Teach Yourself Borland C++ in 14 Days - portal

70 Day 3
NEW TERM
NEW TERM
Dynamic allocation means that memory required for an object is allocated from the
heap.
The heap in a Windows program refers to all of your computer’s virtual memory.
Dynamic Allocation and Pointers
NEW TERM
In a C++ program, memory is allocated dynamically by using the new operator.
I’m going to talk about new a little later in the chapter, but you need a little sampler as I
continue the discussion about pointers. Earlier I talked about structures and used the
mailingListRecord structure as an example. Allocating a structure from the stack looks like
this:
mailingListRecord listArray;
strcpy(listArray.firstName, “Ian”);
strcpy(listArray.lastName, “Spencer”);
// etc.
That’s what I did earlier when I talked about structures. Now I’ll create the array dynamically
rather than locally:
mailingListRecord* listArray;
listArray = new mailingListRecord;
strcpy(listArray->firstName, “Ian”);
strcpy(listArray->lastName, “Spencer”);
// etc.
The first line declares a pointer to a mailingListRecord structure. The next line initializes the
pointer by creating a new instance of a mailingListRecord structure dynamically. This is the
process by which you dynamically create and access objects in C++.
And Now Back to Our Program
Now you begin to see where pointers fit into the scheme of things. When you create an object
dynamically, the new operator returns a pointer to the object in memory. You need that
pointer to be able to do anything with the object. Figure 3.1 illustrates how the pointer points
to the object in memory. Note that although the memory for the dynamically created object
is allocated from heap memory, the actual pointer is a local variable and is allocated from the
stack.