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C++: Memory Problems Logistics Logistics Plan for This Week Memory Logistics • Project C++: Memory Problems – Part 2 (water) due Sunday, Oct 16th • Questions? or When Good Memory Goes Bad Logistics Plan for this week • Final exam • Today: Intro to Memory Management – Good news…bad news • Tomorrow: Memory Problems – Good news • Thursday: Standard Libraries • Last day of finals, November 18th –Bad news • 8am-10am – Room • 01-3338 Memory Leak Memory Leak • A bug in a program that prevents it from class Foo freeing up memory that it no longer needs. { private: • As a result, the program grabs more and int *array_member; more memory until it finally crashes int asize; because there is no more memory left. ... public: • In short: Foo (int size); – Allocating without cleaning up. ~Foo (); } 1 Memory Leak Memory Leak Foo::Foo (int size) : asize (size), array_member (new int[size]) { for (int i=0; i<size; i++) asize array_member[i] = 0; aClass a array_member } 20 ints void f () { // local aClass object stack aClass a (20); ... } heap Pointer Ownership Pointer Ownership • Everything that is a pointer should be • Class members owned – If you allocate it during construction, you – Responsible for cleanup when finished should deallocate during destruction. – Should be known to programmer – Should be by design during implementation. – Should also deallocate during • Copy construction • operator= – Owner and only owner should perform a delete. Pointer Ownership Pointer Ownership // constructor // copy constructor Foo::Foo (int size) : Foo::Foo (const Foo &F) asize (size), array_member (new int[size]) { { if (F != (*this) { for (int i=0; i<size; i++) array_member[i] = 0; delete [] array_member; } array_member = new int[F.asize]; asize = F.asize; // destructor Foo:~Foo () for (int i=0; i<asize; i++) { array_member[i] = F.array_member[i]; delete [] array_member; } } } 2 Pointer Ownership Pointer Ownership // assignment operator Foo &Foo::operator= (const Foo &F) • Pointers returned by functions { – Who should be responsible for memory to if (F != (*this) { delete [] array_member; which these pointers point? array_member = new int[F.asize]; asize = F.asize; for (int i=0; i<asize; i++) array_member[i] = F.array_member[i]; } return (*this); } Pointer Ownership Pointer Ownership class Moo Allocation done in method…caller should be { responsible for pointer. private: char* myID static char anotherId[15]; char * Moo::getID() ... { public: char *id = new char[15]; Moo (); strcpy (id, “I am a cow”); ... char *getID(); return id; } } Pointer Ownership Pointer Ownership Allocation done in constructor…object should be responsible Memory is static…object should be responsible for for pointer….should deallocate in destructor pointer but no deallocation necessary Moo::Moo () : myID (new char[15]) { char Moo::anotherID[15] = “I am a cow”; strcpy (id, “I am a cow”); } char * Moo::getID() char * Moo::getID() { { return anotherID; return myID; } } 3 Pointer Ownership Pointer Ownership Memory is static…object should be responsible for Should not return pointer to local variable pointer but no deallocation necessary char * Moo::getID() char * Moo::getID() { { // This is okay too. // This is not okay. static char idInFunct[50] = “I am a char idInFunct[50] = “I am a cow”; cow”; return idInFunct; return idInFunct; } } Pointer Ownership Pointer Ownership • Pointers returned by functions • Anonymous Objects – Who should be responsible for memory to – An anonymous object is an object in every which these pointers point? sense except that it has no name. • Either caller or object – Used for creating very temporary objects. • Should be clearly designed and documented Point square[] = {Point(0,0),Point(0,1),Point(1,1),Point(1,0) }; Memory Leak / Pointer Pointer Ownership Ownership • Anonymous Objects • Questions? – Beware when anonymous objects are allocated on free store. vector< Card * > hand; hand.push_back( new Card(...) ); hand.push_back( new Card(...) ); hand.push_back( new Card(...) ); : : If vector does not take ownership of the objects stored in it, a memory leak is possible. 4 Dangling Pointers Dangling Pointers • Pointer is pointing to something that it p1 = new Foo; shouldn’t be. : delete p1; • Can happen if: : – If the scope of a pointer extends beyond that of p2 = new Bar; // What if same memory is the object being pointed to //given to p2? • i.e Returning a pointer to a local variable. : – If a dynamically allocated memory cell is freed int i = p1->data; // i contains garbage explicitly and then the pointer pointing to such a space is used in subsequent code. p1->op(...); // p2's object mysteriously changes! Dangling Pointers Dangling Pointer / Double delete c1 • Ways to prevent dangling pointers class Foo { – Do not return pointers to local variables. private: – After calling delete on a pointer, int immediately set it to NULL. *array_member; int asize; ... p1 = new Foo; } c2 delete p1; void moo () p1 = 0; { p2 = new Bar; Foo c1, c2; Free p1->op(...); // core dump! c1 = c2; store } Aka premature deletion Bounds errors Bounds errors • Recall that C++ has no array bounds void foo() { checking. int *a = new int[20]; – Writing past bounds of array will trash aClass *b = new aClass(); unsuspecting memory. ... a a[20] = 23; // b mysteriously // changes } b heap 5 Bounds errors Bounds errors void foo() • This can be quite dangerous for local arrays: Local Variables { int a[20]; a[20] = 23; a Local Variables } Return Address Return Address Function Arguments Function Arguments Dangling Pointers / Bounds errors Bounds errors void foo() • Questions? { char a[20]; strcpy (a, “This string is too long”); } Getting around these problems The Smart Pointer • The smart pointer Smart pointer – Prevents memory leaks and dangling pointers – Wrapper class that owns a pointer to an object ptr • Object keeps a reference count of variables accessing it • When the reference count reaches 0, the object is deleted by the smart pointer. • After deleting object, pointer value set to 0. 6 The Smart Pointer Using smart pointers • The smart pointer should look and act Instead of Use like a regular pointer: – Should support -> void foo() void foo() – Should support * { { MyClass* p(new MyClass); SmartPtr<MyClass> • Should be smarter than your average pointer. p->DoSomething(); p(new MyClass); delete p; p->DoSomething(); – Reduce memory leaks } } – Reduce dangling pointers – Take ownership p will cleanup after itself Smart Pointers Summary • We may be looking at the details of a smart • Memory Woes pointer in Week 9/10 – Memory leak – Dangling Pointer • There is a Smart Pointer class available for use on the project • Double Delete( aka Premature delete) – See choices.html of of Project pages – Pointer Ownership – Overwrite array • Questions? Tomorrow’s Lab • Inheritance • Memory Management Lab in 2 weeks. 7.
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