C++ - Error Handling & Exceptions

Overview

Estimated time: 45–65 minutes

Use exceptions for error reporting in exceptional, non-local situations. Learn try/catch, standard exceptions, noexcept, and exception-safety with RAII.

Learning Objectives

• Throw and catch exceptions; use standard exception types.
• Understand noexcept and when to use it.
• Design code with RAII so resources are released on exception paths.

Prerequisites

• C++ - Files & I/O

Throw and catch

#include <stdexcept>
#include <iostream>
int parse_positive(int x){ if (x <= 0) throw std::invalid_argument("non-positive"); return x; }
int main(){
  try {
    std::cout << parse_positive(1) << "\n";
    std::cout << parse_positive(0) << "\n"; // throws
  } catch (const std::invalid_argument& e) {
    std::cout << "error: " << e.what() << "\n";
  }
}

Expected Output: 1 error: non-positive

noexcept and strong exception safety

#include <vector>
#include <iostream>
void may_throw(){ throw 42; }
void safe() noexcept { /* won't throw */ }
int main(){
  try { may_throw(); } catch(...) { std::cout << "caught\n"; }
  safe();
}

RAII to the rescue

#include <fstream>
#include <string>
#include <iostream>
void write_line(const std::string& path){
  std::ofstream out(path); // closed automatically even if exceptions occur later
  if (!out) throw std::runtime_error("open failed");
  out << "line\n";
}

Beginner Boosters

#include <stdexcept>
#include <string>
#include <iostream>
int to_int(const std::string& s){
  for (char c: s) if (c < '0' || c > '9') throw std::invalid_argument("not a digit");
  return std::stoi(s);
}
int main(){
  try { std::cout << to_int("123") << "\n"; std::cout << to_int("1x") << "\n"; }
  catch(const std::exception& e){ std::cout << e.what() << "\n"; }
}

Common Pitfalls

• Using exceptions for normal control flow; prefer return status for expected outcomes.
• Catching by value (slicing); catch exceptions by const reference.
• Forgetting RAII; leaking resources on throw paths.

Checks for Understanding

1. When should you use exceptions vs return codes?
2. Why catch exceptions by const reference?

Exercises

1. Write a function that opens a file and throws std::runtime_error on failure; write tests using try/catch.
2. Mark a function noexcept and verify the compiler enforces it by preventing throws.