Nil

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The reserved word nil represents the special value of a pointer variable not pointing anywhere 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, someone writes null. The terms “null pointer” or “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:

 1 program nilDemo(input, output, stderr);
 2 var
 3 	loc: pointer;
 4 	chk: array of boolean;
 5 	msg: PChar;
 6 	prc: TProcedure;
 7 	obj: TObject;
 8 begin
 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;
19 end.

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 a Pascal's manner 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 a certain object used to be.

see also