Pointers Dynamic Memory Allocation and Structures Dynamic Data

Pointers, Dynamic Memory Allocation and Structures

Dynamic Data • Suppose we write a program to keep track of a list of students How many student records should we create? • • What if we create too few? What if we create too many? Wouldn’t it be nice to create just as many as we need? !

Pointer Review 1. 2. 3. What is a pointer? How does one declare a pointer variable? When using indirection, what steps are followed to get the specified value *ptr in the ex. below? ie) int *ptr, x, y = 5; ptr = &y; x = *ptr;

Dynamic Memory Allocation • Stack: Area where function data is allocated and reclaimed as program executed • Heap: Area C++ sets aside assuming that the programmer will ask for more memory as program executes

Dynamic Memory Allocation type *var = new type – allocate a chunk of memory large enough to store something of type – new returns address of location of memory allocated delete(var) – free up the memory pointed to by var – otherwise -- memory leak!

Example string *s = new string(“hello”); cout << “The string is “ << *s << endl; cout << “The length is “ << (*s). length() << endl; *s += “ world”; cout << “Now the string is “ << *s << endl; delete s;

Common Errors • Stale Pointers – string *s = new string(“hello”); – string *t = s; – delete s; //t is stale • Double Delete – delete t; //already deleted? • Dynamic memory allocation isn’t so interesting with primitive types…

Parallel Arrays • Inventory – One array keeps track of cost – One array keeps track of number in inventory • What if there were many parallel items to keep track of? • Would you want to keep track of 10, 20, 30 parallel arrays? Why not?

Structures • Group many record items into one element Inventory: Item Name: Shirt Item Number: 1232 Cost: $20. 00 Num in Inventory: 10

Structures struct inventory_item { string name; int number; double cost; int num_in_inventory; };

Structures struct new_type{ type name; … type name; }; • • goes after #include and #define – not inside of main type can be another structure data type struct name{ string first; string last; }; struct student{ name full_name; int id; };

Declaring and Initializing • structure declaration does not allocate any memory inventory_item shirts; . name. number. cost. num_in_inventory …

Declaring and Initializing • Dot member operator -- used to access an item in the structure shirts. number = 1232; shirts. cost = 20. 00; shirts. num_in_inventory = 10; shirts. name = “shirts”; . name . number. cost. num_in_inventory … 1232 20. 00 10

Accessing • Print “There are 10 of item: shirt left in the inventory. ”

Accessing • Print “There are 10 of item: shirt left in the inventory. ” cout << “There are “ << shirts. num_in_inventory << “ of item: “ << shirts. name << “ left in the inventory. ” << endl;

Structures and Functions • Write a function to print all information about an inventory item

Structures and Functions • Write a function to print all information about an inventory item void print_item(inventory_item) { cout << “Item: “ << item. name << endl; cout << “Item Number: “ << item. number << endl; cout << “Cost: “ << item. cost << endl; cout << “Number Remaining: “ << item. num_in_inventory << endl; }

Structures and Functions • Call the function inventory_item shirts; … print_item(shirts);

Structures as Output Parameters • Write a function to “sell” an item by deducting 1 from the number of the item left in the inventory • Return 1 if the sell was successful – 0 otherwise • How would we call this function?

Structures as Output Parameters • How would we call this function? inventory_item shirts; … sell_item(shirts);

Structures as Output Parameters • How would we call this function? inventory_item shirts; int sell_ok; … sell_ok = sell_item(shirts); if(sell_ok) { printf(“Item soldn”); } else { printf(“Problem encountered! Item not sold. n”); }

Structures as Output Parameters int sell_item(inventory_item_t &to_sell) { int sell_ok; if((to_sell. num_in_inventory > 0) { to_sell. num_in_inventory = to_sell. num_in_inventory – 1; sell_ok = 1; } else { sell_ok = 0; } return (sell_ok); }

Arrays of Structures inventory_items[20]; • Access the name of item 5

Arrays of Structures inventory_items[20]; • Access the name of item 5 items[4]. name; items[4]. num_in_inventory = items[4]. num_in_inventory – 1; items[5]. cost = 105. 99; sell_item(items[10]);

Pointers to Structures • inventory_item *item = new inventory_item; • *item. name = “Hats”; //okay?

Pointers to Structures • inventory_item *item = new inventory_item; • *item. name = “Hats”; //okay? NO • (*item). name = “Hats”; • Alternative: -> operator • item->name = “Hats”;

Pointers to Structs with Pointers struct Student{ string *name; int id; }; Student *stu = new Student; stu->name = new string(“Jane”);