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When run on a Windows workstation, this program generates the alert panel shown here. This little window is reporting that the program attempted to access memory location 0040108e. That location is outside the segment of memory that was allocated to the process that is running the program. So the Windows operating system aborted the program.

The run-time error that occurred in Example 6.8 is called an unhandled exception because there is no code in the program to respond to the error. It is possible to include code in C++ programs so that the program won t crash. Such code is called an exception handler. Unlike some other programming languages (e.g., Pascal and Java), the Standard C++ compiler will not allow arrays to be assigned and it will not restrict array indexes from exceeding their bounds. It is the programmer s responsibility to prevent these compile-time and run-time errors. The reward for this extra responsibility is faster, more efficient code. If those benefits are not important to your application, then you should use Standard C++ vector objects instead of arrays. (See 10.) 6.5 PASSING AN ARRAY TO A FUNCTION The code float a[] that declares an array a in the previous examples tells the compiler two things: the name of the array is a, and the array s elements have type float. The symbol a stores the array s memory address. So the code float a[] provides all the information that the compiler needs to declare the array. The size of the array (i.e., the number of elements in the array) does not need to be conveyed to the compiler. C++ requires the same information to be passed to a function that uses an array as a parameter. EXAMPLE 6.9 Passing an Array to a Function that Returns its Sum

int sum(int[],int); int main() { int a[] = { 11, 33, 55, 77 }; int size = sizeof(a)/sizeof(int); cout << "sum(a,size) = " << sum(a,size) << endl; } int sum(int a[], int n) { int sum=0; for (int i=0; i<n; i++) sum += a[i]; return sum; } sum(a,size) = 176 The function s parameter list is (int a[], int n). The function prototype, which is used to declare the function above main() , uses (int[],int); this is the same as in the prototype except that the names of the parameters are omitted. (They can be included.) The function call, which occurs inside main(), uses sum(a,size); this lists the names of the parameters without their types. Note that the actual name of the type for the object a is int[].

When an array is passed to a function, as in the call sum(a,size) in Example 6.9, the value of array name a is actually the memory address of the first element (a[0]) in the array. The function uses that address value to access and possibly modify the contents of the array. So passing an array to a function is similar to passing a variable by reference: the function can change the values of the array s elements. This is illustrated in the next example. EXAMPLE 6.10 Input and Output Functions for an Array

This program uses a read() function to input values into the array a interactively. Then it uses a print() function to print the array: void read(int[],int&); void print(int[],int); int main() { const int MAXSIZE=100; int a[MAXSIZE]={0}, size; read(a,size); cout << "The array has " << size << " elements: "; print(a,size); } void read(int a[], int& n) { cout << "Enter integers. Terminate with 0:\n"; n = 0; do { cout << "a[" << n << "]: "; cin >> a[n]; } while (a[n++] != 0 && n < MAXSIZE); --n; // don't count the 0 } void print(int a[], int n) { for (int i=0; i<n; i++) cout << a[i] << " "; } Enter integers. Terminate with 0: a[0]: 11 a[1]: 22 a[2]: 33 a[3]: 44 a[4]: 0 The array has 4 elements: 11 22 33 44 The read() function changes the values of the array a and the value of the size parameter n. Since n is a scalar variable, it must be passed by reference to allow the function to change its value. Since a is an array variable, it must be passed by value and the function is able to change the values its elements.

Note that the size of the array has to be passed explicitly to the function that processes the array. In C++ a function is unable to compute the size of an array passed to it. Example 6.10 shows that a function can change the values of an array s elements even though the array variable is passed by value. That is possible because the value of the array variable itself is the memory address of the first element of the array. Passing the value of that address to the function gives the function all the information it needs to access and change that part of memory where the array is stored. This is accomplished by a direct calculation of the elements