C++ Tutorial · Advanced C++ · Lesson 18 of 21

C++ Exceptions

throw, try, catch, standard exception types and noexcept.

Exceptions

Throw an exception to signal an error the caller must handle; catch by const reference:

Example 1

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

int parsePort(const std::string& s) {
    long long n = std::stoll(s);   // throws invalid_argument / out_of_range
    if (n < 1 || n > 65535) throw std::out_of_range("port must be 1-65535");
    return static_cast<int>(n);
}

std::optional<std::string> findUser(int id) {
    if (id == 1) return "Ada";
    return std::nullopt;
}

int main() {
    for (const auto& input : {"42", "abc", "99999999999"}) {
        try {
            int port = parsePort(input);   // may throw before anything is printed
            std::cout << "parsed " << port << '\n';
        } catch (const std::invalid_argument& e) {
            std::cout << "invalid: " << e.what() << '\n';
        } catch (const std::out_of_range& e) {
            std::cout << "out of range: " << e.what() << '\n';
        }
    }
    auto maybe = findUser(2);
    std::cout << maybe.value_or("no user") << '\n';
}

Output

parsed 42
invalid: stoll
out of range: port must be 1-65535
no user

Run this on your computer — see “Get Started” for setup.

Standard types in <stdexcept>: std::invalid_argument, std::out_of_range, std::runtime_error. Thanks to RAII, resources are released while the exception propagates. Mark functions that never throw as noexcept.

Many codebases also use std::optional<T> or C++23's std::expected<T, E> for expected failures.

Test yourself

How should you catch exceptions in C++?

By value
By pointer
By const reference
With catch(...) only

Show answer

Answer: By const reference