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Using sizeof to Ensure Portability
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You have seen that structures and unions can be used to create variables of varying sizes, and that the actual size of these variables may change from machine to machine The sizeof unary operator computes the size of any variable or type and can help eliminate machine-dependent code from your programs It is especially useful where structures or unions are concerned
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Borland C++ Builder: The Complete Reference
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For the discussion that follows, keep in mind that C++ Builder has the following sizes for these data types:
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char short int int long int float double long double
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1 2 4 4 4 8 10
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Therefore, the following code will print the numbers 1, 4, 4, and 10 on the screen:
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char ch; int i; float f; printf("%d\n", printf("%d\n", printf("%d\n", printf("%d\n",
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The size of a structure is equal to or greater than the sum of the sizes of its members For example,
struct s { char ch; int i; float f; } s_var;
Here, the sum of the sizes of the individual members is 9 (4+4+1) However, the actual size of s_var might be greater because the compiler is free to align data on word (or paragraph) boundaries This means that the size of an aggregate data type (such as a structure) may be slightly larger than the sum of its parts Manually adding up the lengths of the structure members, for example, may not yield its correct size For C++
sizeof ch); sizeof i); sizeof f); sizeof(long double));
AM FL Y
7:
Structures, Unions, and User-Defined Types
Builder, the size of s_var is 12, because of the reasons just stated Therefore, for maximum portability, you should always use sizeof to determine the size of a structure variable Since the sizeof operator is a compile-time operator, all the information necessary to compute the size of any variable is known at compile time This is especially meaningful for unions because the size of a union is always equal to the size of its largest member For example, consider the following:
union u { char ch; int i; float f; } u_var;
THE FOUNDATION OF C++
The sizeof(u_var) will be 4 bytes long At run time, it does not matter what u_var is actually holding; all that matters is the size of the largest variable it can hold, because the union must be as large as its largest element
typedef
C/C++ allows you to define new data type names using the typedef keyword You are not actually creating a new data type; you are defining a new name for an existing type This process can help make machine-dependent programs more portable; only the typedef statements need to be changed It also can help you document your code by allowing descriptive names for the standard data types The general form of the typedef statement is typedef type newname; where type is any existing data type and newname is the new name for this type The new name you define is an addition to, not a replacement for, the existing type name For example, you could create a new name for float by using
typedef float balance;
This statement tells the compiler to recognize balance as another name for float Next you could create a float variable using balance:
balance past_due;
Here, past_due is a floating-point variable of type balance, which is another word for float
Borland C++ Builder: The Complete Reference
You can also use typedef to create names for more complex types For example:
typedef struct { float due; int over_due; char name[40]; } client; /* here client is the new type name */ client clist[NUM_CLIENTS]; /* define array of structures of type client */
Using typedef can help make your code easier to read and more portable But remember, you are not creating any new data types
8
Input, Output, Streams, and Files
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Borland C++ Builder: The Complete Reference
he C language does not define any keywords that perform input or output Instead, I/O is accomplished through library functions C++ Builder supports three I/O systems:
I The ANSI/ISO Standard C I/O system I The UNIX-like I/O system I Several low-level, platform-specific I/O functions
With a few exceptions, this chapter discusses only the I/O system defined by the ANSI/ISO standard for C The reason for this is twofold First, the ANSI/ISO C I/O system is the most widely used Second, it is fully portable to all platforms The functions that compose the other two systems are covered in Part Two of this book This chapter presents an overview of the ANSI/ISO C I/O system and illustrates the way its core functions work together The ANSI/ISO C I/O library contains a rich and diverse assortment of I/O routines more than can be fully covered here However, the functions in this chapter are sufficient for most circumstances From this point forward, we will refer to the ANSI/ISO C I/O system as simply the C I/O system The prototypes and several predefined types and constants for the C I/O library functions are found in the file <stdioh>
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