| Lesson 8 || Free store operators |
| Objective || Understand the use of the free store operators new and delete. |
C++ Free Store Operators
The unary operators
delete are available to manipulate free store, which is a system-provided memory pool for variables whose lifetime is directly managed by the programmer.
You create the variable by using
new and destroy the object by using
delete. Manipulating free store memory is important for dynamic data structures such as lists and trees. We will look at
delete in the next two lessons. In order to request dynamic memory we use the operator new.
new is followed by a data type specifier and, if a sequence of more than one element is required,
the number of these within brackets . It returns a pointer to the beginning of the new block of memory allocated. Its form is:
pointer = new type
pointer = new type [number_of_elements]
The first expression is used to allocate memory to contain one single element of type type.
The second one is used to assign a block (an array) of elements of type type, where number_of_elements is an integer value representing the amount of these.
The new operator allocates memory from an area called the free store (also known as dynamic memory and heap).
Objects allocated on the free store are independent of the scope from which they are created and "live" until they are destroyed using the delete operator. Vector's constructor allocates some memory on the free store using the
Class hierarchies are different: we tend to allocate them on the free store using new, and we access them through pointers or references. In addition to the initialization of named objects and objects on the free store, constructors are used to initialize temporary objects and to implement explicit type conversion.
There are many situations where stack memory is inappropriate. It is often difficult or impossible to estimate beforehand how large an object should be. For example, how many elements an array needs to contain.
It may also be difficult to determine how many items a program might require. For example, how many nodes will be
contained in a linked list. Finally, it is also common that the lifetime of a value is not tied to procedure entry and exit. For example, when a value is placed into a linked list, the value will continue to exist even after
the insertion function finishes execution. In such cases, local variables on the stack cannot be used, and dynamic allocation of storage is necessary. The heap or free store is the storage area for values explicitly requested using the new operator.
Employee* boss = new Employee("Lin, Lisa", 68000);
C++ How to Program
When you ask for a section of memory using this operator, a memory allocator finds a storage location for the new object in the heap. The memory allocator tells you where the object is located by returning the memory address for the value.
This is termed dynamic memory allocation. Dynamically allocated values are accessed through a pointer, which itself might reside either on the stack or on the heap. The statement above declares a pointer variable named boss that resides on the stack.
The value of the pointer references a data area stored on the heap. Once you are finished with the dynamically allocated memory you must notify the memory allocator that it can be returned to the free store. This is done using the delete operator:
This statement deletes the heap memory that variable boss refers to.