Topic 68 of 91 9. C++ Standard Template Library (STL) Interactive Tutorial

Custom Exceptions in C++: Creating Application-Specific Error Types

7 mins read Last updated: 26 Sep 2026 17,316 views

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Direct Answer & Core Concept

Custom exceptions are created by deriving from std::exception, std::runtime_error, or std::logic_error and overriding the virtual const char* what() const noexcept method (or passing an error message to the std::runtime_error base constructor). This provides descriptive, domain-specific error reporting.

Detailed Explanation & Mechanics

1. Designing Custom Exception Classes:

Inheriting from std::runtime_error is the most common approach because it handles string storage internally:

Architecture Diagram

CUSTOM EXCEPTION HIERARCHY:
       ┌────────────────────────┐
       │   std::runtime_error   │
       └───────────┬────────────┘
                   │
       ┌───────────┴────────────┐
       │   NetworkTimeoutError  │ ──► provides errorCode + what()
       └────────────────────────┘

2. Overriding `what()`:

const char* what() const noexcept override must be marked noexcept to match the standard signature.

Runnable Code Example: Custom Exceptions in C++

C++20 / C++23

main.cpp

Run Online

#include <iostream>
#include <string>
#include <stdexcept>

// Custom Exception inheriting from std::runtime_error
class InsufficientFundsException : public std::runtime_error {
private:
    double currentBalance;
    double requestedAmount;

public:
    InsufficientFundsException(double balance, double requested)
        : std::runtime_error("Insufficient funds for requested transaction."),
          currentBalance(balance),
          requestedAmount(requested) {}

    double getBalance() const noexcept { return currentBalance; }
    double getRequested() const noexcept { return requestedAmount; }
};

void withdraw(double& balance, double amount) {
    if (amount > balance) {
        throw InsufficientFundsException(balance, amount);
    }
    balance -= amount;
    std::cout << "Withdrew $" << amount << ". New balance: $" << balance << "\n";
}

int main() {
    double accountBalance = 250.00;

    try {
        withdraw(accountBalance, 100.00);
        withdraw(accountBalance, 500.00); // Exceeds balance
    }
    catch (const InsufficientFundsException& ex) {
        std::cerr << "[Exception Caught]: " << ex.what() << "\n"
                  << "  Balance: $" << ex.getBalance() << "\n"
                  << "  Attempted: $" << ex.getRequested() << "\n";
    }
    return 0;
}

Common C++ Technical Interview Questions & Answers

Interview Prep

Q: Why inherit from `std::runtime_error` rather than raw `std::exception`?

A: `std::runtime_error` already encapsulates an `std::string` message and implements `.what()`, reducing boilerplate code in your custom exception.

Q: Why must `what()` be declared `noexcept`?

A: In `std::exception`, `what()` is defined as `virtual const char* what() const noexcept;`. In C++, an overriding method cannot have a looser exception specification than the base method.

Key Takeaways, Modern Best Practices & Idioms

C++ Best Practices & Production Rules

• Derive custom exceptions from std::runtime_error (for runtime failures) or std::logic_error (for logic/invariant bugs).
• Include helpful contextual telemetry (error codes, usernames, transaction IDs) in custom exception classes.
• Catch custom exceptions before generic base exceptions in try-catch chains to ensure specific handling.

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