Monday, January 16, 2012

dynamic/runtime binding - 8 restrictions

Here's Another angle to look at runtime binding i.e. dynamic dispatch i.e. virtual function.

For runtime binding to work its magic (via vptr/vtbl), you must, must, must meet all of these conditions.
*) method must be --> non-static.
*) member must be --> non-field. vtbl offers no runtime resolution of FIELD access. See [[java precisely]]
*) host object must be accessed via a --> ref/ptr, not a non-ref variable. P163 [[essential c++]]
*) method's parameter types must be an exact copy-paste from parent to subclass. No subsumption allowed in Java [2]. C++ ARM P210 briefly explains why.

*) method is invoked not during ctor/dtor (c++). In contrast, Java/c# ctor can safely call virtual methods, while the base object is under construction and incomplete!
*) method must be --> virtual, so as to hit the vtbl. In Java, all non-static non-final methods are virtual.
*) the call the the function must NOT be scope-qualified like ptr2->B::virtF(). See P210 ARM
*) the 2 methods (to choose from) must be defined INSIDE 2 classes in a hierarchy. In contrast, a call to 2 overload methods accepting a B param vs a D param respectively will never be resolved at runtime -- no such thing as "argument-based runtime binding". Even if the argument is a D instance, its declared type (B) is always used to statically resolve the method call. This is the **least-understood** restriction among the restrictions. See http://bigblog.tanbin.com/2010/08/superclass-param-subclass-argument.html

If you miss any one condition, then without run/compile-time warnings compiler will __silently__ forgo runtime binding and assume you want compile-time binding.

However, return type of the 2 functions can be different. P208 ARM says it's an error for the two to differ in return type only, but [[c++common knowledge]] P100 gives a clear illustration of clone() method i.e. virtual ctor. See also [[more eff C++]] P126.


[2] equals(SubclassOfObject) is overloading, not overriding. @Override disallowed -- remember Kevin Hein's quiz.

Suppose you have a B extending C, and a B pointer/ref variable o seated at a C object, you won't get runtime binding in these cases --

- if you have a non-static field f defined in both B/C, then o.f is compile-time binding, based on declared type. P40 [[java precisely]]
- if you have a static method m() defined in both B/C, then o.m() is compile-time binding, based on declared type. [1]
- if you have a nonref B variable receiving a C object, then slicing -- u can't access any C part.

[1] That's well-known in java. In C++, You can also "call a static member function using the this pointer of a non-static member function."

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