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    <p>I was thinking in a more OO approach, using the <a href="http://en.wikipedia.org/wiki/State_pattern"><code>State Pattern</code></a>:</p> <h2>The Machine:</h2> <pre><code>// machine.h #pragma once #include "MachineStates.h" class AbstractState; class Machine { friend class AbstractState; public: Machine(int inStockQuantity); void sell(int quantity); void refill(int quantity); int getCurrentStock(); ~Machine(); private: int mStockQuantity; AbstractState* mState; }; // machine.cpp #include "Machine.h" Machine::Machine(int inStockQuantity) : mStockQuantity(inStockQuantity), mState(new Normal()) { } Machine::~Machine() { delete mState; } void Machine::sell(int quantity) { mState-&gt;sell(*this, quantity); } void Machine::refill(int quantity) { mState-&gt;refill(*this, quantity); } int Machine::getCurrentStock() { return mStockQuantity; } </code></pre> <h2>The States:</h2> <pre><code>// MachineStates.h #pragma once #include "Machine.h" #include &lt;exception&gt; #include &lt;stdexcept&gt; class Machine; class AbstractState { public: virtual void sell(Machine&amp; machine, int quantity) = 0; virtual void refill(Machine&amp; machine, int quantity) = 0; virtual ~AbstractState(); protected: void setState(Machine&amp; machine, AbstractState* st); void updateStock(Machine&amp; machine, int quantity); }; class Normal : public AbstractState { public: virtual void sell(Machine&amp; machine, int quantity); virtual void refill(Machine&amp; machine, int quantity); virtual ~Normal(); }; class SoldOut : public AbstractState { public: virtual void sell(Machine&amp; machine, int quantity); virtual void refill(Machine&amp; machine, int quantity); virtual ~SoldOut(); }; // MachineStates.cpp #include "MachineStates.h" AbstractState::~AbstractState() { } void AbstractState::setState(Machine&amp; machine, AbstractState* state) { AbstractState* aux = machine.mState; machine.mState = state; delete aux; } void AbstractState::updateStock(Machine&amp; machine, int quantity) { machine.mStockQuantity = quantity; } Normal::~Normal() { } void Normal::sell(Machine&amp; machine, int quantity) { int currStock = machine.getCurrentStock(); if (currStock &lt; quantity) { throw std::runtime_error("Not enough stock"); } updateStock(machine, currStock - quantity); if (machine.getCurrentStock() == 0) { setState(machine, new SoldOut()); } } void Normal::refill(Machine&amp; machine, int quantity) { int currStock = machine.getCurrentStock(); updateStock(machine, currStock + quantity); } SoldOut::~SoldOut() { } void SoldOut::sell(Machine&amp; machine, int quantity) { throw std::runtime_error("Sold out!"); } void SoldOut::refill(Machine&amp; machine, int quantity) { updateStock(machine, quantity); setState(machine, new Normal()); } </code></pre> <p>I'm not used to program in C++, but this code aparently compiles against GCC 4.8.2 and valgrind shows no leaks, so I guess it's fine. I'm not computing money, but I don't need this to show you the idea.</p> <p>To test it:</p> <pre><code>#include &lt;iostream&gt; #include &lt;stdexcept&gt; #include "Machine.h" #include "MachineStates.h" int main() { Machine m(10); m.sell(10); std::cout &lt;&lt; "Sold 10 items" &lt;&lt; std::endl; try { m.sell(1); } catch (std::exception&amp; e) { std::cerr &lt;&lt; e.what() &lt;&lt; std::endl; } m.refill(20); std::cout &lt;&lt; "Refilled 20 items" &lt;&lt; std::endl; m.sell(10); std::cout &lt;&lt; "Sold 10 items" &lt;&lt; std::endl; std::cout &lt;&lt; "Remaining " &lt;&lt; m.getCurrentStock() &lt;&lt; " items" &lt;&lt; std::endl; m.sell(5); std::cout &lt;&lt; "Sold 5 items" &lt;&lt; std::endl; std::cout &lt;&lt; "Remaining " &lt;&lt; m.getCurrentStock() &lt;&lt; " items" &lt;&lt; std::endl; try { m.sell(10); } catch (std::exception&amp; e) { std::cerr &lt;&lt; e.what() &lt;&lt; std::endl; } return 0; } </code></pre> <p>Output is:</p> <blockquote> <pre><code>Sold 10 items Sold out! Refilled 20 items Sold 10 items Remaining 10 items Sold 5 items Remaining 5 items Not enough stock </code></pre> </blockquote> <p>Now, if you want to add a <code>Broken</code> state, all you need is another <code>AbstractState</code> child. Maybe you'll need to add a <code>broken</code> property on <code>Machine</code> also.</p> <p>To add more products, you must have a map of products and its respective in-stock quantity and so on...</p>
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