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# EiffelBase, The Kernel

In addition to basic concepts close to the language level, the Kernel covers such common needs asinput andoutput,storage andretrieval of objects on persistent storage, fine control overexception handling andmemory management, and access tooperating system facilities. The kernel can be divided into 5 logical clusters of classes:

- The first cluster contains the universal class defining facilities accessible to all other classes: ANY. Every developer-defined class is a descendant of this class.
- The second cluster includes classes whose facilities are directly related to language concepts:
  - Classes describing the basic types: BOOLEAN, CHARACTER, INTEGER, REAL and DOUBLE
  - Arrays: class ARRAY
  - Tuples: classTUPLE
  - Strings: class STRING
  - Basic facilities: class BASIC_ROUTINES
- The third cluster provides input and output facilities:
  - STD_FILES offers basic mechanisms, sufficient for simple input and output.
  - FILE describes the notion of sequential file, viewed as a sequence of characters and fully integrated in the data structure library.
  - DIRECTORY gives properties of directories (files serving as collections of other files).
- The next cluster, through class STORABLE, makes it possible to store object structures on persistent storage and retrieve them later. This facility can also be used to transmit object structures through pipes or over a network.
- The last cluster provides access to internal properties of the compiler and environment, useful for applications that need some fine-tuning of the basic mechanisms:
  - Class EXCEPTIONS (complemented by UNIX_SIGNALS for Unix-type platforms) provides control over the exception handling mechanism, in particular for applications that need to handle different types of exception in different ways.
  - Similarly, classes MEMORY and GC_INFO provide ways to control the garbage collector and tailor it to specific needs.
  - Class ARGUMENTS gives access to the command-line arguments.

# Universal Class and its Features

The Eiffel inheritance mechanism is set up in such a way that every class is a descendant of a Kernel Library class called ANY. The features of this class provide a number of generally applicable facilities covering such needs as comparison, copying and rudimentary input and output.

## The structure of universal classes

Every class which has no inheritance clause is understood to have an inheritance clause of the form

    inherit
    	ANY

As a result, every developer-defined class is a descendant of ANY. You may introduce your own project specific features in ANY so that all the classes of your system will be able to use these features.

## Using the universal classes

If you need to rename or redefine a feature inherited from one of the universal classes, you should include an explicit inheritance clause, as in

    class
    	C
    inherit
       	ANY
          		rename
             			out as basic_out
          		redefine
             			print
          		end
       	...
    feature
     	...
    end

The features of ANY are usable in both qualified and unqualified form. For example, the argumentless function out, which produces a printable representation of any object, may be called under either of the forms

    x := out
    x := a.out

The first call yields a printable representation of the current object; the second, which assumes that a is not void, yields a printable representation of the object attached to a.

## Input and output features

Some of the features of ANY cover common input and output needs.
Feature io, of type STD_FILES, gives access to standard input and output facilities. For example, io . input is the standard input file and io.new_line will print a line feed on the standard output. Feature io is declared as a once function which, when first called, returns the value of an instance of STD_FILES that provides access to the standard input, the standard output and the error output. As a result, io is never void, so that operations such as io . new_line are always possible.
Function out, of type STRING, is a universal mechanism for obtaining a simple external representation of any object. For non-void x of any type, the string x . out is a printable representation of x . This works for x of all types, reference or expanded. For example, if x is an integer expression, x . out is its string representation, such as -897 ; if n is a non-void reference, x . out is (recursively) the concatenation of the result of applying out to the successive fields of the attached object, each labeled by the name of the corresponding attribute. You may redefine out in any class to specify any suitable format for displaying instances of the class. To obtain the default representation regardless of any redefinition of out, use tagged_out, declared as a frozen synonym of the original out.
The call print ( x ) will output the value of x . out on the default output if x is not void, and do nothing otherwise.

## Copy and comparison routines

Procedure copy copies the fields of an object onto those of another. It is used under the form

    target.copy (source)

Here both target and source must be non-void; this means that copy is only good for copying onto an object that already exists. If you need both to allocate a new object and to initialize it as a copy of another, use the function clone. For non-void source, the assignment

    target := clone (source)

starts by creating a new object. If source is void, target will be made void too. The boolean function equal compares two objects for field-by-field equality. This is different from the equality operators = and /= which, in the case of reference types, compare references, not objects.
The function deep_clone produces a duplicate of an entire object structure. The boolean function deep_equal determines whether two object structures are recursively identical. These routines are thedeep counterparts of the shallow copy and equality tests provided by clone and equal.
A class that needs a specific notion of equality and the corresponding copy semantics may redefine copy and is_equal (from which equal follows, since equal ( a , b ) is defined as a . is_equal ( b ) for non-void a ). You will find such redefinitions in a number of classes of the Base libraries. For example an instance of STRING is a string descriptor containing a reference to the actual character sequence, not that sequence itself, so that what the default equal compares and the default copy copies is the descriptor, not the string. Class STRING redefines these routines to yield the semantics normally expected by string clients; the frozen variants standard_copy and standard_equal, originally declared as synonyms to equal and copy, remain available with the default semantics.
The function clone is defined in terms of copy, and so will follow any redefinition of copy. This makes it impossible to change the semantics of one but not of the other, which would be a mistake. The variant standard_clone is defined in terms of standard_copy.

## Type information

The string-valued query generator, applied to any object, returns the name of the object's generating class: the class of which it is an instance. The boolean function conforms_to makes it possible to test dynamically whether the type of an object conforms to that of another - that is to say whether the first one's generator is a descendant of the second one's.
These two features enable clients to ascertain the dynamic type of an entity at runtime. They are only useful for low-level components; the normal mechanism for type-dependent operations is dynamic binding.

## Miscellaneous

The query Void, of typeNONE, denotes a reference that is always void - not attached to any object.
Procedure do_nothing does what its name implies.
Function default also has an empty body; its result type is like Current , so what it returns is the default value of the current type. This is mostly interesting for expanded types, since for reference types the default value is simply a void reference.

# Language-related Facilities

A number of classes offer facilities which are very close to the language level. Here too the book Eiffel: The Language covers the classes in detail, so we can satisfy ourselves with a quick summary; the flat-short forms appear in part C.

## Basic types

The basic types BOOLEAN, CHARACTER, INTEGER, REAL and DOUBLE are defined by classes of the Kernel library.
In reading the class specifications for the numeric types INTEGER, REAL and DOUBLE, you might think that the type declarations are too restrictive. For example the addition operation in class REAL reads

    infix "+" (other: REAL): REAL

but there is actually no problem here. A language convention applicable to all arithmetic expressions, the Balancing rule, states that in any such expression all operands are considered to be converted to the heaviest type, where DOUBLE is heavier than REAL and REAL is heavier than INTEGER. So mixed-type arithmetic, consistent with common practice, is possible and indeed frequent.

## Arrays

To create and manipulate one-dimensional arrays, use class ARRAY of the Kernel Library. Arrays are not primitive language elements; instead, they are handled through class ARRAY. This class is 'normal' in the sense that it may be used just as any other class by client and descendant classes. It is also somewhat special, however, in that the Eiffel compiler knows about it and uses this knowledge to generate efficient code for array operations.
To create an instance of ARRAY, use the creation instruction

    create my_array.make (1, u)

where the arguments indicate the lower and upper bounds. These bounds will then be accessible as my_array . lower and my_array . upper. The number of items is my_array . count; feature capacity is a synonym for count. The class invariant expresses the relation between count, lower and upper.
To access and change the item at index i in array a, you may use features item and put, as in

    x := my_array.item (i)
    my_array.put (new_value, i)

Function item has an infix synonym, infix " @ " , so that you may also write the first assignment above more concisely as

    x := my_array @ i

Features item, infix " @ " and put have preconditions requiring the index ( i in the above calls) to be within the bounds of the array. This means that you can detect bounds violations (which correspond to bugs in the client software) by using a version of class ARRAY compiled with precondition checking on. The bounds of an array may be changed dynamically through procedure resize. Previously entered elements are retained. Rather than an explicit resize, you may use calls to procedure force which has the same signature as put but no precondition; if the index is not within the current bounds force will perform a resize as necessary.

## Optimizing array computations

CAUTION: Although ARRAY benefits from an efficient implementation, its more advanced facilities such as resizing do not come for free. For extensive computations on large arrays, an optimization may be desirable, bypassing these facilities. The technique yields loops that run at about the same speed as the corresponding loops written in C or Fortran (the usual references for array computations). It is of interest for advanced uses only, so that you may safely skip this section on first reading unless your domain of application is numerical computation or some other area requiring high-performance array manipulations.

The optimization relies on the class SPECIAL, used internally by ARRAY but of no direct interest to client developers in most common uses. With the declarations

    my_array: ARRAY [SOME_TYPE]
    direct_access: SPECIAL [SOME_TYPE]

you may use direct_access in lieu of 'my_array' within a critical loop, provided none of the operations may resize the array. Typically, the operations should only include put and item. In such a case you can use the following scheme:

    direct_access:= my_array.area

    	-- The critical loop:
    from
    	   some_initialization
    	   index := some_initial_index
    until
    	   index = some_final_index
    loop
    	   ...
    	   x := direct_access.item (index)
    	   ...
    	   direct_access.put (some_value, index)
    	   ...
    end

This replaces an original loop where the operations were on my_array . Feature area of ARRAY gives direct access to the special object, an instance of SPECIAL, containing the array values. Features put and item are available in SPECIAL as in ARRAY, but without the preconditions; in other words, you will not get any bounds checking. Instances of SPECIAL are always indexed from zero, in contrast with arrays, whose lower bound is arbitrary, 1 being the most common value. But rather than performing index translations (that is to say, subtracting my_array . lower from index throughout the loop) it is preferable to use the following simple technique: if the lower bound 'lb' of my_array is 1 or another small integer, use 0 as lower bound instead when creating my_array , but only use the positions starting at 'lb'. You will waste a few memory positions (0 to lb-1), but will not have to change anything in your algorithm and will avoid costly subtractions.
It is important to note that this optimization, if at all necessary, should at most affect a few loops in a large system. You should always begin by writing your software using the normal ARRAY facilities; then once you have the certainty that the software is correct, if you detect that a large array computation is hampering the efficiency of the system, you may apply the above technique to get the fastest performance out of that computation. The change to the software will be minimal - a few lines - and will be easy to undo if necessary.

## Tuples

A new Kernel Library class is introduced:TUPLE.
Alone among all classes, classTUPLE has a variable number of generic parameters.TUPLE,TUPLE [X],TUPLE [X, Y],TUPLE [X, Y, Z] and so on are all valid types, assuming valid types X, Y, Z and so on.
Conformance rules:

    [CONF1]
    	For n >= 0
    	TUPLE [U1, U2, ..., Un, Un+1] conforms to
    	TUPLE [U1, U2, ..., Un]

(and hence toTUPLE [T1, T2, ..., Tn] if each of the Ui conforms to each of the Ti for 1 <= i <= n.)

In particular all tuple types conform toTUPLE, with no parameter.

    [CONF2]
    	For n >= 0 If *every* one of the types T1, T2, ..., Tn conforms
    	to a type T, then TUPLE [T1, T2, ..., Tn] conforms
    	to ARRAY [T].

Definition: a "tuple type" is any type based on classTUPLE, i.e. any type of the formTUPLE [T1, T2, ..., Tn] for any n (including 0, for which there is no generic parameter).

(Note 1: CONF1 should be understood in terms of the underlying mathematical model. Mathematically,TUPLE [T1, T2, ..., Tn] is the set TUPLEn of all partial functions f from N+ (the set of non-negative integers) to T1 U T2 U ... Tn, such that:

- The domain of f contains the interval 1..n (in other words, f is defined for any i such that 1 <= i <= n).
- For 1 <= i <= n, f (i) is a member of Ti.

With this definition, TUPLEn is indeed a subset of TUPLEn+1, and in particular TUPLE0, the empty set, is a subset of TUPLEn for any n.)

Semantics: an instance ofTUPLE [T1, T2, ..., Tn] is a tuple whose first element is an instance of T1, the second element being an instance of T2 etc. (The precise definition is the mathematical one given in note 1.) Note that there can be more than n elements to the tuple: for example a tuple with first element 5 and second element "FOO" is an instance of all of the following tuple types:TUPLE;TUPLE [INTEGER];TUPLE [INTEGER, STRING].

It may seem restrictive at first to permit only one class,TUPLE, to have an arbitrary number of actual generic parameters. Why not have a general mechanism for declaring any class C so that all of C [X], C [X, Y] etc. are valid? But in fact this is not really a restriction. To obtain this effect without any complicated language convention, just declare C as

C [G -> TUPLE]

and then use the generic derivations

C [TUPLE [X]]
C [TUPLE [X, Y]]

and so on. This also makes it possible to have the effect of some fixed parameters and some variable ones, as in

C [G, H, I -> TUPLE]

so we have all the necessary flexibility.)

## Tuple expressions

Let e1, e2, ..., en be expressions of respective types T1, T2, ..., Tn. Then the expression

[e1, e2, ..., en]

denotes an instance ofTUPLE [T1, T2, ..., Tn], whose first element is e1, the second element being e2 etc.
Tuple expressions can be nested: whereas

[1, 2, 3]

is a tuple with three elements (representing an instance ofTUPLE [INTEGER, INTEGER, INTEGER]),

[1, [2, 3]]

is a tuple with two elements, the second one itself a tuple; the overall expression represents an instance ofTUPLE [INTEGER,TUPLE [INTEGER, INTEGER].
As a special case of tuple expression syntax, the delimiters [ and ] are replaced by parentheses for the tuple representing the actual argument list of a routine call (see section 4).

## Tuple features

The exact specification of classTUPLE will be described in an addition to ELKS. The principal features are:

- count (number of significant elements)
- item (i), with the obvious precondition: the i-th element, of typeANY (since the value of i is not known at compile time); also first, second, third, fourth and fifth, of the appropriate types.
- put (x, i), with the obvious precondition: replace i-th element with x. If argument x is not of the appropriate type Ti there is no effect.
- is_equal: redefined to consider only the first n elements, where n is the smaller length.

Other features under consideration include:

- stripped (i): a tuple of typeTUPLE [T1, T2, Ti-1, Ti+1, ..., Tn], derived from the current one by removing the i-th component, again with the obvious precondition.
- wedged (x, i): a tuple with one more element, inserted at position i.
- infix "+": tuple concatenation
- infix "++": element concatenation; t ++ x is the same thing as t.wedged (x, t.count + 1).

## What have we gained?

First we have solved the only case in the Eiffel language in which an expression has no precisely defined type: polymorphic manifest arrays. We don't have manifest arrays any more, but manifest tuples, with a precisely defined type. No incompatibility is introduced thanks to rule CONF2. The original syntax for manifest arrays, Result := <<e1, e2, ..., en>>, will continue to be supported.
Second, we can define functions that return multiple results. This is a quite significant increase in expressive power. No common language has that. (You have to go to Lisp and functional languages.) Just defineTUPLE [...] as the result type; in the function, you will write things like

Result := [e1, e2, ..., en]

Also, from a theoretical viewpoint, feature calls are simpler and more homogeneous: every feature takes exactly one tuple as argument and returns exactly one tuple as a result. (Either of these tuples may be empty: the first for a feature with no argument, the second for a procedure.) The syntax for a call becomes

    Feature Arguments

with Arguments defined as

    Tuple_expression

where the Tuple_expression uses the form given in section 2 but with the outermost [ and ] delimiters replaced by parentheses to conform to usual practice. So the call

    f (a, b, c)

which we continue to think of as having three arguments a, b and c, formally has only one tuple argument [a, b, c]. This is of course not to be confused with a call of the form

    g ([a, b, c])

which has one argument (a tuple with three elements) in both the ordinary and the formal sense.

## Active, iterators, numerical applications, introspection

For a set of important applications of tuples see the book chapter onagents and iterators which also covers aspects of numerical software and related topics following from the tuple mechanism.

## Temporary limitations

The implementation of tuples has the following limitations:

- Conformance of ARRAY types toTUPLE types is not yet fully supported.
- ClassTUPLE does not have features such as first and second. You must use item and, in most cases, an assignment attempt.

## Strings

Strings are handled by class STRING, similar in many respects to ARRAY. Strings are of arbitrary size. The make creation procedure takes an integer argument, as in:

    s, s1, s2, s3: STRING
    	...
    create s.make (30)

The argument indicates the number of characters for the initial allocation. This is not an absolute limit: the string will automatically grow or shrink as a result of future operations. You may always request a resizing explicitly by calling procedure resize.

## String descriptor

The object attached at run-time to an entity such declared of type STRING is not the actual sequence of characters but a string descriptor, which contains a reference to the actual string contents.

As a result, four assignment or assignment-like operations are possible:

- A1 s1 := s
- A2 s2 . share ( s )
- A3 s3 := clone ( s )
- A4 s4 . copy ( s )

As illustrated below, A1 is a reference assignment: s1 will be attached to the same descriptor as s. A2 keeps the descriptors distinct, but make them refer to the same sequence of characters. A3 uses the redefinition of clone for class STRING: s3 will be attached to a new string, completely distinct from the string attached to s1 although made of identical characters. A4 has almost the same effect asA3, but is only applicable if s4 was not void, and will override the existing descriptor rather than creating a new one.

fig. 1: Effect of string assignment and copy operations

BASIC_ROUTINES provides a number of conversion functions, such as charconv.

# Files, Input, Output

A few classes of the Kernel Library support file manipulation, input and output: STD_FILES, FILE, DIRECTORY and UNIX_FILE_INFO. For simple applications it suffices to use STD_FILES, but to understand the concepts better it is preferable to look first at the other two.

## General files

FILE describes the notion of sequential file viewed as a data structure which fits in the general taxonomy of EiffelBase.
The class declaration defines files as unbounded sequences of characters. This means that you will find in FILE all the operations on sequential data structures that you have come to know and love by reading this documentation - at least, all that apply. Just as stacks and linked lists, files have put, extend, has, item and so on. More specific to files are the typed input and output operations. For output, you will find put_character, put_integer, put_real, put_double and put_string, as well as new_line. For input you will find read_integer and its co-conspirators.

Caution: Note the application to input features of the command-query separation principle.
The input features such as read_integer do not by themselves return a result; they set the values of queries such as last_integer. So the normal way to read is through two operations:
my_file . read_integer
new_value := my_file . last_integer

Queries are available to determine the status of a file, in particular exists, is_readable, is_executable, is_writable, is_creatable, is_closed, is_open_read and so on.

Caution: You will notice in the flat-short form that all these queries except the first have exists as a precondition. This precondition is good for efficiency since it saves an existence test - a relatively expensive operation - when you know that a certain file exists. But it also means that if you have any doubt about the file's existence you must use the queries in the style
if my_file . exists and then my_file . is_readable then ...

FILE is a deferred class. Various implementations are possible. A quite detailed one is PLAIN_TEXT_FILE, which adds many features for accessing reading and writing data from/to a file.
UNIX_FILE_INFO describes objects that contain internal information, such as protection mode and size, about a file.
The class DIRECTORY describes those files which are directories - nodes in the tree describing the file structure.

## Basic input and output

Regardless of the operating system that you use, for simple input and output STD_FILES is sufficient. You may inherit from that class to gain direct access to its features; or you may declare an entity of type STD_FILES. But remember that a feature of this type is always available: io, from class ANY. Thanks to this feature you may include simple input and output in any class, with instructions such as

    io.put_string ("My message")

STD_FILES defines three default files through features input, output and error. These features are Once functions, so that the first reference to any one of them will automatically create the corresponding file descriptor and open the associated file.
To simplify the writing of common input and output operations, the most frequently used features of class FILE - for reading and writing integers, reals and so on, as discussed next - have been repeated in STD_FILES so as to apply to the default input and output. Procedure put_string in the example at the beginning of this section is typical: it writes its output on the standard output. More generally, STD_FILES has all theput_xxx,read_xxx andlast_xxx features of FILE.

# PERSISTENCE, STORAGE AND RETRIEVAL

Most object-oriented applications need the ability to store object structures on persistent storage for later retrieval, and to transfer such object structures to other applications.
Class STORABLE addresses this need.

## Persistence completeness

A fundamental requirement on object persistence mechanisms is the Persistence Completeness rule, stated as follows in Eiffel: The Language:
Whenever a routine of class STORABLE stores an object into an external file, it stores with it the dependents of that object. Whenever one of the associated retrieval routines retrieves a previously stored object, it also retrieves all its dependents.
Storing an object just by itself would usually result in wrong semantics: most objects contain references to other objects, which must also be stored and retrieved with it. The persistence completeness rule ensures that this is always the case. It also means, of course, that the features of STORABLE must do much more than simple input and output; they must perform complete traversals of object structures.

## Using the storage and retrieval facilities

Class STORABLE is meant to be used as ancestor. You can use its features in any descendantC; for example a routine ofC may contain a call of the form basic_store ( my_descriptor ) .
The effect of this call will be to store the current object and all its dependents into the file denoted by my_descriptor.
Although basic_store and other procedures of STORABLEwill in general process objects of many different types, only the generating class of the structure's initial object,C in our example, needs to be a descendant of STORABLE.

## Varieties of store operations

Two variants of the store operation are supported: basic store and general store. Basic store produces more compact structures in the resulting files, and is slightly faster; but the resulting structure is dependent on the system which executes the store operation ('System' is taken here, as elsewhere in this documentation, in its Eiffel sense of an executable assembly of classes, compiled together with the help of a configuration file.) This means that you can use procedure basic_store to store an object structure during an execution of a system if you will only retrieve it later in that execution, or in a
subsequent execution of the same system. If you need to store objects from a certain system and then retrieve them from a different system, possibly running on a different computer, use general_store. The result of general_store is still platform-dependent; the representation of numbers, in particular, is determined by the underlying machine architecture. A third variant called independent_store will be added in the future to STORABLE, using an entirely platform-independent storage format; then a system running on a computer with a certain architecture will be able to retrieve, without any explicit conversion operation, object structures stored by a system running on a machine of a completely different architecture. With general_store the storing and retrieving computers must have compatible architectures.

## Retrieval

You only need to be aware of the difference between basic and general store at storage time. The stored structure will always be available through feature retrieved; this feature will figure out, from the format of the stored structure, whether it was stored by basic_store or general_store, and will decode it accordingly.
Feature retrieved returns a result of type STORABLE and is typically used through an assignment attempt of the form

    x ?= retrieved (my_descriptor)

The assignment attempt is necessary because retrieved returns a result of type STORABLE whereas the type of x will be based on a proper descendant of STORABLE. If the structure in the file has been corrupted and retrieved is unable to do its job, it will trigger an exception. The code for that exception in class EXCEPTIONS (which inherits it from EXCEP_CONST and is discussed in the next section, together with the notion of exception code) isRetrieve_exception.

## Recoverable storable

Sometime you will be in a position where the schema of a class will have changed between the time you stored your object and the time you are trying to retrieve it. Such changes include:

- class name changed
- attributes have been added or removed
- attributes have been renamed
- attributes type have changed

The storable mechanism allows you to retrieve the old version of the object only if it was saved using the independent_store facility. Each time you retrieve an object of a certain base class whose schema has changed, the featurecorrect_mismatch will be called. This feature is defined inANY and by default will raise an exception. To handle the mismatch, you need to redefinecorrect_mismatch in the base class whose schema has been changed. For example in EiffelBase,HASH_TABLE has changed between version 5.1 and version 5.2 to use SPECIAL rather thanARRAYfor its internal data storage. To retrieve a 5.1 version ofHASH_TABLE, you need to definecorrect_mismatch as following:

    correct_mismatch is
    		-- Attempt to correct object mismatch during retrieve using `mismatch_information'.
    	local
    		array_content: ARRAY [G]
    		array_keys: ARRAY [H]
    		array_marks: ARRAY [BOOLEAN]
    	do
    		-- In version 5.1 and earlier, `content', `keys' and `deleted_marks'
    		-- where of base class ARRAY. In 5.2 we changed it to be a SPECIAL for
    		-- efficiency reasons. In order to retrieve an old HASH_TABLE we
    		-- need to convert those ARRAY instances into SPECIAL instances.

    			-- Convert `content' from ARRAY to SPECIAL
    		array_content ?= mismatch_information.item ("content")
    		if array_content /= Void then
    			content := array_content.area
    		end

    			-- Convert `keys' from ARRAY to SPECIAL
    		array_keys ?= mismatch_information.item ("keys")
    		if array_keys /= Void then
    			keys := array_keys.area
    		end

    			-- Convert `deleted_marks' from ARRAY to SPECIAL
    		array_marks ?= mismatch_information.item ("deleted_marks")
    		if array_marks /= Void then
    			deleted_marks := array_marks.area
    		end

    		if content = Void or keys = Void or deleted_marks = Void then
    			-- Could not retrieve old version of HASH_TABLE. We throw an exception.
    			Precursor {TABLE}
    		end
    	end

Note the use ofmismatch_information, this is a once feature ofANY of type MISMATCH_INFORMATION which behaves like aHASH_TABLE. The keys of the table are the names of the attributes on which a mismatch occurred and the values are the corresponding object fields as they were originally stored. In this particular case ofHASH_TABLE we know that the previous version was anARRAY, so we do an assignment attempt and if it succeeds we assign itsarea to the corresponding attribute ofHASH_TABLE.

If a class name changed, then you need to create an instance ofCLASS_NAME_TRANSLATIONS, it behaves like aHASH_TABLE where the keys represent the old name, and the value the new name. This instance needs to be created before the call to retrieved.

# Access To Internal Properties

In some applications you may need to fine-tune the exception handling and memory management mechanisms. You may also need a simple way to access command-line arguments. In less common cases you may require low-level access to internal properties of objects.

## Exception handling

Class EXCEPTIONS enables you to control the handling of exceptions. UNIX_SIGNALS, discussed next, complements it for the special case of fine-grain signal handling on Unix or Unix-like platforms. Both are meant to be inherited by any class that needs their facilities.
The basic exception mechanism treats all exceptions in the same way. In some cases it may be useful to discriminate in a Rescue clause between the various possible causes.
Class EXCEPTIONS provides the features to do this. Each kind of exception has an integer code, which you can use through several features:

- The integer-valued query exception which gives the code of the latest exception.
- Queries which determine the general nature of the latest exception: is_signal which determines whether the exception was an operating system signal; is_developer_exception which determines whether it was explicitly caused by a raise, as explained next; assertion_violation.
- Query recipient_name which gives the name of the exception's recipient - the routine that was interrupted by the exception.

The class also provides a set of constant integer-valued attributes which denote the various possible codes, such as No_more_memory, Routine_ failure and Precondition_violation. So you can test the value of exception against these codes if you need to ascertain the precise nature of an exception. To keep EXCEPTIONS simple these constant attributes are declared in a class EXCEP_CONST, of which EXCEPTIONS is an heir.
Another occasional requirement is for a mechanism to trigger an exception explicitly. Procedure raise answers this needs; the argument, a string, is the tag chosen for the exception. The code in this case is Developer_exception; the query is_developer_exception will return true; and the tag is accessible through feature tag_name.
You will notice in the interface specification for EXCEPTIONS that for some properties of the latest exception there are two features, one with a name such as exception or recipient_name as seen above and the other with a name prefixed by original_: original_exception, original_recipient_name.

CAUTION: The reason for the presence of these pairs is that the immediately visible cause of a routine interruption may not be the real one. Assume that routiner from classC, which has a Rescue clause, callss fromD with no Rescue clause, and that some call executed bys causes a precondition violation. Becauses has no Rescue clause of its own,s will fail. Up the call chain, the first routine that has a Rescue clause -r itself, or one of its own direct or indirect callers - may process the exception; but if it examines the exception code through attribute exception it will get the value of Routine_failure. This may be what you want; but to handle the situation in a finer way you will usually need to examine the code for the original exception, the one that interrupteds. This code will be accessible through the attribute original_exception, which in this case will have the value ofPrecondition, the exception code for precondition violations. So you have the choice between exploring the properties of the original exception, or those of the resulting routine failures. Just make sure you know what you are looking for.

As you will see from the header comments in the flat-short form of class EXCEPTIONS, the queries that return detailed information about an exception, such as assertion_violation, all give an answer determined by original_exception rather than exception, since when the two are different (that is to say, when you handle the exception in a routine other than the original recipient) the value of exception is alwaysRoutine_failure and there is nothing more to say about it.

## Signal handling

The features of class EXCEPTIONS enable you to determine whether a certain exception is a signal - an operating system event such as may result from a child process that disappears, a window that is resized, a user that hits the Break key and many others. But they do not give you more details because the exact set of possible signals is highly platform-dependent.
Class UNIX_SIGNALS complements EXCEP_CONST by providing codes for the signals of Unix and similar systems, such as Sigkill for the 'kill' signal and Sigbus for bus error.
Query is_defined ( some_signal ) , where some_signal is an integer code, will determine whether some_signal is supported on the platform.
A class whose routines need to perform specific processing depending on the nature of signals received should inherit from UNIX_SIGNALS, or a similar class for another platform.
Because signal codes are platform-dependent, the features of UNIX_SIGNALS are implemented as once functions - computed on the first call - rather than constants, although this makes no difference to clients.

## Memory management

Class MEMORY, like EXCEPTIONS, is meant to be used as an ancestor by classes that need its facilities. It offers a number of features for controlling memory management and fine-tuning the garbage collection mechanism, a key component of the Eiffel Software environment.
One of the most useful features in this class is dispose. This procedure describes actions to be applied to an unreachable object just before the garbage collector reclaims it. By default, as declared in MEMORY, the procedure does nothing; but you may redefine it in a proper descendant of MEMORY to describe dispose actions. Normally such actions will involve freeing external resources: for example a class describing file descriptors may redefine dispose so that whenever a descriptor object is garbage-collected the corresponding file will be closed.

Caution: This example is typical of proper uses of dispose.In a dispose procedure you should not include any instruction that could modify the Eiffel object structure, especially if some objects in that structure may themselves have become unreachable: these instructions could conflict with the garbage collector's operations and cause catastrophic behavior. The legitimate use of dispose redefinitions is for disposing of non-Eiffel resources.

Other features of MEMORY provide direct control over the operation of the garbage collector. You can in particular stop garbage collection through a call to collection_off, and restart it through a call to collection_on. By default, garbage collection is always on (a testimony to its authors' trust in its efficiency). Garbage collection is normally incremental, so as not to disrupt the application in a perceptible way. To start a complete garbage collection mechanism - reclaiming all unused objects - call procedure full_collect. The remaining features of MEMORY enable finer control of the collection mechanism and are useful in special cases only. You will even find a free procedure providing brave (and competent) developers with a mechanism for reclaiming individual objects manually.
MEM_INFO, the result type for query memory_statistics in MEMORY, describes objects containing information collected about memory usage. The features of GC_INFO provide statistics about the garbage collector's operation.

## Command-line arguments

Writing, assembling and compiling a system yields an executable command. The system's users will call that command with arguments. These are normally provided in textual form on the command line, as in

    your_system arg1 arg2 arg3

although one may conceive of other ways of entering the command arguments, such as tabular or graphical form-filling. In any case the software must be able to access the values passed as command arguments.
A language mechanism is available for that purpose: the Root Class rule indicates that the creation procedure of the root class may have a single argument (in the Eiffel sense of argument to a routine) of type ARRAY [STRING ] . The corresponding array of strings will be initialized at the beginning of the system's execution with the values entered as arguments to that execution of the command.
Although this facility suffices in many cases, it is not always convenient if you suddenly need to access the command arguments in a class that is far-away from the root. An alternative mechanism, class ARGUMENTS, is available. Once again, this is a class from which you should inherit if you need its facilities. It has just two exported features:

- argument_count, a non-negative integer, is the number of command arguments.
- argument (i), a string, is the i-th command argument. Here i must be between 0 and argument_count; the convention is that for i = 0 the result is the name of the command itself.