Static and dynamic polymorphism

Polymorphism means “Many forms”
OO Purists – “Only virtual methods”
Liberal C++ guys – Either virtual methods (Behavior), or templates (Types), or
even function overloading for that matter (Behavior)
Generally accepted – virtual method based, and template based
OVERVIEW

Runtime/Dynamic polymorphism (Based on virtual methods)
Compile time/Static polymorphism (Based on templates)
These 2 are largely orthogonal techniques, but many ways where one could be
replaced with the other…and that’s where the confusion
KINDS OF POLYMORPHISM

ANY OF THEM WORKS – EXAMPLE
// Using virtual method based polymorphism
// lock is a virtual method. SingleThreading and Multi-
threading are 2 derived classes which define lock()
differently
void func(BaseThreadingPolicy * pThreadingPolicy)
{
pThreadingPolicy->lock();
};
// Using template based polymorphism
SingleThreading and Multi-threading both implement a lock()
method, but the needn’t have any virtual methods, nor do
they need to derive from a common base class
template <class T> func(T* pThreadingPolicy)
{
pThreadingPolicy->lock();
};

In the previous slide we have shown that templates aren’t restricted to
polymorphism of type alone, but they also can be used for polymorphism of
behavior
In fact the template version in the previous example yields much faster code than
the virtual method equivalent
Much prevailing confusion about which technique to use in what situation
ANY OF THEM WORKS - CONTD

Runtime binding between abstract type and concrete type
Enforces a common interface for all derived types via the base class
Enforces an ‘Is-A’ relationship
Used extensively in OO frameworks
Templates eating into parts of its territory
RUNTIME POLYMORPHISM

Compile time binding between abstract type and concrete
type
Concrete types need not belong to the same inheritance
hierarchy. They just need to meet the ‘Constraints’
imposed by the generic type. So, more generic than the
virtuals
Foundation of ‘Generic programming’ or programming
based on ‘Concepts’ (Eg: STL)
A very ‘Happening area’ in C++ right now (See TR1,
Boost, C++0x...)
COMPILE TIME POLYMORPHISM

Templates –
Buy us generality in software, without the ‘Abstraction penalty’
Are type safe
Are non-intrusive
Allow both reference and value semantics unlike virtuals
WHY ALL THE
EXCITEMENT
ABOUT
TEMPLATES?

DESIGN OF REUSABLE DATA
STRUCTURES – INHERITANCE OR
TEMPLATES?
IndexIndex Inheritance basedInheritance based
approachapproach
Template based approachTemplate based approach
1 Slower (virtuals) Faster
2 Non type-safe
(Crash!)
Type safe
3 Intrusive (put an
int/char inside a class
and derive from base)
Non-Intrusive
4 Reference semantics
only (leaks, crashes..)
Value or Reference
semantics, both allowed

So, templates lead to ‘Faster, Safer, and Easier to reuse’ data structures than
inheritance
Using inheritance for designing generic data structures, is a common mistake in
C++
Dates back to days where compiler support was bad for templates
SURE CHOICE - TEMPLATES

Code and structure reuse from base classes
OO Frameworks
Framework implementation requires ‘Heterogeneous containers’
Reactor example
Often depend on polymorphic return types
Factory example
Can’t do that with templates
WHY INHERITANCE THEN?

Don’t overuse inheritance (Implementing data structures using inheritance is an
example of misuse of inheritance)
Evaluate if your design could be done using templates, they are faster and safer
than virtuals.
Read up on templates and be aware of its pitfalls (Another class of mine will deal
with that)
CHOOSING YOUR
POLYMORPHISM

Templates and Inheritance can be combined sometimes
Eg 1: Singleton pattern implementation
Eg 2: Reusable object counter
Eg 3: Reducing template code bloat
Weakness of inheritance – Base classes don’t know the
type of their derived classes
Static attributes in base class are ‘Shared’
Weakness of templates – They don’t lead to an ‘Is-A’
relationship
VANILLA OR
STRAWBERRY? I
THINK I’LL
HAVE BOTH

Best of both worlds
Have base class know the derived class type by using templates [So, getInstance in the
base class is able to create a derived class instance]
Enforce ‘Is-A’ relationship using inheritance [Eg: Make ctor private in the base class, so
nobody can instantiate a derived class instance as well]
SINGLETON
USING THE VAN-
BERRY FLAVOR

Synergy again
Base classes need to share the static attributes
Use templates to get over that issue – One base class generated for each derived class
Use inheritance for the ‘Is-A’ relationship [Whenever the class ctor is called, the base class
ctor is called, likewise for the dtor, increment and decrement operations happen
automatically]
OBJECT COUNTER

Each class that has a virtual method has an associated
‘vtable’
A ‘vtable’ is just an array of function pointers containing
pointers to virtual method implementations for that class
Each object of any such class, has a pointer to the
associated vtable
The compiler creates the required vtables and initializes
each object to point to the associated vtable all ‘Under the
hood’
COST OF RUNTIME
POLYMORPHISM

ILLUSTRATION
Class Base
{
public:
Base();
virtual void func1();
virtual ~Base():
};
Class Derived
{
public:
Derived();
virtual ~Derived();
};

ILLUSTRATION (CONTD)
Pointer to Base::func1
Pointer to Base::~Base
vtable for the Derived class
Pointer to Derived::func1
Pointer to Derived::~Derived
vtable for the Base class

The compiler would have converted all your virtual method calls
Your call: pDerived->func2();
It’s become: (*pDerived->vptr[1])(pDerived)
As we see above, the vptr stored by the compiler in the derived object is looked
up, an offset (+1) added to get to the second virtual method in the class, and the
function is called
WHAT HAPPENS AT RUNTIME?

Direct cost – vtable lookup (Put at about 5-10% overhead as opposed to non-
virtual methods)
Indirect cost –
Cannot inline virtual methods (Makes a big difference)
Each objects needs to store extra pointer (An issue for fine grained objects – say 1000000
link element objects, each containing 4 bytes extra!)
SO, WHAT’S THE COST?

Well, this gets worse when MI (Multiple Inheritance is
used)
Now, the deal is 15-20% overhead for MI for method
defined in the 2nd
and onwards base class. There’s no
penalty for methods in the first base class we derive from
Ensure your most frequently called methods are from the
FIRST base class you derive from if you use MI, order
your base classes accordingly
IS THAT ALL?

ZERO runtime cost (Abstraction without abstraction penalty)
However …
Coders not aware of how templates work ‘Under the hood’ end up creating code bloat
Developer turn around time increased due to longer compiles (This could be avoided too)
COST OF
COMPILE TIME
POLYMORPHISM

Code bloat happens because compilers cannot do
‘Commonality and variability analysis’
If the code body is the same for a set of template
argument values, the compiler fails to see that and
generates multiple classes for each argument value
Eg: list<int *>, list<char *, list<MyClass*> all of these
may have the same underlying implementation of a
‘Container of POINTERS’, but the compiler generates 3
different class definitions for these
TEMPLATE CODE BLOAT

A coder who knows this, does the following
Give a partial template specialization for POINTER types
Have that derive from a void * based container which has most of the implementation in
it
This specialized class will have simple inline methods that ‘Delegate’ to the base class and
gets work done
TEMPLATE CODE BLOAT
(CONTD)

So, we get the following result –
The thin inline wrappers in the specialized class offer type safety
Majority of the code body is in the non-template base class and hence no duplication of it
The thin wrappers are all inline hence no performance overhead as well
This is called the ‘Hoisting idiom’
TEMPLATE CODE BLOAT
(CONTD)

Happens mainly because the template method definitions have to be in the
header file
They may in turn pull in other headers and hence lead to large include sizes and
hence more compile time
More work to the compiler in case one uses template meta-programming
LONGER COMPILATION TIME

To reduce compile times, we can use the ‘Explicit instantiation’ technique in
cases where large includes are warranted
Here basically you give the method body in a .cpp file and put only the template
class definition in the .h file, but explicitly instantiate the template in the .cpp file
where the methods are defined for all types expected
WORKAROUND

Static and dynamic polymorphism

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Static and dynamic polymorphism