Free Pascal supports the Delphi implementation of
the PChar type. PChar is defined as a pointer to a Char type, but allows additional
operations. The PChar type can be understood best as the Pascal equivalent of a C-style
null-terminated string, i.e. a variable of type PChar is a pointer that points to an array of type
Char, which is ended by a null-character (#0). Free Pascal supports initializing of PChar
typed constants, or a direct assignment. For example, the following pieces of code are
equivalent:
program one;
var p : PChar;
begin
P := 'This is a null-terminated string.';
WriteLn (P);
end.
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Results in the same as
program two;
const P : PChar = 'This is a null-terminated string.'
begin
WriteLn (P);
end.
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These examples also show that it is possible to write the contents of the string to a file of type
Text. The strings unit contains procedures and functions that manipulate the PChar type as in the
standard C library. Since it is equivalent to a pointer to a type Char variable, it is also possible to
do the following:
Program three;
Var S : String[30];
P : PChar;
begin
S := 'This is a null-terminated string.'#0;
P := @S[1];
WriteLn (P);
end.
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This will have the same result as the previous two examples. Null-terminated strings cannot be
added as normal Pascal strings. If two PChar strings mustt be concatenated; the functions from the
unit strings must be used.
However, it is possible to do some pointer arithmetic. The operators + and - can be used to do
operations on PChar pointers. In table (3.5), P and Q are of type PChar, and I is of type
Longint.
| Table 3.5: | PChar pointer arithmetic |
| Operation | Result |
| P + I | Adds I to the address pointed to by P. |
| I + P | Adds I to the address pointed to by P. |
| P - I | Substracts I from the address pointed to by P. |
| P - Q | Returns, as an integer, the distance between 2 addresses |
| | (or the number of characters between P and Q) |
| |
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