The STL and professional C++ · Lesson 19 of 21

Exceptions

throw, try and catch, the standard error types, and how RAII fits in.

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When a function hits something it can't continue with (bad input, a missing file), it can throw an exception. Execution stops at once and unwinds outward to the nearest matching catch. With no catch at all, the program terminates.

C++

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

double average(const std::vector<int>& v) {
    if (v.empty()) throw std::invalid_argument("average of an empty list");
    int sum = 0;
    for (int x : v) sum += x;
    return static_cast<double>(sum) / v.size();
}

int main() {
    try {
        std::cout << average({80, 90}) << "\n";
        std::cout << average({}) << "\n";          // throws
        std::cout << "never printed\n";
    } catch (const std::invalid_argument& e) {
        std::cout << "bad input: " << e.what() << "\n";
    }

    std::vector<int> v = {1, 2, 3};
    try {
        std::cout << v.at(10) << "\n";
    } catch (const std::out_of_range& e) {
        std::cout << "out of range\n";
    }
    std::cout << "still running\n";
    return 0;
}

std::exception
- std::logic_error
  - std::invalid_argument
  - std::out_of_range
- std::runtime_error
  - std::overflow_error
- std::bad_alloc (new failed)

The standard types form a tree: catch (const std::exception& e) catches any of them.

What's the output?

#include <iostream>
#include <stdexcept>
struct Guard {
    ~Guard() { std::cout << "G"; }
};
void risky() {
    Guard g;
    std::cout << "1";
    throw std::runtime_error("x");
    std::cout << "2";
}
int main() {
    try { risky(); } catch (const std::exception&) { std::cout << "C"; }
    std::cout << "\n";
}

What will this code print?

Exercise

Write int toPercent(int part, int whole) that throws std::invalid_argument when whole is zero and otherwise returns part * 100 / whole. In main: print toPercent(3, 4), then try toPercent(1, 0), catch the exception and print error.

Expected output:

75
error

C++

#include <iostream>
#include <stdexcept>

int toPercent(int part, int whole) {
    return part * 100 / whole;
}

int main() {
    std::cout << toPercent(3, 4) << "\n";
    
    return 0;
}

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#include <iostream> #include <stdexcept> #include <string> #include <vector> double average(const std::vector<int>& v) { if (v.empty()) throw std::invalid_argument("average of an empty list"); int sum = 0; for (int x : v) sum += x; return static_cast<double>(sum) / v.size(); } int main() { try { std::cout << average({80, 90}) << "\n"; std::cout << average({}) << "\n"; // throws std::cout << "never printed\n"; } catch (const std::invalid_argument& e) { std::cout << "bad input: " << e.what() << "\n"; } std::vector<int> v = {1, 2, 3}; try { std::cout << v.at(10) << "\n"; } catch (const std::out_of_range& e) { std::cout << "out of range\n"; } std::cout << "still running\n"; return 0; }

#include <iostream> #include <stdexcept> struct Guard { ~Guard() { std::cout << "G"; } }; void risky() { Guard g; std::cout << "1"; throw std::runtime_error("x"); std::cout << "2"; } int main() { try { risky(); } catch (const std::exception&) { std::cout << "C"; } std::cout << "\n"; }