : : ~Trace (){ count << “Exiting “ << name << end1; }

  27. Program Example: procedures void procedureOne (){ Trace dummy("Procedure One"); …. procedureTwo(); // proc one invokes proc two } void procedureTwo (){ Trace dummy("Procedure Two"); …. If (x < 5) { Trace dumTrue("x test is true"); ….. } else { Trace dumFalse("x test is false"); ….. } ….. }

  28. Auto_ptr Class • There is an object that must dynamically allocate another memory value in order to perform its intended task • But the lifetime of the dynamic value is tied to the lifetime of the original object; it exists as long as the original objects exists, and should be eliminated when the original object ceases to exist. • To simplify the management of memory, the standard library implements a useful type named auto_ptr.

  29. Reference Count • Reference Count: the count of the number of pointers to a dynamically allocated object. • Ensure the count is accurate: whenever a new pointer is added the count is incremented, and whenever a pointer is removed the count is decremented.

  30. Reference Count (Table)

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