CSCI 235

Reference Parameters ​

Chapter 6

Variable Aliases ​

The & can be used to create a new name (i.e., alias or reference) for an existing variable. For example:

cpp
int num = 5;
int& ref = num; // alias to num
int copy = num; // a copy of num

cout << num << '\n'; // outputs 5
cout << ref << '\n'; // outputs 5
cout << copy << '\n'; // outputs 5

ref = 10; // changes num to 10, because ref is just another name for num

cout << num << '\n'; // outputs 10
cout << ref << '\n'; // outputs 10
cout << copy << '\n'; // outputs 5, because it is a copy of the original

A reference must be initialized when it is declared, and it remains an alias for that same variable. Assigning a new value through the reference changes the variable; it does not make the reference refer to a different variable.

We use & not only for variables but also for function parameters. A reference parameter acts as another name for the actual parameter, so the function can modify the original variable.

Two Types of Function Parameters ​

There are two ways to send information to a function using parameters: Pass-by-Value and Pass-by-Reference.

  1. Value parameter: a formal parameter that receives a copy of the content of the corresponding actual parameter

    • The formal parameter has its own copy of the data.
    • During execution, the function manipulates the data stored in its own memory space.
    • The copy is lost when the function call ends.

  2. Reference parameter: a formal parameter that acts as another name for the corresponding actual parameter

    • Changes to the formal parameter will change the corresponding actual parameter.
      • The function works with the original variable, not a copy.
    • Reference parameters are useful in three situations:
      1. When a function needs to provide more than one result.
      2. When changing the actual parameter.
      3. When passing a large object by reference avoids copying it. Use a const reference when the function only needs to read the object; this avoids copying without allowing the function to modify it.

For example, only the reference parameter changes the variable in the calling function:

cpp
void addOneByValue(int number)
{
    number++;
}

void addOneByReference(int& number)
{
    number++;
}

int count = 5;
addOneByValue(count);     // count is still 5
addOneByReference(count); // count is now 6

A read-only function can use a const reference parameter:

cpp
void printName(const string& name)
{
    cout << name << '\n';
}

The function can read name but cannot change it.

Memory Allocation for Parameters ​

  • When a function is called, memory for its formal parameters and its local variables is allocated in the function data area.

  • For a value parameter, the actual parameter’s value is copied into the formal parameter’s memory cell.

    • Changes to the formal parameter do not affect the actual parameter’s value.

  • For a reference parameter, the formal parameter refers to the same variable as the actual parameter.

    • Both names refer to the same object.
    • During execution, changes made to the formal parameter’s value permanently change the actual parameter’s value.

Design Guideline

Reference parameters can be used to provide additional results from a function. For simple functions, prefer returning a value when that is enough; later, you will see other ways to return multiple related values (using structs).