Throwing exceptions in C++ - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this C++ code with exception throwing?

Consider the following C++ program. What will it print when run?

C++

#include <iostream>
#include <stdexcept>

int main() {
    try {
        throw std::runtime_error("Error happened");
        std::cout << "No error" << std::endl;
    } catch (const std::runtime_error& e) {
        std::cout << "Caught: " << e.what() << std::endl;
    }
    return 0;
}

A Caught: Error happened

B No error

C Caught: std::runtime_error

D Program crashes with uncaught exception

💡 Hint

Remember that throwing an exception transfers control to the matching catch block.

The throw statement raises an exception. The try block stops executing at the throw, so "No error" is never printed. The catch block catches the std::runtime_error and prints its message.

❓ Predict Output

intermediate

What error does this code produce?

What happens when this C++ code runs?

C++

#include <iostream>
#include <string>

void f() {
    throw 42;
}

int main() {
    try {
        f();
    } catch (const std::string& e) {
        std::cout << "Caught string: " << e << std::endl;
    }
    return 0;
}

A No output, program runs normally

B Caught string: 42

C Compilation error due to catch type mismatch

D Program terminates with uncaught exception

💡 Hint

Check if the catch block matches the thrown exception type.

The function f() throws an int (42). The catch block tries to catch a std::string, which does not match. Since no matching catch block exists, the program terminates with an uncaught exception.

🔧 Debug

advanced

Why does this code fail to catch the exception?

Identify the reason the exception is not caught in this C++ code snippet.

C++

#include <iostream>
#include <stdexcept>

int main() {
    try {
        throw "Error";
    } catch (const std::exception& e) {
        std::cout << "Caught exception: " << e.what() << std::endl;
    }
    return 0;
}

A Cannot catch a const char* with std::exception reference

B Missing semicolon after throw statement

C std::exception does not have what() method

D Throwing string literals is not allowed

💡 Hint

Think about the type of the thrown object and the catch parameter type.

The code throws a string literal (type const char*). The catch block tries to catch a const std::exception&. These types do not match, so the catch block will not catch the thrown exception. This is a logical error but compiles. However, if the code tries to call e.what() on a non-std::exception, it would be invalid. Here, the catch block will not catch the thrown exception, so the program terminates.

❓ Predict Output

advanced

What is the output of nested try-catch with rethrow?

What will this C++ program print?

C++

#include <iostream>
#include <stdexcept>

int main() {
    try {
        try {
            throw std::logic_error("Logic error");
        } catch (const std::logic_error& e) {
            std::cout << "Inner catch: " << e.what() << std::endl;
            throw;  // rethrow
        }
    } catch (const std::exception& e) {
        std::cout << "Outer catch: " << e.what() << std::endl;
    }
    return 0;
}

A Outer catch: Logic error

B Inner catch: Logic error

C

Inner catch: Logic error
Outer catch: Logic error

D Program crashes with uncaught exception

💡 Hint

Remember that throw; rethrows the current exception to the outer catch block.

The inner catch block catches the std::logic_error and prints its message. Then it rethrows the same exception. The outer catch block catches it again and prints the message. So both messages appear.

🧠 Conceptual

expert

Which option correctly describes exception object slicing in C++?

Consider throwing a derived class exception object but catching by value as a base class. What happens?

A The catch block receives the full derived class object with all info intact

B The caught exception object is sliced to base class, losing derived class info

C The program crashes due to slicing at runtime

D Catching by value is not allowed in C++

💡 Hint

Think about what happens when an object is copied by value from a derived to a base class type.

When catching exceptions by value as a base class, the derived class part is sliced off. This means the catch block only sees the base class part, losing any extra data or behavior from the derived class. This is why catching exceptions by reference is recommended.

Practice

1. What does the throw keyword do in C++?

easy

A. It defines a new function.

B. It catches an exception.

C. It signals an error by raising an exception.

D. It declares a variable.

Solution

1. Step 1: Understand the purpose of throw

   The throw keyword is used to signal that an error or exceptional condition has occurred in the program.

2. Step 2: Differentiate from other keywords

   throw does not catch exceptions (that's catch), nor does it define functions or declare variables.

3. Final Answer:

   It signals an error by raising an exception. -> Option C

4. Quick Check:

   throw = signal error [OK]

Hint: Remember: throw means "raise an error" [OK]

Common Mistakes:

• Confusing throw with catch
• Thinking throw declares variables
• Assuming throw defines functions

2. Which of the following is the correct syntax to throw an integer exception with value 10?

easy

A. throw int 10;

B. throw(int 10);

C. throw new int(10);

D. throw 10;

Solution

1. Step 1: Recall correct throw syntax

   In C++, to throw an exception, you use throw followed by the value or object, without parentheses or type keywords.

2. Step 2: Analyze each option

   throw 10; uses throw 10; which is correct. throw(int 10); incorrectly declares a type inside parentheses, invalid syntax. throw int 10; incorrectly adds type keyword. throw new int(10); throws a pointer, which is allowed but not the simplest form here.

3. Final Answer:

   throw 10; -> Option D

4. Quick Check:

   Simple throw syntax = throw value; [OK]

Hint: Use throw followed directly by value, no parentheses [OK]

Common Mistakes:

• Adding parentheses after throw
• Including type keyword with throw
• Throwing pointers when not needed

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

try {
  throw "Error occurred";
} catch (const char* msg) {
  std::cout << msg << std::endl;
}

medium

A. Error occurred

B. Runtime crash

C. No output

D. Compilation error

Solution

1. Step 1: Understand the throw and catch block

   The code throws a string literal "Error occurred" and catches it as a const char* pointer.

2. Step 2: Check the catch block output

   The catch block prints the caught message using std::cout, so it will output the string "Error occurred" followed by a newline.

3. Final Answer:

   Error occurred -> Option A

4. Quick Check:

   Thrown string caught and printed = "Error occurred" [OK]

Hint: Thrown string literals caught as const char* print directly [OK]

Common Mistakes:

• Expecting compilation error due to throw
• Thinking catch won't match string literal
• Assuming no output without explicit return

4. Identify the error in this code snippet:

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

medium

A. No error, code is correct

B. Multiple catch blocks for same exception type

C. Catch block missing parameter type

D. Throwing wrong data type

Solution

1. Step 1: Analyze the try and catch blocks

   The code throws an integer 5, which is caught by the first catch block for int. The second catch block is a catch-all for any other exceptions.

2. Step 2: Check for syntax or logic errors

   Multiple catch blocks are allowed and correctly ordered from specific to general. The catch blocks have correct parameter types. Throwing an int matches the first catch.

3. Final Answer:

   No error, code is correct -> Option A

4. Quick Check:

   Proper catch blocks for thrown int = correct code [OK]

Hint: Multiple catch blocks allowed; order matters [OK]

Common Mistakes:

• Thinking multiple catch blocks cause error
• Missing parameter type in catch
• Throwing incompatible data type

5. Consider this function that throws exceptions based on input: <pre>void checkValue(int x) { if (x < 0) throw std::string("Negative value"); else if (x == 0) throw 0; else throw 3.14; } What is the best way to catch all exceptions thrown by checkValue and print a message for each type accordingly?

hard

A. Use a single catch block with catch(...) only.

B. Use separate catch blocks for std::string, int, and double types.

C. Use only catch(int) block.

D. Use try without any catch block.

Solution

1. Step 1: Identify exception types thrown

   The function throws three different types: std::string, int, and double.

2. Step 2: Determine how to catch each type

   To handle each exception properly, separate catch blocks for each type are needed. Using only catch(...) catches all but cannot distinguish types. Catching only int misses others. No catch block causes program termination on exception.

3. Final Answer:

   Use separate catch blocks for std::string, int, and double types. -> Option B

4. Quick Check:

   Multiple catch blocks for different types = best practice [OK]

Hint: Catch each thrown type separately for clear handling [OK]

Common Mistakes:

• Using only catch(...) and missing type info
• Catching only one exception type
• Not using any catch block