Sunday, September 6, 2009

Changi (citi) c++ IV - onMsg dispatcher - STL, slicing, cloning

void onMsg(Price& const price){ // need to persist to DB in arrival order
// Message rate is very high, so I chose async producer-consumer pattern like
    dispatch(price); // then return;

// Q: should I dispatch to multiple queues?
// A: Requirement says arrival order, so i decided to use a single queue.

// The actual price object could be a StockPrice or an OptionPrice. To be able to persist the option-specific attributes, the queue consumer must figure out the actual type.

// Also, whoever feeding us the price object may modify its state concurrently. I decided to make a deep copy of the object.

// Q: when to make a copy, in onMsg or the consumer? I personally prefer the latter, but let's say we need to adopt the former. Here's a poor solution --

   //Instead of dispatch(price)
   dispatchPBClone(price); // slicing a StockPrice down to a Price. Here's my new solution --

   dispatchPBPointer(price.cloneAndReturnPtr());

// clone() is a virtual method in StockPrice and OptionPrice.
//Q: clone() should allocate memory on the heap, but where is the de-allocation?
//A: the consumer. This is an unusual memory mgmt protocol. Standard is same function allocates and deallocates.

// Q: why not pass by reference?
// A: possible, but remember the STL queue can't accept a reference type as a type argument (see other blog post). You must eventually feed it a pointer.

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