How do I pattern match (std::visit) std::expected (when available) in C++?

Pattern matching with std::expected using std::visit is a powerful way to handle various outcomes of operations in modern C++. With the release of C++20, std::expected provides a way to represent values which may either be a valid result or an error. Below is an example of how to utilize pattern matching effectively with std::expected and std::visit.

Here’s a simple example of how to do that:


    #include <variant>
    #include <expected>
    #include <iostream>

    // Define a simple expected type
    using MyExpected = std::expected<int, std::string>;

    // Sample function which returns std::expected
    MyExpected divide(int numerator, int denominator) {
        if (denominator == 0)
            return std::unexpected("Division by zero");
        return numerator / denominator;
    }

    int main() {
        auto result = divide(10, 0);

        // Use std::visit to pattern match on std::expected
        std::visit([](auto&& arg) {
            using T = std::decay_t<decltype(arg)>;
            if constexpr (std::is_same_v<T, int>) {
                std::cout << "Result: " << arg << std::endl;
            } else {
                std::cout << "Error: " << arg << std::endl;
            }
        }, result);
        
        return 0;
    }

C++ std::expected std::visit error handling pattern matching modern C++

How do I pattern match (std::visit) std::expected (when available) in C++?

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How do I pattern match (std::visit) std::expected (when available) in C++?

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