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So far we need something like the following declarations: Initialization is part of the job of new and different types require different work — new may even require different arguments for different types: Since constructor and destructor are type-specific and do not change, we pass both to new as part of the type description. Note that constructor and destructor are not responsible for acquiring and releasing the memory for an object itself — this is the job of new and delete.
The constructor is called by new and is only responsible for initializing the memory area allocated by new. For a string, this does involve acquiring another piece of memory to store the text, but the space for struct String itself is allocated by new. This space is later freed by delete. First, however, delete calls the des- tructor which essentially reverses the initialization done by the constructor before delete recycles the memory area allocated by new. Therefore, revising the declarations shown in section 2.
What should this pointer point to? If all we have is the address of an object, this pointer gives us access to type-specific information for the object, such as its destructor function. It seems likely that we will soon invent other type-specific functions such as a function to display objects, or our comparison function differ , or a function clone to create a complete copy of an object.
Therefore we will use a pointer to a table of function pointers. Looking closely, we realize that this table must be part of the type description passed to new , and the obvious solution is to let an object point to the entire type description: Looking down this list, we notice that every function works for the object through which it will be selected.
Only the constructor may have to cope with a partially initialized memory area.
We call these functions methods for the objects. Calling a method is termed a message and we have marked the receiving object of the message with the parameter name self. Since we are using plain C functions, self need not be the first parameter. Many objects will share the same type descriptor, i. We call all objects with the same type descriptor a class; a single object is called an instance of the class. So far a class, an abstract data type, and a set of possible values together with operations, i.
An object is an instance of a class, i. Conventionally speaking, an object is a value of a particular data type. The constructor is called by new for a new memory area which is mostly uninitialized: This is why we initialize this pointer already in new: The object is created at run time and the dashed pointers are then inserted.
We force a conversion of p which treats the beginning of the object as a pointer to a struct Class and set the argument class as the value of this pointer. Next, if a constructor is part of the type description, we call it and return its result as the result of new , i. Section 2. Note that only explicitly visible functions like new can have a variable parame- ter list.
Since we might later want to share the original parameters among several functions, we pass the address of ap to the constructor — when it returns, ap will point to the first argument not consumed by the constructor. This is used to call the destructor if any exists. Here, self plays the role of p in the previous picture. We force the conversion using a local variable cp and very carefully thread our way from self to its description: If the constructor decides to cheat, the destructor thus has a chance to correct things, see section 2.
If an object does not want to be deleted, its destructor would return a null pointer. All other methods stored in the type description are called in a similar fashion. In each case we have a single receiving object self and we need to route the method call through its descriptor: For the moment at least, we guard against null pointers.
In any case, differ illustrates why this technique of calling functions is called dynamic linkage or late binding: We will call differ a selector function. It is an example of a polymorphic func- tion, i. Once we implement more classes which all contain. We can view selectors as methods which themselves are not dynamically linked but still behave like polymorphic functions because they let dynamically linked functions do their real work. Polymorphic functions are actually built into many programming languages, e.
This phenomenon is called overloading: There is no clear distinction here: Methods can be polymorphic without having dynamic linkage. As an example, consider a function sizeOf which returns the size of any object: Notice the difference: They are derived from the correspond- ing components of struct Class by simply removing one indirection from the declarator.
Here is the application: We show the size of a String object — not the size of the text controlled by the object — and we check that two different texts result in different strings. Finally, we check that a copy is equal but not identical to its original and we delete the strings again.
Dynamic linkage helps to clearly identify which functions need to be written to implement a new data type. The constructor retrieves the text passed to new and stores a dynamic copy in the struct String which was allocated by new: The destructor frees the dynamic memory controlled by the string.
Since delete can only call the destructor if self is not null, we do not need to check things: This can easily be done by calling new: If we really compare two distinct strings, we try strcmp: All these methods are static because they should only be called through new , delete , or the selectors.
The methods are made available to the selectors by way of the type descriptor: In order to properly initialize the type descriptor, it also includes the private header new.
An atom is a unique string object; if two atoms contain the same strings, they are identical. Atoms are very cheap to compare: Atoms are more expensive to construct and destroy: Before the con- structor saves a text it first looks through the list to see if the same text is already stored.
Otherwise we insert the new string object into the circular list and set its reference count to 1. The destructor prevents deletion of an atom unless its reference count is decre- mented to zero. Otherwise we clear the circular list marker if our string is the last one or we remove our string from the list. With this implementation our application from section 2. In particular, a descriptor contains a pointer to a constructor which initializes the memory area allocated for the object, and a pointer to a destructor which reclaims resources owned by an object before it is deleted.
We call all objects sharing the same descriptor a class. An object is an instance of a class, type-specific functions for an object are called methods, and messages are calls to such functions.
We use selector functions to locate and call dynamically linked methods for an object. Through selectors and dynamic linkage the same function name will take dif- ferent actions for different classes. Such a function is called polymorphic. Polymorphic functions are quite useful. They provide a level of conceptual abstraction: A cheap and very convenient debugging tool is a polymorphic function store to display any object on a file descriptor.
This is difficult for Set because we can no longer record in the set elements to which set they belong. There should be more methods for strings: Things get interesting if we also deal with substrings. Atoms are much more efficient, if we track them with a hash table. Can the value of an atom be changed?
Does it make any difference what we pass to new? Rather than writing a few functions, each with a big switch to handle many special cases, we can write many small functions, one for each case, and arrange for the proper function to be called by dynamic linkage. This often simplifies a routine job and it usually results in code that can be extended easily. As an example we will write a small program to read and evaluate arithmetic expressions consisting of floating point numbers, parentheses and the usual opera- tors for addition, subtraction, and so on.
Normally we would use the compiler gen- erator tools lex and yacc to build that part of the program which recognizes an arith- metic expression. This book is not about compiler building, however, so just this once we will write this code ourselves.
If things go wrong, we simply read the next input line. Here is the main loop: If error is called somewhere in the program, longjmp continues execution with another return from setjmp. In this case, the result is the value passed to longjmp , the error is counted, and the next input line is read. The exit code of the program reports if any errors were encountered.
At the end of a line token is zero: White space is ignored and for a leading digit or decimal point we extract a floating point number with the ANSI-C function strtod. Any other character is returned as is, and we do not advance past a null byte at the end of the input buffer.
If we have detected a number, we return the unique value NUMBER and we make the actual value available in the global variable number. If we do not use yacc we recognize expressions by the method of recursive descent where grammatical rules are translated into equivalent C functions. For example: A grammatical rule like sum: Alternatives are translated into switch or if statements, iterations in the grammar produce loops in C.
The only problem is that we must avoid infinite recursion. If we recognize it, we must call scan 0 to advance in the input and store a new symbol in token. If we only want to perform simple arithmetic on numerical values, we can extend the recognition functions and compute the result as soon as we recognize the operators and the operands: If we want to build a system that can handle more complicated expressions we need to store expressions for later processing.
In this case, we can not only per- form arithmetic, but we can permit decisions and conditionally evaluate only part of an expression, and we can use stored expressions as user functions within other expressions.
All we need is a reasonably general way to represent an expression. The conventional technique is to use a binary tree and store token in each node: We need to introduce a union to create a node in which we can store a numerical value and we waste space in nodes representing unary operators. Additionally, process and delete will contain switch state- ments which grow with every new token which we invent.
Instead, we place some declarations in a header file value. As soon as token is consumed we need to call scan 0. All we require is that the vari- ous nodes start with a pointer for the dynamic linkage. This pointer is entered by new so that delete can reach its node-specific function: Variables or constants can even remain behind — they simply would do nothing in response to delete.
If we want to emit a postfix version of the expression, we would add a character string to the struct Type to show the actual operator and process would arrange for a single output line indented by a tab: It is represented as a structure with a double information field: Here are two examples: This is usually a bit tricky, depending on who is responsible for emitting the parentheses.
In addition to the operator name used for postfix output we add two numbers to the struct Type: As an example consider A binary node such as an addition is given the precedence of its superior and a flag indicating whether parentheses are needed in the case of equal precedence.
In any case, if our description has. Dynamic linkage has helped to divide a problem into many very simple functions. The resulting program is easily extended — try adding comparisons and an operator like?
Here is a quick test for one of the classes we would like to have: As usual, delete will recycle our point and by convention we will allow for a destructor. Since we expect to work with other graphical objects — hence the switch in the test program — we will provide dynamic linkage for draw. If we implement each graphical object relative to its own reference point, we will be able to move it simply by applying move to this point.
Therefore, we should be able to do without dynamic linkage for move. It replaces selectors such as differ introduced in section 2. Once again, object-orientation has helped us to identify precisely what we need to do: If we stick with two-dimensional, Cartesian coordinates, we choose the obvious representation: Drawing happens a bit differently, but moving only requires that we change the coordinates of the center.
This is where we would normally crank up our text editor and perform source code reuse. We make a copy of the implementation of points and change those parts where a circle is different from a point. The necessary actions are identical for a point and a circle: However, in one case, move works on a struct Point, and in the other case, it works on a struct Circle.
If move were dynamically linked, we could pro- vide two different functions to do the same thing, but there is a much better way. Consider the layout of the representations of points and circles: The picture shows that every circle begins with a point. If we derive struct Circle by adding to the end of struct Point, we can pass a circle to move because the initial part of its representation looks just like the point which move expects to receive and which is the only thing that move can change.
Here is a sound way to make sure the initial part of a circle always looks like a point: Information hiding demands that we should never reach into the base structure directly; therefore, we use an almost invisible underscore as its name and we declare it to be const to ward off careless assignments.
This is all there is to simple inheritance: It is perfectly sound to pass a circle to move: While we can pass points as well as circles to move , it is not really a polymorphic function: The situation is different for a dynamically linked method like draw. Let us look at the previous picture again, this time with the type descriptions shown expli- citly: Circle size ctor dtor draw struct Class When we up-cast from a circle to a point, we do not change the state of the circle, i.
Consequently, the circle viewed as a point still has Circle as a type description because the pointer in its. A subclass inherits the statically linked methods of its superclass — those methods operate on the part of the subclass object which is already present in the superclass object.
A subclass can choose to supply its own methods in place of the dynamically linked methods of its superclass. If inherited, i. Consider the declarations for move and draw: The only difference is that we declare a statically linked method like move as part of the abstract data type interface in Point.
Static linkage is more efficient because the C compiler can code a subroutine call with a direct address, but a function like move cannot be overwritten for a subclass. Dynamic linkage is more flexible at the expense of an indirect call — we have decided on the overhead of calling a selector function like draw , checking the arguments, and locating and calling the appropriate method.
Additionally, the macro needs the declaration of struct Class which we have thus far made avail- able only to class implementations and not to the entire application.
Unfortunately, we pretty much decide things when we design the superclass. While the function calls to the methods do not change, it takes a lot of text editing, possibly in a lot of classes, to switch a function definition from static to dynamic linkage and vice versa. Beginning in chapter 7 we will use a simple preprocessor to simplify coding, but even then linkage switching is error-prone.
In case of doubt it is probably better to decide on dynamic rather than static linkage even if it is less efficient. Generic functions can provide a useful concep- tional abstraction and they tend to reduce the number of function names which we need to remember in the course of a project. If, after implementing all required classes, we discover that a dynamically linked method was never overwritten, it is a lot less trouble to replace its selector by its single implementation, and even waste its slot in struct Class, than to extend the type description and correct all the initiali- zations.
The representation of a circle is declared in a second header file, Circle. For a subclass it includes the representation file of the superclass so that we can derive the representation of the subclass by extending the superclass: The point is certainly not constant — move will change its coordinates — but the const qualifier guards against accidentally overwriting the components. The representation file Circle. Finally, the implementation of a circle is defined in the source file Circle.
From an information hiding perspective this is not such a good idea. While reading coordinate values should not create major problems we can never be sure that in other situations a subclass implementation is not going to cheat and modify its superclass part directly, thus potentially playing havoc with its invariants. Efficiency dictates that a subclass reach into its superclass components directly. Information hiding and maintainability require that a superclass hide its own representation as best as possible from its subclasses.
If we opt for the latter, we should provide access functions for all those components of a superclass which a subclass is allowed to look at, and modification functions for those components, if any, which the subclass may modify. If we declare them in the representation file for the superclass, which is only included in the implementations of subclasses, we can use macros, because side effects are no problem if a macro uses each argument only once. As an example, in Point.
The technique is to up-cast the pointer into our superclass and reference the interesting component there. Outside the implementation of a subclass we can only use statically linked methods for access and modification functions.
We cannot resort to macros because the internal representation of the superclass is not available for the macros to reference. Information hiding is accomplished by not providing the representa- tion file Point. The macro definitions demonstrate, however, that as soon as the representa- tion of a class is available, information hiding can be quite easily defeated. Here is a way to conceal struct Point much better. Inside the superclass implementation we use the normal definition: Your email address will not be published.
Skip to content. Principles of Object Oriented Programming 2. Tokens, Expressions, and Control Structures 4. Classes and Objects 6. Constructors and destructor 7. Operator Overloading and Type Conversions 8. Extending Classes 9. Pointers, Virtual Functions, and Polymorphism Working with Files Templates Exception Handling Introduction to the Standard Template Library Manipulating Strings