Tài liệu Module 7 More Data Types and Operators docx

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Each of the other specifiers is examined here.
auto
The auto specifier declares a local variable. However, it is rarely (if ever) used, because local variables
are auto by default. It is extremely rare to see this keyword used in a program. It is a holdover from the
C language.
CRITICAL SKILL 7.2: extern
All the programs that you have worked with so far have been quite small. However, in reality, computer
programs tend to be much larger. As a program file grows, the compilation time eventually becomes
long enough to be annoying. When this happens, you should break your program into two or more
separate files. Then, changes to one file will not require that the entire program be recompiled. Instead,
you can simply recompile the file that changed, and link the existing object code for the other files. The
multiple file approach can yield a substantial time savings with large projects. The extern keyword helps
support this approach. Let’s see how.
In programs that consist of two or more files, each file must know the names and types of the global
variables used by the program. However, you cannot simply declare copies of the global variables in
each file. The reason is that your program can only have one copy of each global variable. Therefore, if
you try to declare the global variables needed by your program in each file, an error will occur when the
linker tries to link the files. It will find the duplicated global variables and will not link your program. The
solution to this dilemma is to declare all of the global variables in one file and use extern declarations in
the others, as shown in Figure 7-1.
File One declares x, y, and ch. In File Two, the global variable list is copied from File One, and the extern
specifier is added to the declarations. The extern specifier allows a variable to be made known to a
module, but does not actually create that variable. In other words, extern lets the compiler know what
the types and names are for these global variables without actually creating storage for them again.
When the linker links the two modules together, all references to the external variables are resolved.
While we haven’t yet worried about the distinction between the declaration and the definition of a
variable, it is important here. A declaration declares the name and type of a variable. A definition causes
storage to be allocated for the variable. In most cases, variable declarations are also definitions.
However, by preceding a variable name with the extern specifier, you can declare a variable without
defining it.

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A variation on extern provides a linkage specification, which is an instruction to the compiler about how
a function is to be handled by the linker. By default, functions are linked as C++ functions, but a linkage
specification lets you link a function for a different type of language. The general form of a linkage
specifier is shown here:
extern “language” function-prototype
where language denotes the desired language. For example, this specifies that myCfunc( ) will have C
linkage:
extern "C" void myCfunc();
All C++ compilers support both C and C++ linkage. Some may also allow linkage specifiers for FORTRAN,
Pascal, or BASIC. (You will need to check the documentation for your compiler.) You can specify more
than one function at a time using this form of the linkage specification:
extern “language” { prototypes
}
For most programming tasks, you won’t need to use a linkage specification.
CRITICAL SKILL 7.3: static Variables
Variables of type static are permanent variables within their own function or file. They differ from global
variables because they are not known outside their function or file. Because static affects local variables
differently than it does global ones, local and global variables will be examined separately.
static Local Variables
When the static modifier is applied to a local variable, permanent storage for the variable is allocated in
much the same way that it is for a global variable. This allows a static variable to maintain its value
between function calls. (That is, its value is not lost when the function returns, unlike the value of a

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normal local variable.) The key difference between a static local variable and a global variable is that the
static local variable is known only to the block in which it is declared.
To declare a static variable, precede its type withthe word static. For example, this statement declares
count as a static variable:
static int count;
A static variable may be given an initial value. For example, this statement gives count an initial value of
200:
static int count = 200;
Local static variables are initialized only once, when program execution begins, not each time the block
in which they are declared is entered.
The static local variable is important to functions that must preserve a value between calls. If static
variables were not available, then global variables would have to be used—opening the door to possible
side effects.
To see an example of a static variable, try this program. It keeps a running average of the numbers
entered by the user.

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Here, the local variables sum and count are both declared as static and initialized to 0. Remember, for
static variables the initialization only occurs once—not each time the function is entered. The program
uses running_avg( ) to compute and report the current average of the numbers entered by the user.
Because both sum and count are static, they will maintain their values between calls, causing the
program to work properly. To prove to yourself that the static modifier is necessary, try removing it and
running the program. As you can see, the program no longer works correctly, because the running total
is lost each time running_avg( ) returns.
static Global Variables
When the static specifier is applied to a global variable, it tells the compiler to create a global variable
that is known only to the file in which the static global variable is declared. This means that even though
the variable is global, other functions in other files have no knowledge of it and cannot alter its contents.
Thus, it is not subject to side effects. Therefore, for the few situations where a local static variable
cannot do the job, you can create a small file that contains only the functions that need the global static
variable, separately compile that file, and use it without fear of side effects. For an example of global

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static variables, we will rework the running average program from the preceding section. In this version,
the program is broken into the two files shown here. The function reset( ), which resets the average, is
also added.



Here, sum and count are now global static variables that are restricted to the second file. Thus, they can
be accessed by both running_avg( ) and reset( ) in the second file, but not elsewhere. This allows them

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to be reset by a call to reset( ) so that a second set of numbers can be averaged. (When you run the
program, you can reset the average by entering –2.) However, no functions outside the second file can
access those variables. For example, if you try to access either sum or count from the first file, you will
receive an error message.
To review: The name of a local static variable is known only to the function or block of code in which it is
declared, and the name of a global static variable is known only to the file in which it resides. In essence,
the static modifier allows variables to exist that are known to the scopes that need them, thereby
controlling and limiting the possibility of side effects. Variables of type static enable you, the
programmer, to hide portions of your program from other portions. This can be a tremendous
advantage when you are trying to manage a very large and complex program.
Ask the Expert
Q: I have heard that some C++ programmers do not use static global variables. Is this true?
A: Although static global variables are still valid and widely used in C++ code, the C++ Standard
discourages their use. Instead, it recommends another method of controlling access to global variables
that involves the use of namespaces, which are described later in this book. However, static global
variables are widely used by C programmers because C does not support namespaces. For this reason,
you will continue to see static global variables for a long time to come.
CRITICAL SKILL 7.4: register Variables
Perhaps the most frequently used storage class specifier is register. The register modifier tells the
compiler to store a variable in such a way that it can be accessed as quickly as possible. Typically, this
means storing the variable either in a register of the CPU or in cache memory. As you probably know,
accessing the registers of the CPU (or cache memory) is fundamentally faster than accessing the main
memory of the computer. Thus, a variable stored in a register will be accessed much more quickly than if
that variable had been stored in RAM. Because the speed by which variables can be accessed has a
profound effect on the overall speed of your programs, the careful use of register is an important
programming technique.
Technically, register is only a request to the compiler, which the compiler is free to ignore. The reason
for this is easy to understand: there are a finite number of registers (or fast-access memory), and these
may differ from environment to environment. Thus, if the compiler runs out of fast-access memory, it
simply stores the variable normally. Generally, this causes no harm, but of course the register advantage
is lost. You can usually count on at least two variables being optimized for speed. Since only a limited
number of variables can actually be granted the fastest access, it is important to choose carefully those
to which you apply the register modifier. (Only by choosing the right variables can you gain the greatest
increase in performance.) In general, the more often a variable is accessed, the more benefit there will

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be to optimizing it as a register variable. For this reason, variables that control or are accessed within
loops are good candidates for the register specifier.
Here is an example that uses register variables to improve the performance of the summation( )
function, which computes the summation of the values in an array. This example assumes that only two
variables will actually be optimized for speed.

Here, the variable i, which controls the for loop, and sum, which is accessed inside the loop, are
specified as register. Since they are both used within the loop, both benefit from being optimized for
fast access. This example assumed that only two variables could actually be optimized for speed, so n
and nums were not specified as register because they are not accessed as often as i and sum within the
loop. However, in environments in which more than two variables can be optimized, they too could be
specified as register to further improve performance.

1. A static local variable ___________ its value between function calls.

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2. You use extern to declare a variable without defining that variable. True or false?
3. What specifier requests that the compiler optimize a variable for speed?

Ask the Expert
Q: When I tried adding the register specifier to a program, I saw no change in performance. Why not?
A: Because of advances in compiler technology, most compilers today will automatically optimize
your code. Thus, in many cases, adding the register specifier to a declaration might not result in any
performance increase because that variable is already optimized. However, in some cases, using register
is still beneficial because it lets you tell the compiler which variables you think are the most important to
optimize. This can be valuable for functions that use a large number of variables, all of which cannot be
optimized. Thus, register still fulfills an important role despite advances in compiler design.
CRITICAL SKILL 7.5: Enumerations
In C++, you can define a list of named integer constants. Such a list is called an enumeration. These
constants can then be used anywhere that an integer can. Enumerations are defined using the keyword
enum and have this general format:
enum type-name { value-list } variable-list;
Here, type-name is the type name of the enumeration. The value-list is a comma-separated list of names
that represent the values of the enumeration. The variable-list is optional because variables may be
declared later using the enumeration type name.
The following fragment defines an enumeration called transport and two variables of type transport
called t1 and t2:
enum transport { car, truck, airplane, train, boat } t1, t2;
Once you have defined an enumeration, you can declare additional variables of its type using its name.
For example, this statement declares one variable, called how, of enumeration transport:
transport how;
The statement can also be written like this:
enum transport how;
However, the use of enum here is redundant. In C (which also supports enumerations), this second form
was required, so you may see it used in some programs.
Assuming the preceding declarations, the following gives how the value airplane:

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how = airplane;
The key point to understand about an enumeration is that each of the symbols stands for an integer
value. As such, they can be used in any integer expression. Unless initialized otherwise, the value of the
first enumeration symbol is 0, the value of the second symbol is 1, and so forth. Therefore,
cout << car << ' ' << train;
displays 0 3.
Although enumerated constants are automatically converted to integers, integers are not automatically
converted into enumerated constants. For example, the following statement is incorrect:
how = 1; // Error
This statement causes a compile-time error because there is no automatic conversion from integer to
transport. You can fix the preceding statement by using a cast, as shown here:
how = (transport) 1; // now OK, but probably poor style
This causes how to contain the value truck, because it is the transport constant associated with the
value 1. As the comment suggests, although this statement is now correct, it would be considered to be
poor style except in unusual circumstances.
It is possible to specify the value of one or more of the enumerated constants by using an initializer. This
is done
by
following the symbol with an equal sign and an integer value. When an initializer is used, each symbol
that appears after it is assigned a value 1 greater than the previous initialization value. For example, the
following statement assigns the value of 10 to airplane:
enum transport { car, truck, airplane = 10, train, boat };
Now, the values of these symbols are as follows:





car
0
truck
1
airplane
10
train
11
boat
12


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One common, but erroneous, assumption sometimes made about enumerations is that the symbols can
be input and output as a string. This is not the case. For example, the following code fragment will not
perform as desired:
// This will not print "train" on the screen. how = train; cout << how;
Remember, the symbol train is simply a name for an integer; it is not a string. Thus, the preceding code
will display the numeric value of train, not the string “train”. Actually, to create code that inputs and
outputs enumeration symbols as strings is quite tedious. For example, the following code is needed in
order to display, in words, the kind of transportation that how contains:

Sometimes it is possible to declare an array of strings and use the enumeration value as an index in
order to translate the value into its corresponding string. For example, the following program prints the
names of three types of transportation:

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