Friday, December 12, 2008

std::string field and other non-ptr fields in a c++ class

Background -- I feel this is a fundamental but overlooked design consideration.

In java, any non-primitive field is a ptr. In c++, std::string is a common field type. No ptr needed. But What other non-ptr fields are common? Let's exclude primitive types like double or "bool" (shorter spelling than java "boolean")

I feel any class like Address can be used as the nonref type of a field myAddr in a new class Customer. Customer ctor needs to allocate this field, but is it allocated on stack or heap? It depends on the context.

- If you allocate the entire Customer object on heap, then myAddr field is also on heap
- ditto for stack. You can trace the steps of myAddr's allocation and there's no malloc().

Either way, upon Customer de-allocation, myAddr is automatically /bulldozed/ since myAddr is on the real estate of the Customer object. sizeof(Customer) includes sizeof(Address), which is not the case for pimpl.

Now we know nonref field like qq{ Address myAddr; } is the c++ default whereas qq{ Address * myAddrPtr; } is the pimpl/java/c# version.

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