Difference between revisions of "Nil"
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{{Nil}} | {{Nil}} | ||
− | The [[Reserved word|reserved word]] <syntaxhighlight lang="pascal" | + | The [[Reserved word|reserved word]] <syntaxhighlight lang="pascal" inline>nil</syntaxhighlight> represents the special [[Constant|value]] of a pointer [[Variable|variable]] not pointing anywhere in particular. |
− | In [[FPC]] it is implemented as <syntaxhighlight lang="pascal" | + | In [[FPC]] it is implemented as <syntaxhighlight lang="pascal" inline>pointer(0)</syntaxhighlight> (the numeric value <syntaxhighlight lang="pascal" inline>0</syntaxhighlight>), however the programmer is not supposed to use this fact. |
− | In other programming languages, e.g in C, you write <syntaxhighlight lang="C" | + | In other programming languages, e. g. in C, you write <syntaxhighlight lang="C" inline>null</syntaxhighlight>. |
The terms “null pointer” and “nil pointer” are used interchangeably, even among [[Standard Pascal|Pascal]] programmers. | The terms “null pointer” and “nil pointer” are used interchangeably, even among [[Standard Pascal|Pascal]] programmers. | ||
− | There are two popular explanations of <syntaxhighlight lang="pascal" | + | There are two popular explanations of <syntaxhighlight lang="pascal" inline>nil</syntaxhighlight>’s etymology. |
− | One says, <syntaxhighlight lang="pascal" | + | One says, <syntaxhighlight lang="pascal" inline>nil</syntaxhighlight> is short for the Latin word “nihil” meaning “nothing”. |
− | The other suggests, <syntaxhighlight lang="pascal" | + | The other suggests, <syntaxhighlight lang="pascal" inline>NIL</syntaxhighlight> is an English acronym standing for “not in list.” |
− | Maybe, since the German word „Null“ stands for the digit “zero”, and in order to avoid confusion, or to distinguish between the concept and value, the word <syntaxhighlight lang="pascal" | + | Maybe, since the German word „Null“ stands for the digit “zero”, and in order to avoid confusion, or to distinguish between the concept and value, the word <syntaxhighlight lang="pascal" inline>nil</syntaxhighlight> was chosen. |
+ | At any rate, this does not have any implications while programming. | ||
== Assignment compatibility == | == Assignment compatibility == | ||
− | <syntaxhighlight lang="pascal" | + | <syntaxhighlight lang="pascal" inline>nil</syntaxhighlight> can be of course [[Becomes|assigned]] to a [[Pointer|<syntaxhighlight lang="pascal" inline>pointer</syntaxhighlight>]] variable, but also to other types, which are in fact pointers, but their usage is more convenient. |
+ | For instance [[Dynamic array|dynamic arrays]] or [[Class|classes]]: | ||
<syntaxhighlight lang="pascal" line highlight="10,12,14,16,18"> | <syntaxhighlight lang="pascal" line highlight="10,12,14,16,18"> | ||
Line 37: | Line 39: | ||
</syntaxhighlight> | </syntaxhighlight> | ||
− | Note, assigning <syntaxhighlight lang="pascal" | + | Note, assigning <syntaxhighlight lang="pascal" inline>nil</syntaxhighlight> to a dynamic array is virtually equivalent to the [[Procedure|procedure]] invocation <syntaxhighlight lang="pascal" inline>setLength(dynamicArrayVariable, 0)</syntaxhighlight>. |
− | The values of the array are lost, if the reference count of <syntaxhighlight lang="pascal" | + | The values of the array are lost, if the reference count of <syntaxhighlight lang="pascal" inline>dynamicArrayVariable</syntaxhighlight> has hit zero. |
− | However, there is no comparable mechanism for other types, e.g. assigning <syntaxhighlight lang="pascal" | + | However, there is no comparable mechanism for other types, e. g. assigning <syntaxhighlight lang="pascal" inline>nil</syntaxhighlight> to a <syntaxhighlight lang="pascal" inline>class</syntaxhighlight> or <syntaxhighlight lang="pascal" inline>pointer</syntaxhighlight> variable ''will not'' release (i. e. de-allocate) the memory that is possibly been occupied by the referenced structure. |
== Application == | == Application == | ||
− | In Pascal you usually do not write expressions like <syntaxhighlight lang="pascal" | + | In Pascal you usually do not write expressions like <syntaxhighlight lang="pascal" inline>pointerVariable =: nil</syntaxhighlight> but use more explanatory [[Identifier|identifiers]]. |
− | The [[Function|function]] [[Assigned|<syntaxhighlight lang="pascal" | + | The [[Function|function]] [[Assigned|<syntaxhighlight lang="pascal" inline>system.assigned</syntaxhighlight>]] will be replaced by the exact same expression, but conceals the fact a variable is (''implemented'' as) a pointer. |
+ | So its usage is optional. | ||
− | The [[Routine|routine]] [[FreeAndNil|<syntaxhighlight lang="pascal" | + | The [[Routine|routine]] [[FreeAndNil|<syntaxhighlight lang="pascal" inline>SysUtils.FreeAndNil</syntaxhighlight>]] will call a class’s <syntaxhighlight lang="pascal" inline>free</syntaxhighlight> routine and assign <syntaxhighlight lang="pascal" inline>nil</syntaxhighlight> to the handed pointer (variable of type <syntaxhighlight lang="pascal" inline>class</syntaxhighlight>). |
Although it is a good idea, to clear pointers which do not point to valid objects anymore, this can make debugging more difficult since there is no pointer available pointing to the address at which a certain object used to be. | Although it is a good idea, to clear pointers which do not point to valid objects anymore, this can make debugging more difficult since there is no pointer available pointing to the address at which a certain object used to be. | ||
== See also == | == See also == | ||
− | * [[^|<syntaxhighlight lang="pascal" | + | * [[^|<syntaxhighlight lang="pascal" inline>^</syntaxhighlight>]] |
− | * {{Doc|package=RTL|unit=system|identifier=returnnilifgrowheapfails|text=<syntaxhighlight lang="pascal" | + | * {{Doc|package=RTL|unit=system|identifier=returnnilifgrowheapfails|text=<syntaxhighlight lang="pascal" inline>system.returnNilIfGrowHeapFails</syntaxhighlight>}} |
− | * <syntaxhighlight lang="pascal" | + | * <syntaxhighlight lang="pascal" inline>{$objectChecks on}</syntaxhighlight> |
* [[Nullable types|nullable types]] | * [[Nullable types|nullable types]] |
Latest revision as of 17:06, 20 January 2022
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The reserved word nil
represents the special value of a pointer variable not pointing anywhere in particular.
In FPC it is implemented as pointer(0)
(the numeric value 0
), however the programmer is not supposed to use this fact.
In other programming languages, e. g. in C, you write null
.
The terms “null pointer” and “nil pointer” are used interchangeably, even among Pascal programmers.
There are two popular explanations of nil
’s etymology.
One says, nil
is short for the Latin word “nihil” meaning “nothing”.
The other suggests, NIL
is an English acronym standing for “not in list.”
Maybe, since the German word „Null“ stands for the digit “zero”, and in order to avoid confusion, or to distinguish between the concept and value, the word nil
was chosen.
At any rate, this does not have any implications while programming.
Assignment compatibility
nil
can be of course assigned to a pointer
variable, but also to other types, which are in fact pointers, but their usage is more convenient.
For instance dynamic arrays or classes:
1program nilDemo(input, output, stderr);
2var
3 loc: pointer;
4 chk: array of boolean;
5 msg: PChar;
6 prc: TProcedure;
7 obj: TObject;
8begin
9 // point to nothing
10 loc := nil;
11 // clears dynamic array
12 chk := nil;
13 // empty string
14 msg := nil;
15 // procedural variable not referencing any procedure
16 prc := nil;
17 // loses reference to object
18 obj := nil;
19end.
Note, assigning nil
to a dynamic array is virtually equivalent to the procedure invocation setLength(dynamicArrayVariable, 0)
.
The values of the array are lost, if the reference count of dynamicArrayVariable
has hit zero.
However, there is no comparable mechanism for other types, e. g. assigning nil
to a class
or pointer
variable will not release (i. e. de-allocate) the memory that is possibly been occupied by the referenced structure.
Application
In Pascal you usually do not write expressions like pointerVariable =: nil
but use more explanatory identifiers.
The function system.assigned
will be replaced by the exact same expression, but conceals the fact a variable is (implemented as) a pointer.
So its usage is optional.
The routine SysUtils.FreeAndNil
will call a class’s free
routine and assign nil
to the handed pointer (variable of type class
).
Although it is a good idea, to clear pointers which do not point to valid objects anymore, this can make debugging more difficult since there is no pointer available pointing to the address at which a certain object used to be.
See also
^
system.returnNilIfGrowHeapFails
{$objectChecks on}
- nullable types