Question

Consider the following \(\mathrm{C}++\) code: double balance; try { cout << "Enter the balance: "; cin >> balance; cout << endl; if (balance < 1000.00) throw balance; cout << "Leaving the try block." << endl; } catch (double x) { cout << "Current balance: " << x << endl << "Balance must be greater than 1000.00" << endl;} a. In this code, identify the try block. b. In this code, identify the catch block. c. In this code, identify the catch block parameter and its type. d. In this code, identify the throw statement.

Short answer

a. Try block: Lines 2-9. b. Catch block: Lines 10-13. c. Catch parameter: `double x`. d. Throw statement: `throw balance;`.

Step-by-step solution

Identifying the Try Block

The try block in C++ is initiated with the keyword `try` and ends before the corresponding `catch` block. In the provided code, the try block starts at line 2 (after the `try` keyword) and ends at line 9 (before the `catch` is called).

Identifying the Catch Block

The catch block follows the try block, and is initiated with the keyword `catch`. In this code, the catch block starts at line 10 and encloses the code block until line 13, where it finishes after displaying an error message.

Identifying the Catch Block Parameter and Type

The catch block has a parameter enclosed in parenthesis, indicating the exception type and a variable. In this code it is `catch (double x)`, meaning the catch block will handle exceptions of type `double` and will use `x` as the variable to refer to the exception.

Identifying the Throw Statement

A throw statement is used in a try block to signal an exception and transfer control to a catch block. In the given code, the throw statement is `throw balance;`, which occurs if the balance value is less than 1000.00 at line 7.

Key concepts

try block

In C++, the try block is where you contain code that you wish to monitor for exceptions. It's enclosed by the `try` keyword, immediately followed by a pair of curly braces. Within this block, any code that potentially throws an exception should be placed. For example, if a user input does not meet certain conditions, you can write logic inside the try block to trigger an exception.

The idea is to separate code that might go wrong from the normal flow, simplifying the main program structure.

• Starts with `try {`
• Ends right before the `catch` block
• Where exceptions can be thrown

In the given code, the try block begins immediately after the `try` keyword and includes statements that accept and validate user input for `balance`. The statement `throw balance;` is also inside this block, indicating an exceptional scenario if the balance is deemed invalid.

catch block

After the try block, the catch block is where you define handlers for exceptions caught inside the try block. It starts with the keyword `catch` and includes an exception handler inside curly braces. You can handle multiple types of exceptions by chaining different catch blocks after a single try block.

• Starts with `catch`
• Follows immediately after the try block
• Contains logic to address or log exceptions

In this code example, the catch block begins at `catch (double x)`, which means it is set up to catch exceptions of the type `double`. It includes logic to inform the user about the encountered issue, printing a specific message that mentions the current `balance` and clarifies why an exception was thrown.

throw statement

A throw statement is used within a try block to signal that an exception has occurred. This is a special C++ mechanism to "throw" or trigger an exception, transferring control to a matching catch block. When you execute a throw statement, the flow of the program jumps out of the try block directly into the catch block.

Here's when a throw statement becomes useful: whenever a condition is met that indicates a certain anomaly, like an "out of bounds" error or a value that violates business logic.

• Occurs inside a try block
• Transfers control to a catch block
• Syntax: `throw expression;`

In the provided code, `throw balance;` is used to throw an exception when the balance is less than 1000.00. This statement effectively communicates to the program that something has gone wrong, prompting an immediate response by the catch block.

catch block parameter

The catch block parameter allows the catch block to handle the specific exception value or data triggered by a throw statement. This parameter is specified inside the parentheses following the `catch` keyword. The type specified in the catch block must match the type of the thrown exception for it to handle it correctly.

• Defines type of exception to handle
• Helps access the exact thrown value
• Can be a primitive data type or an object

For instance, in the code `catch (double x)`, the parameter `(double x)` tells C++ that this catch block is designed to handle exceptions of type `double`. The variable `x` is used to access the exception value within the block. Here, it captures the thrown `balance` to output a user-friendly message that informs the user of the erroneous balance input.