Thursday, 3 December 2020

OOP - Recovering lost class types

 Normally casting is used to convert classes to other classes, however this operation can break the type system.

For example, a "List" may use the class "ListItem". And the "ListItem" is then inherited by the "Item" class. So we may store "Item" classes in the "List" class.

Using casting we could recover the "Item" class from the "ListItem" class like this:


ListItem* ptr = list.front();

Item* item = (ListItem*)ptr;


However here, we have over-ruled the type system to enforce the cast from ListItem to Item. We need a way to perform this operation without breaking the type system rules.





One method is to store the type information in the base class, to be used later to reconstruct the derived class is, for example:

class ListItem {


    bool isItem = false; // stored type information

    bool isLinkedItem = false;



class Item : public ListItem {


    Item(ListItem* listItem) {

        // reconstruct this class using p_list_item details    



    // new method declared not accessible in ListItem class.

    void Print() {




ListItem* listItem = list.front();

if (listItem->isItem) {                           // check type information

    Item* item = new Item(listItem);              // reconstruct


    delete listItem;                                  // delete old object

} else if (ptr->isLinkedItem) {

    // .etc


This is more desirable since the type system is preserved, which means that our code can be statically verified by the compiler. Also, it has an effect on how the classes are designed. Since now our type information is being manually specified, we can potentially perform more complicated operations depending on the base types-type members, for example:

if (listItem->isItem && !listItem->isLinked && listItem->isBack) { // reconstruct // }

Friday, 13 November 2020

Abstraction function, representation invariant

Abstraction Function & Rep invariant

The abstraction functions map values in a class to the abstract concept's required values. The representation invariants are those values that satisfy the abstraction function. By asserting the abstraction function we create the link (abstraction) from ints and string to our concept. This can be seen as giving the class meaning.

The abstraction funtion is like a filter that only allows values past which correctly describe the abstract concept.
Figure 1 - RatNum ADT


Below ZeroNumber is the concept we want to create. We represent it with a float however the concept requires that it only represents the number 0. Therefore we assert the abstraction function on the inValue and discard any values which are not rep invariants, this results in our classes concept (of being zero) being enforced.

class ZeroNumber {
    ZeroNumber(float inValue) {
        assert(inValue == 0.0f);     (2. Assertion for rep invariants)
        value = inValue;
    float value;  (1. Representation)

The abstraction function here is that the ZeroNumber class must be a number that can only be 0.

The rep invariants here are all values of (inValue) that are 0.0f.  In order for abstration function to hold the assertion (2) must hold since only rep invariants are allowed.


  • Object-oriented Software Construction, 1st Edition Bertrand Meyer

Tuesday, 10 November 2020

Object interface design - telling it to do something vs doing something to it

 An object represents a concept otherwise known as a "thing" or noun. Traditionally, we're taught that you can create an object then tell it what to do, however, this design (coming from Alan Kay) actually results in a mess of global state (which is the usual complaint about OOP). A more appropriate view is a little more complicated, we can create an object but we must define what we can do TO it.

For example you can Squeeze an Orange, therefore the object Orange has the Squeeze method. Traditionally it would be modelled as Person.Squeeze(Orange) however, here you aren't actually doing something TO the person object you are telling the person to do SOMETHING.

Friday, 6 November 2020

Mixing DDD , OOP and Modularity

The world map is a model of the Earth, it tries to solve a specific problem of planning shipping routes. As you can see Greenland is much much bigger than it is in reality, but this is because the purpose of the map is to find angles to sail, not to display the relative sizes of parts of the world. This is the most important concept for DDD because when we write software (using OOP in particular, more on this later) we are actually solving a problem by creating a model (since OOP based upon concepts and 'real things')  

For example when creating a program that models an air conditioning fan (it could be anything, a network, architecture, command processor) we need to find out what problem exactly we are trying to solve with the model. You can try to model the fan taking into account everything such as its weight, colour, year of purchase.etc but if you place a problem ontop of the model, you can really simplify your model down to an elegant solution. This is very important since software complexity is the major battle we have when creating programs.

In software we aim to create highly modular programs. We do this because it makes the software easier to create and maintain since we only have to think about a small set of things at a time.

Using the OOP paradigm, objects are supposed replicate real-life things (as Bertrand Meyer puts) as a "cousin twice removed" from reality. So, to modularize our OOP programs into simplified models provides a perfect match. 

  1. OOP for modelling concepts as objects.
  2. Models made from objects for solving a problem.
  3. Models divided into contexts to modularize our software.

Some links

Here is the talk Eric gives that explains it:

It's very difficult to summarise such a large topic but here are some links that explain the concepts.



Tuesday, 3 November 2020

OOP Tell dont ask and SRP


Tell dont ask and SRP seem to be conflicting principles, one says to make the object do the work and the other says that each object should only change for one reason (meaning it must be responsible for 1 logical part). So how do we design our programs to conform to both of these principles when an object theoretically does more than one thing?

Protected state, released state

Tell don't ask is actually about querying the state an object, making a decision on that queries state, and then changing the state of that object. So its not saying that you can't query the objects, but just that state changes must happen from within. In relation to Commands and Queries, Queries cannot change the internal state or release write-access to any of the internal state which fits with the tell don't ask principle.

Creating tiny objects

The SRP is about making each object do one thing, it is self explanitory in that a single object must have a defined singular purpose. Often it requires a very high level of granularity which can be found by using very specific bounded contexts e.g. namespaces/nested classes.

Well formed objects, well formed designs

We can combine both together by recognizing that the Tell-Dont-Ask does not mean that you cannot query data from it. So we can create small, axiomatic objects that can be queried by other small objects for the information they need.

Shopping for an objects interface

One problem is that its hard to decide what should be exported by the object and what shouldn't. There are many opinions on how it should be done, but, i find that Bertrand Meyers explanation is most mathematically sound. . The subject is huge and there are hundreds of sources of information from everywhere!

Friday, 30 October 2020

OOP Object interfaces and decomposing the blob class


The interface of the object should provide meaningful services to clients. A problem arises when too many jobs are assigned to one class of object for example a client object:

class Client {


    void Authenticate();

    void SendMessageToChat();

    void JoinChatRoom();

    void LeaveChatRoom();

    void JoinGroup();

    // etc


Looks fine and useful but why does the client contain the information to manage groups and chat rooms?

Interface beauty and implementation blob

This object looks fine from the interface point of view and incredibly useful in that all the functions you need are in one place. But, it suggests a problem with the implementation behind the interface. The problem doesn't seem to be the interface semantically/syntactically but a problem in terms of information.

Since the Client's interface is the set of operations on the Client object, why does the client object contain the information to manage chat rooms, groups and all the other objects? The Client object holds way too much information about the rest of the system. This is known as a "God class" or "Blob".

Decomposing the blob

A solution for this would be to seperate the client object into further important concepts:

class Server;
class User;

class Client {


    void Authenticate(Server& server);


class ChatRoom {


    void Join(User& user);

    void Leave(User& user);

    void Post(User& user, string message);


class Group {


    void Join(User& user);

    void Leave(User& user);


Here the information is spread across meaningful objects in the system which is more desireable since now each object can be thought of and maintained individually (for example assigning multiple people on each object instead of all on the client object).

Paraphrasing the collaboration, Subject not object

Additionally there error made in the design in the original client class is that the client object is not viewed as the subject of the methods calls. For example; the object method SendMessageToChat() could be paraphrased as "tell the client to send a message to the chat" or "the client can send a message to the chat" but in these phrases the subject is Chat not Client. This is a confusing notion however its simplifed if you remember that an object is the subject of its methods and wording the class collaboration appropriatly. The original client seems to be a functional decomposition instead of a data abstraction view.


When creating objects, always remember that the object is the subject and to carefully paraphrase the collaborations.

Tuesday, 20 October 2020

Commands, queries and side-effects (object interface design)

Commands and queries

"A machine has 2 types of buttons, command and query buttons. When a command button is pressed, the machine performs work and its state is modified. When a query button is pressed, the machine returns some information about it." - Bertrand Meyer

Command corresponds to procedures, they do not return any results. For example:

class MyMachine {


    void SetName(const char * name); // is a command/procedu


Query corresponds to functions or attributes, for example:

class MyMachine {


    void SetName(const char * name);     // command/procedure

    const char * GetName() const;        // query/function        

    bool ProcessingReady() const;        // query/attribute


    const char * name;


Side effects

 "A side effect on an object is an operation that may change at least one attribute of the object."

The above command and query view is important with relation to side effects since  according to Bertrand Meyer in his book XXX (Chapter 7.7.2 "Commands and queries", Page 134)...

"The clean seperation between procedures and functions averts many of the pitfalls of traditional programming. Side-effect-producing functions, which have been elevated by some langauges (C seems to be the most extreme example) to the status of an institution, conflict with the classical notion of function in mathematics. A mathematical function will always give the same result when applied to the same argument."


From this information, we can create better class interfaces and keep them "more true" to the mathmatical foundation of abstract data type theory. This means that our code is closer to "formal verification" than guesswork.

OOP - Recovering lost class types

 Normally casting is used to convert classes to other classes, however this operation can break the type system. For example, a "List&q...