#include <cassert>
#include <stdexcept>

class Balance {
    int cents_;
public:
    explicit Balance(int cents) : cents_(cents) {
        if (cents < 0) throw std::invalid_argument("negative balance");
    }
    bool withdraw(int amount) {
        if (amount < 0 || amount > cents_) return false;
        cents_ -= amount;
        return true;
    }
    int value() const { return cents_; }
};

struct Payment {
    virtual bool pay(int cents) = 0;
    virtual ~Payment() = default;
};

class Wallet final : public Payment {
    Balance balance_;
public:
    explicit Wallet(int cents) : balance_(cents) {}
    bool pay(int cents) override { return balance_.withdraw(cents); }
    int remaining() const { return balance_.value(); }
};

int main() {
    Wallet wallet(100);
    Payment& payment = wallet;
    const bool paid = payment.pay(30);
    const bool rejected = payment.pay(80);
    assert(paid && !rejected);
    assert(wallet.remaining() == 70);
}