Fundamentals of Programming Session 23 Instructor: Reza Entezari-Maleki Email: entezari@ce.sharif.edu 1 Fall 2013 These slides have been created using Deitel’s slides Sharif University of Technology
Outlines C++ Inline Functions References and Reference Parameters Default Arguments Classes and Objects Defining a Member Function with a Parameter Data Members, set Functions and get Functions 2
C++ C++ improves on many of C’s features and provides object-oriented-programming (OOP) capabilities that increase software productivity, quality and reusability. This section revisits the addition program of Fig. 2.8 and illustrates several important features of the C++ language as well as some differences between C and C++. C file names have the .c (lowercase) extension. C++ file names can have one of several extensions, such as .cpp , .cxx or .C (uppercase). 3
Header Files The C++ Standard Library is divided into many portions, each with its own header file. The header files contain the function prototypes for the related functions that form each portion of the library. The header files also contain definitions of various class types and functions, as well as constants needed by those functions. Figure 15.2 lists common C++ Standard Library header files. 4
Header Files … 5
C++ … 6
Inline Functions Implementing a program as a set of functions is good from a software engineering standpoint, but function calls involve execution-time overhead. C++ provides inline functions to help reduce function call overhead — especially for small functions. The trade-off is that multiple copies of the function code are inserted in the program (often making the program larger) rather than there being a single copy of the function to which control is passed each time the function is called. The compiler can ignore the inline - qualifier and 7 typically does so for all but the smallest functions.
Inline Functions … 8
Inline Functions … 9
Inline Functions … Lines 4 – 6 are using statements that help us eliminate the need to repeat the std:: prefix. From this point forward, each C++ example contains one or more using statements. In place of lines 4 – 6, many programmers prefer to use using namespace std; C++ also provides type bool for representing boolean (true/false) values. The two possible values of a bool are the keywords true and false . 10
References and Reference Parameters Two ways to pass arguments to functions in many programming languages are pass-by-value and pass-by- reference. This section introduces reference parameters — the first of two means that C++ provides for performing pass-by- reference. A reference parameter is an alias for its corresponding argument in a function call. For example, the following declaration in a function header int int &count when read from right to left is pronounced ― count is a reference to an int . ‖ 11
References and Reference Parameters … In the function call, simply mention the variable by name to pass it by reference. Then, mentioning the variable by its parameter name in the body of the called function actually refers to the original variable in the calling function, and the original variable can be modified directly by the called function. As always, the function prototype and header must agree. Figure 15.5 compares pass-by-value and pass-by-reference with reference parameters. 12
References and Reference Parameters … 13
References and Reference Parameters … 14
References and Reference Parameters … References can also be used as aliases for other variables within a function (although they typically are used with functions as shown in Fig. 15.5). For example, the code in int cou count nt = = 1; ; // // decl eclare are int integ eger er var variabl able e cou count nt in int &cR cRef ef = c = count unt; ; // // cre create te cR cRef ef as as an a n ali lias as for for cou count nt cR cRef ef++; ++; // // inc increm remen ent c t coun ount (u (usi sing ng its its ali alias as cRef cRef) increments variable count by using its alias cRef . Reference variables must be initialized in their declarations, as we show in line 9 of both Fig. 15.6 and Fig. 15.7, and cannot be reassigned as aliases to other variables. 15
References and Reference Parameters … 16
References and Reference Parameters … 17
References and Reference Parameters … Returning references from functions can be dangerous. When returning a reference to a variable declared in the called function, the variable should be declared static within that function. Otherwise, the reference refers to an automatic variable that is discarded when the function terminates; such a variable is ―undefined‖ and the program’s behavior is unpredictable. References to undefined variables are called dangling references. 18
Default Arguments It’s not uncommon for a program to invoke a function repeatedly with the same argument value for a particular parameter. In such cases, the programmer can specify that such a parameter has a default argument, i.e., a default value to be passed to that parameter. When a program omits an argument for a parameter with a default argument in a function call, the compiler rewrites the function call and inserts the default value of that argument to be passed as an argument in the function call. 19
Default Arguments … Default arguments must be the rightmost (trailing) arguments in a function’s parameter list. When calling a function with two or more default arguments, if an omitted argument is not the rightmost argument in the argument list, then all arguments to the right of that argument also must be omitted. Default arguments should be specified with the first name — typically, occurrence of the function in the function prototype. If the function prototype is omitted because the function definition also serves as the prototype, then the default 20 arguments should be specified in the function header.
Default Arguments … 21
Default Arguments … 22
Classes and Objects In C++, we begin by creating a program unit called a class to house a function. A function belonging to a class is called a member function. In a class, you provide one or more member functions that are designed to perform the class’s tasks. You must create an object of a class before you can get a program to perform the tasks the class describes. That is one reason C++ is known as an object-oriented programming (OOP) language. Messages are sent to an object. Each message is known as a member-function call and tells a member function of the object to perform its task. 23 This is often called requesting a service from an object.
Classes and Objects … We begin with an example (Fig. 16.1) that consists of class GradeBook (lines 8 – 16) that an instructor can use to maintain student test scores, and a main function (lines 19 – 23) that creates a GradeBook object. Function main uses this object and its member function to display a message on the screen welcoming the instructor to the grade-book program. 24
Classes and Objects … 25
Classes and Objects … 26
Classes and Objects … Function main is always called automatically when you execute a program. Most functions do not get called automatically. You must call member function displayMessage explicitly to tell it to perform its task. public: The access-specifier label contains the keyword public is an access specifier. Indicates that the function is ―available to the public‖— that is, it can be called by other functions in the program (such as main ), and by member functions of other classes (if there are any). Access specifiers are always followed by a colon ( : ). 27
Classes and Objects … Each function in a program performs a task and may return a value when it completes its task. When you define a function, you must specify a return type to indicate the type of the value returned by the function when it completes its task. Keyword void to the left of the function name displayMessage is the function’s return type. Indicates that displayMessage will not return any data to its calling function when it completes its task. The name of the member function, displayMessage , follows the return type. By convention, function names begin with a lowercase first letter and all subsequent words in the name begin with a capital letter. 28
Classes and Objects … Empty parentheses indicate that a member function does not require additional data to perform its task. The first line of a function definition is commonly called the function header. Every function’s body is delimited by left and right braces ( { and } ). The body of a function contains statements that perform the function’s task. 29
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