std::exception

Learn std::exception and the what() method.

std::exception base type

std::exception is the base type of C++'s standard exception hierarchy. Types like runtime_error, invalid_argument, out_of_range are all derived from it. This lets a single catch block catch all standard exceptions.

catch (const std::exception& e) {
    cout << e.what();
}

Here e.what() returns the exception's description string. Catching by base type reference avoids copying and catches derived types without losing their details.

Using the what() method

The what() method returns a string pointer describing the exception's cause. This string is the text given to the exception constructor. It is useful for debugging and user reporting.

try {
    throw std::runtime_error("file not found");
} catch (const std::exception& e) {
    cout << "Error: " << e.what() << endl;
}

Here e.what() becomes "file not found". This pattern is the standard way to tell the user what went wrong.

Exercise Print the message with what()

Add #include <stdexcept>. Inside a try block, write throw std::runtime_error("file not found");. In a catch (const std::exception& e) block, print the message with cout << e.what() << endl;.

Show example solution

#include <iostream>
#include <stdexcept>
using namespace std;

int main() {
    try {
        throw std::runtime_error("file not found");
    } catch (const std::exception& e) {
        cout << e.what() << endl;
    }
    return 0;
}

Test yourself

Which method of std::exception returns the description string?
- message()
- why()
- what()
- text()
Show answer

what() — what() returns the description string.
Match each exception type with its scenario

runtime_error

runtime error
invalid_argument

invalid input
out_of_range

out of bounds

Fill in the blank: what() call

try {
    throw std::runtime_error("x");
}
catch (const std::exception& e) {
    cout << e.____();
}

Show answer

try {
    throw std::runtime_error("x");
}
catch (const std::exception& e) {
    cout << e.what();
}

Next lesson: Custom exceptions

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