Declaration: what changed
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Title: Declaration
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| 1 | + | A **declaration** introduces the compiler to new [[Identifier|identifiers]]. It is an agreement between the programmer and compiler, that a certain symbol henceforth has a fixed meaning. | |
| 2 | + | ||
| 3 | + | ## characteristics | |
| 4 | + | ||
| 5 | + | Declarations do not have any (explicit) impact on a compiled program: For example, a declaration of a variable informs the compiler about a new identifier associated with a data type. However, neither the variable’s name, nor the data type are stored in the [[Binary|compiled binary file]]. | |
| 6 | + | ||
| 7 | + | Nevertheless, implicitly the compiler *reserves* enough *memory* and ensures only *legal operations* are performed in compliance with the data type. Thus declarations ensure the compiler can process the source code. | |
| 8 | + | ||
| 9 | + | ## types | |
| 10 | + | ||
| 11 | + | [[Pascal]] knows following types of declarations: | |
| 12 | + | ||
| 13 | + | - modules | |
| 14 | + | - [[Constant|constants]] | |
| 15 | + | - resource strings (non-standard extension) | |
| 16 | + | - [[Type|data types]] | |
| 17 | + | - [[Variable|variables]] | |
| 18 | + | - [[Label|labels]] (legacy) | |
| 19 | + | - [[Routine|routines]] | |
| 20 | + | ||
| 21 | + | ## difference to definitions | |
| 22 | + | ||
| 23 | + | Declarations merely tell the compiler “there is something”. In contrast to that, *definitions* elaborate what “something” is. All definitions will (eventually) change the program state. Declarations do not change the program state. | |
| 24 | + | ||
| 25 | + | - In a [[Program|program]] the program header is the declaration, the subsequent [[Block|block]] *defines* the program. | |
| 26 | + | - The same applies for routines. | |
| 27 | + | - Labels are declared in the `label` section | |
| 28 | + | ||
| 29 | + | ` systemCrash;` | |
| 30 | + | ||
| 31 | + | </syntaxhighlight> | |
| 32 | + | ||
| 33 | + | Their definition, i. e. actually associating this identifier with an address, occurs later in the source code: | |
| 34 | + | ||
| 35 | + | ``` pascal | |
| 36 | + | begin | |
| 37 | + | … | |
| 38 | + | if somethingIsWrong then | |
| 39 | + | begin | |
| 40 | + | goto systemCrash; | |
| 41 | + | end; | |
| 42 | + | … | |
| 43 | + | systemCrash: | |
| 44 | + | … | |
| 45 | + | ``` | |
| 46 | + | ||
| 47 | + | - Constants are declared and defined in one brush: In | |
| 48 | + | ||
| 49 | + | ` answer = 42;` | |
| 50 | + | ||
| 51 | + | </syntaxhighlight> | |
| 52 | + | ||
| 53 | + | the information “`answer` is an integer constant” is the declaration. The information “`answer` equals `42`” is the definition. The same applies to resource strings. | |
| 54 | + | ||
| 55 | + | - Data types are frequently defined implicitly. The following only *declares* a data type `point`: | |
| 56 | + | ||
| 57 | + | ` point = record`\ | |
| 58 | + | ` x, y: integer;`\ | |
| 59 | + | ` end;` | |
| 60 | + | ||
| 61 | + | </syntaxhighlight> | |
| 62 | + | ||
| 63 | + | The definition of `point` is invisible: Which operations are allowed on `point` is not written in this piece of code. Nevertheless, the [[Becomes|assignment]] of a `point` value to a `point` variable works out of the box, and further operations on `point` can be defined using [[Operator overloading|operator overloading]]. | |
| 64 | + | ||
| 65 | + | - The same applies to variable declarations (except there is no possibility to define operators on anonymous data types). The *definition* of a variable is an [[Becomes|assignment]]. | |
| 66 | + | ||
| 67 | + | In summary, breach of agreed *declarations* cause [[compile-time error|compile-time errors]], whereas *wrong definitions* cannot be caught by the compiler. | |
| 68 | + | ||
| 69 | + | --- | |
| 70 | + | *From the Free Pascal wiki, page "Declaration", as it stood on 2021-08-12 after a change by Kai Burghardt, out of the wiki's dump of 2024-05-26. Copyright stays with the author who wrote it, according to the old wiki's [[Licensing]] page. If you have concerns about this material, [message the admin](/wiki-contact).* | |
| 71 | + | Category: Glossary |