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Subroutines/Functions in C++

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• Types of functions in C++
  • There are two kinds of functions (just like Java) in C++:

    Member functions that are defined inside a class These functions (always) have an implict (hidden) parameter that has the type of that class A member function in C++ is the equivalent of the member function in Java Stand-alone functions that are not defined inside any class These functions do not have the implict (hidden) parameter A member function in C++ is the equivalent of the class (static) function in Java

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  • We will discuss the "stand-alone" functions here.

    The member functions are very similar to the ones in Java, so you should be familiar with them already

    The member functions will be discussed later when we discuss classes

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  • Nomenclature:

    Function means "stand-alone" functions In fact, the term stand-alone function does not exist in C++ literature I coined it to high-light the difference between a member function and a non-member function A non-member function in C++ is usually called an "ordinary" function

    From now on, I will not used my coined phrase stand-alone function, but simply: function...

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• Defining Functions in C++
  • Function definition is the creation of a function.

    The function definition contain all the information on a function:

    Its name (how it identify the function) The number and type of parameters (what it needs to do its work) The type of output (return) value produced What does it do (the code of the function)

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  • Syntax used to define a function:

    ReturnType MySubrName(type1 Var1, type1 Var2, ...) { *** LOCAL Variable definitions *** Subroutine Body (statements) }

    Basically, it is the same syntax as in Java, except the function definition is not contained in any class

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  • Example:

    double cube( double x ) { return( x * x * x ); }

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• Invoking a function
  • The syntax to invoke a function in C++ is the same as Java, except:

    There is no classname prefix before the function name in the function invocation syntax

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  • Example:

    double r, x; x = 4; r = cube(x);

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  • Example Program: (Demo above code)                                                
    • Prog file: click here

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• The need to declare a function
  • Consider the following program (same as the demo prog above, but the order of the functions is switched around)

    int main(int argc, char *argv[]) { double r, x; x = 4; r = cube(x); // Error: cube() undefined !!! } double cube( double x ) { return( x * x * x ); }

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  • Example Program: (Demo above code)                                                
    • Prog file: click here

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  • The reason the program fails to compile is the violation of the (what I call) "prime directive":

    Every identifier in a C++ program must be defined or declared before it can be used

    To make the above program work (without re-arranging the order), we must declare the function cube() before it is used:

    int main(int argc, char *argv[]) { double r, x; ****** Declare cube() x = 4; r = cube(x); // Error: cube() undefined !!! } double cube( double x ) { return( x * x * x ); }

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• Declaring functions in C++

  • Function declaration is the description of a function

    The description is the information that the C++ compiler need to know in order to translate the program

  • The compiler needs the following information:

    The name of the function (you need to find the function) The number and type of parameters (you need to know how many value and what kind to give the function) The type of output value produced (you need to know what to do with the output - e.g., possible conversion)

    Examples: function declaration

    [extern] double cube( double ) ; cube() has one parameter [extern] int myFunc( int, short, double ); myFunc() has three parameters

    The use of extern in a function declaration is optional.

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  • Example:

    int main(int argc, char *argv[]) { double r, x; double cube(double); // ****** Declare cube() x = 4; r = cube(x); // Error: cube() undefined !!! } double cube( double x ) { return( x * x * x ); }

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  • Example Program: (Demo above code)                                                
    • Prog file: click here

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  • NOTE:

    C++ allows the user to include dummy parameter names in a function declaration Example: double cube(double x); instead of: double cube(double);

    It "looks nicer"...

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• Multiple program files
  • Recall that variable declaration was necessary to compile a program that consists of multiple program files - see: click here

  • This is also true for functions

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  • Example:

    File 1: cube.C	File 2: main.C
    double cube( double x ) { return( x * x * x ); }	int main(int argc, char *argv[]) { double r, x; double cube(double); // Declare x = 4; r = cube(x); }