CS106B'Handout'
Functions)and)Pass)by)Reference)
Function Prototypes
As we learned in lecture, C++ computations are carried out in the context of functions. A function is a
named section of code that performs a specific operation. Function prototypes tell the compiler
everything it needs to know about how to call each function when it appears in code. C++ requires
prototype declarations so the compiler can check whether calls to functions are compatible with the
function definitions.
Function Definitions
Programs should be broken down into several smaller functions. Good decomposition leads to code
that is clear, logical and easy to understand. All functions have a body that consists of a return, a
name and potentially parameters. Here is a simple function example:
int$absoluteValue(int$n)${
$$if$(n$<$0)${
$$$$return$-n;
$$}
$$return$n;
}
return:$int
name:$absoluteValue
parameters:$n
Pass by Value
In C++, whenever you pass a variable from one function to another as a parameter, the function gets
a copy of the variable. Assigning a new value to the parameter as part of the function changes the
local copy but has no permanent effect on the variable. Consider the following example:
void$setToZero(int$n)${
$$n$=$0;
}
After the function finishes, n will no longer equal
zero. It will only equal zero in the scope of this
function.
Pass by Reference
In C++, if you declare a parameter with an ampersand (&) after its type, instead of passing a copy of
its value, it will link the caller and callee functions to the same variable in memory. Passing a variable
by reference ensures that any changes made to the variable will persist outside of the scope of the
function. This style of parameter passing is generally used when a function needs to return more than
one value to the calling program. Let’s revisit the setToZero function:
void$setToZero(int&$n)${
$$n$=$0;
}
After the function finishes, n will still be equal to
zero. The caller of setToZero will see n as zero
after the function returns.
