C++ Primer Plus

Consider the function getname() in the program in Listing 4.22. It uses cin to place an input word into the temp array. Next, it uses new to allocate new memory to hold the word. Including the null character, the program needs strlen(temp) + 1 characters to store the string, so that’s the value given to new. After the space becomes available, getname() uses the standard library function strcpy() to copy the string from temp to the new block. The function doesn’t check to see whether the string fits, but getname() covers that by requesting the right number of bytes with new. Finally, the function returns pn, the address of the string copy.

In main(), the return value (the address) is assigned to the pointer name. This pointer is defined in main(), but it points to the block of memory allocated in the getname() function. The program then prints the string and the address of the string.

Next, after it frees the block pointed to by name, main() calls getname() a second time. C++ doesn’t guarantee that newly freed memory is the first to be chosen the next time new is used, and in this sample run, it isn’t.

Note in this example that getname() allocates memory and main() frees it. It’s usually not a good idea to put new and delete in separate functions because that makes it easier to forget to use delete. But this example does separate new from delete just to show that it is possible.

To appreciate some of the more subtle aspects of this program, you should know a little more about how C++ handles memory. So let’s preview some material that’s covered more fully in Chapter 9.

Automatic Storage, Static Storage, and Dynamic Storage

C++ has three ways of managing memory for data, depending on the method used to allocate memory: automatic storage, static storage, and dynamic storage, sometimes called the free store or heap. Data objects allocated in these three ways differ from each other in how long they remain in existence. We’ll take a quick look at each type. (C++11 adds a fourth form called thread storage that we’ll discuss briefly in Chapter 9.)

Automatic Storage

Ordinary variables defined inside a function use automatic storage and are called automatic variables. These terms mean that the variables come into existence automatically when the function containing them is invoked, and they expire when the function terminates. For example, the temp array in Listing 4.22 exists only while the getname() function is active. When program control returns to main(), the memory used for temp is freed automatically. If getname() returned the address of temp, the name pointer in main() would be left pointing to a memory location that would soon be reused. That’s one reason you have to use new in getname(). Actually, automatic values are local to the block that contains them. A block is a section of code enclosed between braces. So far, all our blocks have been entire functions. But as you’ll see in the next chapter, you can have blocks within a function. If you define a variable inside one of those blocks, it exists only while the program is executing statements inside the block.

Automatic variables typically are stored on a stack. This means that when program execution enters a block of code, its variables are added consecutively to the stack in memory and then are freed in reverse order when execution leaves the block. (This is called a last-in, first-out, or LIFO, process.) So the stack grows and shrinks as execution proceeds.

Static Storage

Static storage is storage that exists throughout the execution of an entire program. There are two ways to make a variable static. One is to define it externally, outside a function. The other is to use the keyword static when declaring a variable:

static double fee = 56.50;

Under K&R C, you can initialize only static arrays and structures, whereas C++ Release 2.0 (and later) and ANSI C allow you to initialize automatic arrays and structures, too. However, as you may have discovered, some C++ implementations do not yet implement initialization for automatic arrays and structures.

Chapter 9 discusses static storage in more detail. The main point you should note now about automatic and static storage is that these methods rigidly define the lifetime of a variable. Either the variable exists for the entire duration of a program (a static variable) or it exists only while a particular function is being executed (an automatic variable).

Dynamic Storage