CPSC 252 Dynamic Memory Allocation Page 1 Dynamic memory allocation Our first IntVector class has some serious limitations the capacity is fixed at MAX_SIZE elements to change capacity, we change MAX_SIZE in the header then we recompile the program – not pretty! We get around this using dynamic memory allocation memory is given to an object at run-time each object gets what it needs as it needs it dynamic memory requires the use of C++ pointers

CPSC 252 Dynamic Memory Allocation Page 2 Pointers A pointer is a data type whose value is the address of an object in memory char* charPtr; char myChar = ‘A’; We “point to” an object using the & or “address of” operator: charPtr = &myChar; We can change the value of myChar indirectly using the * or “dereferencing” operator: *charPtr = ‘B’; charPtr myChar ‘A’ charPtr myChar ‘B’ charPtr myChar ‘A’ ???

CPSC 252 Dynamic Memory Allocation Page 3 charPtr char2char1 ‘B’ ‘A’ charPtr char2char1 ‘B’ ‘C’ Constant pointers and constant objects char char1 = ‘A’; char char2 = ‘B’; char* const charPtr = &char1; the variable charPtr is a constant pointer to char the address in charPtr must be defined by initialization the address in charPtr cannot be changed afterward charPtr = &char2; you can change the object pointed to *charPtr = ‘C’;

CPSC 252 Dynamic Memory Allocation Page 4 Pointers to constant objects char char1 = ‘A’; const char char2 = ‘B’; const char* charPtr = &char1; the variable charPtr is a pointer to a constant char the address in charPtr can be changed any time the character pointed to cannot be changed *charPtr = ‘C’; char2 = ‘D’; you can change the object pointed to and char1 itself charPtr = &char2; char1 = ‘E’; charPtr char2char1 ‘B’ ‘A’ charPtr char2char1 ‘B’ ‘E’

CPSC 252 Dynamic Memory Allocation Page 5 Reference types char myChar = ‘A’; char myChar = ‘B’; char& charRef1 = myChar1; const char& charRef2 = myChar2; “like” a constant pointer, but better! charRef1 is a reference to the character myChar1 it stores the address of the variable that it is referencing this address cannot be changed once it has been assigned automatic dereference (no * operator ) charRef1 = charRef2; but not for a constant reference charRef2 = ‘C’; charRef1 myChar1 ‘A’ charRef2 myChar2 ‘B’ charRef2 myChar2 ‘B’ charRef1 myChar1 ‘B’

CPSC 252 Dynamic Memory Allocation Page 6 Reference parameters Reference variables are often used as function parameters formal parameter become references to actual parameters (more about paramter-passing in a future lecture) void swap( int& num1, int& num2 ) { int temp; temp = num1; num1 = num2; num2 = temp; } int x = 2, y = 5; swap( x, y );

CPSC 252 Dynamic Memory Allocation Page 7 Dynamic memory allocation (after learning pointers) C++ has three kinds of variables: automatic variables allocated and de-allocated as needed static variables allocated for the lifetime of the execution dynamic variables allocated and de-allocated explicitly during execution as specified by the program Dynamically memory allocation allows: memory efficiently (use only what you need) array (and object) sizes that vary at run time more sophisticated data structures and algorithms

CPSC 252 Dynamic Memory Allocation Page 8 The new operator new returns the address of a newly allocated object int* intPtr; intPtr = new int; We can also allocate an array of integers: int capacity; cout > capacity; int* data = new int[ capacity ]; for( int index = 0; index > data[ index ]; intPtr data ???

CPSC 252 Dynamic Memory Allocation Page 9 Garbage collection Dynamic memory must be released when no longer required some languages have automatic garbage collection C++ requires explicit garbage collection by the program memory is released using the delete operator: delete intPtr; delete[] data; delete operator does not delete the pointer it releases the memory to which the pointer points delete operator will not change the pointer it still points to the memory that was just released! this is a problem

CPSC 252 Dynamic Memory Allocation Page 10 Dangling pointers An illegal pointer dereference results if the pointer is: not initialized (it could point anywhere) NULL (its value is zero so it points nowhere) referencing an object that no longer exists (dangling pointer) referencing outside the object it points to Pointer arithmetic causes the last case: int* p = new int[3]; *(p + 4) = p[5]; both attempts dereference outside the dynamic array so they are illegal pointer dereferences

CPSC 252 Dynamic Memory Allocation Page 11 Avoiding dangling pointers After using the delete operator: it is often a good idea to set the corresponding pointer to 0 the built-in value NULL is equal to zero delete intptr; intptr = NULL; this avoids accidentally dereferencing a dangling pointer A pointer whose value is NULL is a null pointer we often use a “ground” symbol to denote a NULL pointer intptr

CPSC 252 Dynamic Memory Allocation Page 12 How to avoid dangling pointers release memory only when there is only one pointer to it! int* intPtr1; int* intPtr2; intPtr1 = new int; *intPtr1 = 7; intPtr2 = intPtr1; delete intPtr1; intPtr1 = NULL; intPtr2intPtr1 7 intPtr2intPtr1 ??? intPtr2intPtr1 ??? intPtr2intPtr1 ??? intPtr2intPtr1 ??? intPtr2intPtr1 7 ???

CPSC 252 Dynamic Memory Allocation Page 13 Memory leaks If there is no pointer pointing to a dynamically allocated block of memory, there is a memory leak int* intPtr; intPtr = new int; *intPtr = 7; intPtr = new int; *intPtr = 5; intPtr ??? intPtr 7 7 5

CPSC 252 Dynamic Memory Allocation Page 14 Invoking delete with and without the [] What memory is released when a pointer is to an array? int* data = new int[10]; delete [] data; delete operator releases all 10 words of memory int* data = new int[10]; delete data; delete operator still releases all 10 words of memory but only one destructor is invoked (for the first object) The only difference is whether 1 or 10 destructors are called