Exception handling flow in C++

Introduction

Exception handling flow helps your program deal with unexpected problems without crashing. It lets you catch errors and decide what to do next.

When reading a file that might not exist.

When dividing numbers and the divisor could be zero.

When converting user input to a number that might be invalid.

When working with network connections that can fail.

Syntax

C++

try {
    // code that might cause an exception
} catch (const ExceptionType& e) {
    // code to handle the exception
} catch (...) {
    // code to handle any other exceptions
}

The try block contains code that might cause an error.

The catch blocks handle specific or any exceptions thrown in the try block.

Examples

This detects divide by zero, throws a standard exception, and catches it.

C++

try {
    int divisor = 0;
    if (divisor == 0) {
        throw std::runtime_error("Division by zero error!");
    }
    int x = 10 / divisor;
} catch (const std::exception& e) {
    std::cout << "Error: " << e.what() << std::endl;
}

This throws and catches an integer exception.

C++

try {
    throw 5;
} catch (int e) {
    std::cout << "Caught an int: " << e << std::endl;
}

This catches any exception not caught by earlier catch blocks.

C++

try {
    // risky code
} catch (...) {
    std::cout << "Caught an unknown exception." << std::endl;
}

Sample Program

This program asks the user for a number and divides 100 by it. If the user enters zero, it throws an exception and catches it to show a friendly message. The program then continues running.

C++

#include <iostream>

int main() {
    try {
        std::cout << "Enter a number to divide 100 by: ";
        int divisor;
        std::cin >> divisor;
        if (divisor == 0) {
            throw "Division by zero error!";
        }
        int result = 100 / divisor;
        std::cout << "Result is: " << result << std::endl;
    } catch (const char* msg) {
        std::cout << "Exception caught: " << msg << std::endl;
    }
    std::cout << "Program continues after exception handling." << std::endl;
    return 0;
}

Important Notes

Only code inside the try block can throw exceptions caught by the following catch blocks.

You can have multiple catch blocks to handle different exception types.

The catch(...) block catches any exception not caught by previous blocks.

Summary

Use try to wrap code that might cause errors.

Use catch to handle those errors and keep your program running.

Exception handling flow helps your program stay safe and user-friendly.

Practice

1. What is the main purpose of using try and catch blocks in C++?

easy

A. To speed up the program execution

B. To handle errors and keep the program running smoothly

C. To declare variables

D. To create loops

Solution

1. Step 1: Understand the role of try block

   The try block contains code that might cause errors or exceptions during execution.

2. Step 2: Understand the role of catch block

   The catch block handles those errors so the program does not crash and can continue running.

3. Final Answer:

   To handle errors and keep the program running smoothly -> Option B

4. Quick Check:

   Exception handling = handle errors safely [OK]

Hint: Try-catch blocks catch errors to avoid crashes [OK]

Common Mistakes:

• Thinking try-catch speeds up code
• Confusing try-catch with loops
• Using try-catch to declare variables

2. Which of the following is the correct syntax to catch an exception of type int in C++?

easy

A. catch (int e) { /* handle error */ }

B. catch int (e) { /* handle error */ }

C. catch { int e; /* handle error */ }

D. catch (e int) { /* handle error */ }

Solution

1. Step 1: Recall correct catch syntax

   The catch block must specify the exception type in parentheses followed by a variable name, like catch (int e).

2. Step 2: Identify syntax errors in other options

   The other options contain syntax errors such as reversed type and variable order, incorrect keyword placement, or missing parentheses with improper declaration.

3. Final Answer:

   catch (int e) { /* handle error */ } -> Option A

4. Quick Check:

   Correct catch syntax = catch (type var) [OK]

Hint: Catch syntax: catch (type variable) { } [OK]

Common Mistakes:

• Omitting parentheses in catch
• Swapping type and variable order
• Missing variable name in catch

3. What will be the output of the following code?

try {
  throw 5;
  std::cout << "No Exception" << std::endl;
} catch (int e) {
  std::cout << "Caught exception: " << e << std::endl;
}

medium

A. Program crashes

B. No Exception

C. Caught exception: 5

D. No output

Solution

1. Step 1: Understand the throw statement

   The code throws an integer exception with value 5, so control jumps to the catch block immediately.

2. Step 2: Identify which catch block runs

   The catch block for int e catches the exception and prints the message with the value 5.

3. Final Answer:

   Caught exception: 5 -> Option C

4. Quick Check:

   Throw 5 caught by catch(int) = output message [OK]

Hint: Throw jumps to matching catch, skipping rest of try [OK]

Common Mistakes:

• Expecting code after throw to run
• Thinking program crashes without catch
• Ignoring exception value in output

4. Identify the error in this code snippet:

try {
  int x = 10 / 0;
} catch (double e) {
  std::cout << "Caught double exception" << std::endl;
}

medium

A. Division by zero causes a runtime error but no exception is thrown

B. Catch block should catch int, not double

C. Try block syntax is incorrect

D. No error, code runs fine

Solution

1. Step 1: Analyze division by zero in C++

   Integer division by zero causes undefined behavior or runtime error but does not throw a C++ exception automatically.

2. Step 2: Check catch block type

   The catch block expects a double exception, but no exception is thrown, so it never runs.

3. Final Answer:

   Division by zero causes a runtime error but no exception is thrown -> Option A

4. Quick Check:

   Integer divide by zero = runtime error, no catch [OK]

Hint: Integer divide by zero crashes, no catch thrown [OK]

Common Mistakes:

• Assuming division by zero throws exception
• Mismatching catch type with thrown exception
• Thinking try block syntax is wrong

5. Given this code, what will be the output?

try {
  try {
    throw std::string("Error");
  } catch (int e) {
    std::cout << "Caught int" << std::endl;
  }
} catch (std::string &e) {
  std::cout << "Caught string: " << e << std::endl;
}

hard

A. Caught int

B. Program crashes

C. No output

D. Caught string: Error

Solution

1. Step 1: Trace inner try-catch

   The inner try throws a std::string exception. The inner catch expects int, so it does not catch this exception.

2. Step 2: Trace outer catch block

   The exception propagates to the outer catch, which catches std::string &e and prints the message.

3. Final Answer:

   Caught string: Error -> Option D

4. Quick Check:

   Inner catch mismatch, outer catch handles string [OK]

Hint: Exception bubbles up until matching catch found [OK]

Common Mistakes:

• Assuming inner catch catches all exceptions
• Ignoring exception propagation
• Expecting program crash on uncaught inner exception