Mission Impossible / How to create datatypes which cannot contain invalid state

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For details, see: https://en.cppreference.com/w/cpp/utility/optional

std::optional

#include <iostream>
#include <optional>
void print_value(std::optional<int> optional_value) {
    if (optional_value) {
        std::cout << optional_value.value() << "\n";
    } else {
        std::cout << "n/a\n";
    }
}
std::optional<int> read_number() {
    return 5;
}
int main() {
    auto input_number = std::optional<int>{};
    print_value(input_number);
    input_number = 5;
    print_value(input_number);
    if (auto opt_num = read_number()) {
        std::cout << opt_num.value() << "\n";
    }
    std::cout << read_number().value_or(7) << "\n";
}

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