Parameters in C++

Parameters in C++
An Introduction for Java Programmers

Prof. David Bernstein
James Madison University

Computer Science Department

bernstdh@jmu.edu

Terminology

Formal Parameters:
- The parameters in the specification/declaration of the function
Actual Parameters:
- The parameters in the execution/invocation of the function

Basics

Call by Value:
- A copy of the variable is passed into the function.
- The value of the copy can be changed but the changes are not "passed back to" the caller
Call by Reference:
- A reference is passed to the function
- Any changes made are made to the variable being referred to (and, hence, are effectively "passed back to" the caller)

Syntax

Specification/Declarations:
- type name([type param]...) // Specification for "Call by Value"
- type name([type &param]...) // Specification for "Call by Reference"
Executing/Invoking/Calling a Function:
- name([param]...) // void return type
- result = name([param]...) // non-void return type

Constant Arguments

Constant call by value:
- A copy of the variable is passed into the function and the value of the copy can not be changed
- Syntax: name(const type param)
Constant call by reference:
- A reference is passed to the function and the value of the variable being "referred to" cannot be changed
- This is preferred to using call by value when passing classes (and structs) because it is faster (i.e., the system does not have to make a copy of the object)
- Syntax: name(const type &param)

Passing Pointers

An Observation:
- It is possible to pass a pointer to an object by value
Implications:
- A copy of the pointer is made and passed to the function
- The function has access to the members of the object being pointed to but cannot change the pointer itself

Another Use of the const Qualifier

Syntax:
- classname::function( ... ) const
Purpose:
- Prevents the method from making changes to the attributes of the object

An Example:

        void Account::printout() const
        {
          cout << "Actual Balance: " << balance;

          balance = 0.00; // Generates a compile-time error
        }

Copy Constructors Reconsidered

Recall:
- A default implementation is provided by the compiler (but it will only work properly for simple classes)
An Observation:
- A working copy constructor is necessary for an object to be passed by value
An Implication:
- You should always include an implementation of a working copy constructor

The Return

Return by Value:
- A copy of the value is made and returned
Return by Reference:
- A reference is returned (which can lead to serious problems if a local variable is returned since the local variable goes out of scope immediately after the return and the reference may, as a result, not refer to anything)