Template and OOAD can help design genericity in different ways. One can not replace the other.
The template way is to provide templatized data structures and templatized algorithms and have iterators that connect the two. Based on the category of iterator that algorithm decides at compile time as to which implementation of the algorithm is most efficient for this data structure and operates on it. It may also decide that a particular algorithm is not best suited for a datastructure.
For example std::sort accepts two random access iterators and sorts everything between them. If passed iterators to a range in a linked list you will get a complier error. The algorithm can also choose the most efficient implementation based on the iterator category. When copying one range to another using std::copy, if you passed a random access iterator, the entire chunk between them is copied with a native memmove or some such function. If passed a less powerful iterator, it iterate and copy the range as expected. It can vary based on the implementation in your STL library, but that's the general idea.
If you look at it, the algorithms are generic, they don't know which data structure they are operating on. On the other hand the each one of the data structures did not have to provide these algorithms as member functions. So, the commonality is lifted out as generic algorithms.
OOAD on the other hand is about using a generic interface in the user code and the implementation detail is pushed to run time. OOAD helps where you expect conformance to an interface. This can not be done with the generic programming techniques above. If you want to ensure a Thread object should have a run function in it, you will have to write a abstract base class with a pure virtual function called run() and based on the kind of thread the implementation detail can vary as late as run time.
As a general rule iterators and algorithms are used when youo have a collection of data to operate on and they can be present in different kids of datastructures. OOAD is a more structural concept which concentrates on how a class hierarchy is designed and what functionality a class must provide, etc. One of the drawbacks of OOAD is that sometimes you may run into a situation where you a derived class is a kind of a base class, but you do not want to provide the functionality that you are enforced to.
The template way is to provide templatized data structures and templatized algorithms and have iterators that connect the two. Based on the category of iterator that algorithm decides at compile time as to which implementation of the algorithm is most efficient for this data structure and operates on it. It may also decide that a particular algorithm is not best suited for a datastructure.
For example std::sort accepts two random access iterators and sorts everything between them. If passed iterators to a range in a linked list you will get a complier error. The algorithm can also choose the most efficient implementation based on the iterator category. When copying one range to another using std::copy, if you passed a random access iterator, the entire chunk between them is copied with a native memmove or some such function. If passed a less powerful iterator, it iterate and copy the range as expected. It can vary based on the implementation in your STL library, but that's the general idea.
If you look at it, the algorithms are generic, they don't know which data structure they are operating on. On the other hand the each one of the data structures did not have to provide these algorithms as member functions. So, the commonality is lifted out as generic algorithms.
OOAD on the other hand is about using a generic interface in the user code and the implementation detail is pushed to run time. OOAD helps where you expect conformance to an interface. This can not be done with the generic programming techniques above. If you want to ensure a Thread object should have a run function in it, you will have to write a abstract base class with a pure virtual function called run() and based on the kind of thread the implementation detail can vary as late as run time.
As a general rule iterators and algorithms are used when youo have a collection of data to operate on and they can be present in different kids of datastructures. OOAD is a more structural concept which concentrates on how a class hierarchy is designed and what functionality a class must provide, etc. One of the drawbacks of OOAD is that sometimes you may run into a situation where you a derived class is a kind of a base class, but you do not want to provide the functionality that you are enforced to.
No comments:
Post a Comment