Function (C++)

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Contents 1 Overview 2 Function Declaration 2.1 Default Parameters 2.2 Function Overloading 3 Function Definition 4 Invoking Functions in Other Files 5 Parameter Passing 5.1 Call-By-Value 5.2 Call-By-Reference 5.3 Call-By-Constant Reference 6 Main Function

Overview

• C++ provides functions as well as object orientated programming
• entities may exist outside of any object
  • it is possible to define functions outside of any class
• a function is called by name
  • e.g. functionx();
• each function consists of a function prototype and function definition

Function Declaration

   <return type> <function name> ( <parameter list> );

where

• <return type> is a data type
• <function name> is the name of the function
• <parameter list> is a comma separated list of parameters in the format:
  • <data type> <parameter name>

There is no function body, and the prototype ends with a semi-colon, no brace block

Default Parameters

• default values for parameters can be specified in the function declaration

   int printChar(string pString, int position==1); 

Function Overloading

• several functions can have the same name as long as the signature of the functions are different

signature

the function declaration without the return type

Function Definition

   <return type> <function name> ( <parameter list> ){
   
       <function body>
   }

where

• <return type> is a data type
• <function name> is the name of the function
• <parameter list> is a comma separated list of parameters in the format:
  • <data type> <parameter name>
• <function body> is the statements that perform the function and return the correct data type.

Invoking Functions in Other Files

• this is valid so long as the declaration is visible
• we can define functions using 2 files
  • a .hpp file
    • a header file containing the function declaration
  • a .cpp file
    • contains the function definition
• any file can use a function by including the the header file

Parameter Passing

Call-By-Value

• C++ uses call-by-value by default
  • argument values are copied into the parameter
    • hence the original argument is unaffected by changes
  • when objects are passed
    • a copy of the object is made, not a copy of the reference as in Java

Call-By-Reference

• a reference variable is an alias of an entity and is declared by using &
• mostly used with functions
  • more efficient
    • just a reference to an object is copied, the object referred to is not copied
  • allows the actual arguments to be changed

• it is illegal to pass a constant to a reference parameter
• actual arguments must be type compatible with formal parameters without typecast

Call-By-Constant Reference

• guarantees the argument is not changed when passed by call-by-reference
• used when call-by-value is not suitable due to large objects
  • call-by-constant reference is more efficient

• use both const and & modifiers

Main Function

• each C++ program must have a main function

2 formats:

no parameters

   int main () 
       { return 0; }

2 parameters

       int main (int argc, char *argv[]){
           
           cout << "Run program" << argv[0] << endl;
           return 0; 
       }

argc

the total number of arguments from the command line, including the program name

argv[]

an array of pointers to characters, which stores the arguments as strings