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.
Results in the same as
program two; const P : PChar = 'This is a null-terminated string.'; begin WriteLn (P); end.
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; P : PChar; begin S := 'This is a null-terminated string.'#0; P := @S; WriteLn (P); end.
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 must 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 (3.6), P and Q are of type PChar, and I is of type Longint.
|P + I||Adds I to the address pointed to by P.|
|I + P||Adds I to the address pointed to by P.|
|P - I||Subtracts I from the address pointed to by P.|
|P - Q||Returns, as an integer, the distance between two addresses|
|(or the number of characters between P and Q)|