#include <cassert>
#include <iostream>

class Sample {
    const int value_ = 3;
    mutable bool ready_ = false;
    mutable int cached_ = 0;
public:
    int doubled() const {
        if (!ready_) {
            cached_ = value_ * 2;
            ready_ = true;
        }
        return cached_;
    }
};

int main() {
    const Sample sample;
    assert(sample.doubled() == 6);
    assert(sample.doubled() == 6);
    int original = 4;
    auto next = [original]() mutable { return ++original; };
    const int captured_value = next();
    assert(captured_value == 5 && original == 4);
    std::cout << sample.doubled() << ' ' << captured_value << ' '
              << original << '\n';
}