Question

Give an example of a C++ code fragment that performs an array reference that is possibly out of bounds, and if it is out of bounds, the program catches that exception and prints an appropriate error message.

Short answer

Use try-catch blocks to handle out-of-bounds errors when accessing array elements in C++.

Step-by-step solution

Initialize the Array and Variables

Create an integer array and a variable to hold the index for referencing the array. For simplicity, the array can have a fixed size of 5.

Start a Try Block

Begin a try block using the `try` keyword. This will enable catching exceptions that might occur during array access.

Attempt Array Access

Within the try block, attempt to access an element of the array using the index variable. This is where the potential out-of-bounds access might occur.

Implement Custom Exception Handling

Use a catch block to handle any exceptions. In the catch block, print an error message indicating that the index is out of bounds.

Key concepts

Array Access

In C++, arrays are a collection of elements stored sequentially in memory. An array can be defined with a fixed size, for example, int arr[5]; creates an integer array with five elements. Each element can be accessed via an index starting from 0 up to the array size minus one. So, for the array arr[5], valid indices are 0 through 4.

Accessing elements is straightforward using the following syntax: arr[index]. For instance, arr[2] gets the third element. Accessing an array element beyond these valid indices can cause undefined behavior or errors.

Out of Bounds Error

An 'out of bounds' error occurs when an attempt is made to access an element outside the valid range of indices. For example, arr[6] for an array arr[5] is out of bounds because valid indices are from 0 to 4. This can result in unexpected behavior or a runtime error.

In C++, accessing an out-of-bounds index does not directly throw an exception. Instead, it results in undefined behavior, which could mean accessing and modifying another area's memory, leading to potential program crashes and security issues.

Try and Catch Blocks

C++ provides a powerful mechanism to handle exceptions, ensuring the program behaves predictably even when errors occur. This is achieved using the try and catch blocks.
wrap any code that might throw an exception in a try block. Immediately following this block, you provide one or more catch blocks to handle specific types of exceptions and maintain program stability.

Here's an example of how to use these blocks in case of array access:

 int arr[5] = {1, 2, 3, 4, 5}; int index = 6; // Potential out-of-bounds access try { int value = arr[index]; } catch (const std::out_of_range& e) { std::cout << 'Index out of bounds error: ' << e.what() << std::endl; }

Error Message Handling

Handling errors efficiently ensures a seamless user experience and stable application behavior. When an out-of-bounds access is detected inside a catch block, it's crucial to inform the user about the nature of the error clearly.

Use descriptive and user-friendly messages so an end-user understands what went wrong and possibly how to fix it. For instance, printing 'Index out of bounds. Please provide an index between 0 and 4.' is much clearer than a generic error message.

Here's how you might handle the error message in your code:

try { int value = arr[index]; } catch (const std::out_of_range& e) { std::cout << 'Index out of bounds. Please provide an index between 0 and 4.' << std::endl; }