AnsiString: what changed
Revision 4 to 5. Removed lines have a minus sign; added lines have a plus sign.
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| 1 | 1 | **`AnsiString`** is a variable-length string [[Data type|data type]]. It can store characters that have a size of one Byte. | |
| 2 | 2 | ||
| 3 | 3 | ## implementation | |
| 4 | 4 | ||
| 5 | 5 | In [[FPC]] an `AnsiString` is implemented as a [[Pointer|pointer]]. It is a managed data type. As such it is initialized with [[Nil|nil]] as soon as it enters the scope. Memory for the character sequence is dynamically allocated and freed. | |
| 6 | 6 | ||
| 7 | 7 | An `AnsiString` points to the *first* character. This facilitates interfacing to libraries or foreign functions expecting [[PChar|pChar strings]]. For that, an `AnsiString` always concludes with a null [[Byte]]. *In [[Pascal]]*, this terminating null Byte has no significance as to the string’s value (including its length). An `AnsiString` always entails some management data *before* the first character. These are | |
| 8 | 8 | ||
| 9 | 9 | - a code page | |
| 10 | 10 | - the size of a character | |
| 11 | 11 | - a reference count | |
| 12 | 12 | - the length of the string. | |
| 13 | 13 | ||
| 14 | - | ||
| 15 | - | ||
| 16 | - | ||
| 14 | + | | `253` | `233` | `0` | `1` | `0` | `0` | `0` | `1` | `0` | `0` | `0` | `3` | `'F'` | `'o'` | `'o'` | `#0` | | |
| 15 | + | |---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---| | |
| 16 | + | | code page | | maximum character size | | reference count | | | | length | | | | payload | | | complimentary Null | | |
| 17 | + | | pointer points here ⤴ | | | | | | | | | | | | | | | | | |
| 17 | 18 | ||
| 18 | 19 | Only the length field has significance in Pascal. In Pascal, an `AnsiString` may contain `#0` characters. | |
| 19 | 20 | ||
| 20 | 21 | An `AnsiString` can furthermore be associated with a code page (since [[FPC New Features 3.0.0|3.0.0]]). | |
| 21 | 22 | ||
| 22 | 23 | ## application | |
| 23 | 24 | ||
| 24 | 25 | The data type `AnsiString` can be used like any other string data type. You may [[Becomes|assign]] string literals to an `AnsiString` variable as normal. String values can be compared ([[Equal|=]]) just as usual. The entire pointer-characteristic is transparent. | |
| 25 | 26 | ||
| 26 | 27 | Characters in `AnsiString` have a 1-based index. `myAnsiString[1]` refers to the first character. | |
| 27 | 28 | ||
| 28 | 29 | > **Note:** The linear character index is only guaranteed to work for strings that have a maximum character size of `1`. That means, using an integer index for example on an UTF-8 encoded string (not exclusively containing ASCII characters) will produce erroneous results. | |
| 29 | 30 | ||
| 30 | 31 | The [`length` function](https://www.freepascal.org/docs-html/rtl/system/length.html), and for that matter also `high`, will return a string’s length by examining the length data field. | |
| 31 | 32 | ||
| 32 | 33 | Because an `AnsiString` is essentially a pointer, *copying* strings of this type is fast, since only the reference is copied and the reference count increased. Modifications may trigger a COW. | |
| 33 | 34 | ||
| 34 | 35 | ## caveats | |
| 35 | 36 | ||
| 36 | 37 | - The compiler directive [[$H|{$longStrings on}]] (or `{$H+}`) [[Defensive programming techniques|aliases string]] (without a specified length) to `AnsiString`. | |
| 37 | 38 | - `AnsiString` as a *managed* data type introduces a certain overhead. See [[Avoiding implicit try finally section]] for more explanations. | |
| 38 | 39 | - The [[SizeOf|sizeOf]] value of an `AnsiString` variable is merely the size of a pointer. | |
| 39 | 40 | - Assigning an empty string *to an `AnsiString` variable will in fact assign `nil` to the variable and, if the reference count hit zero, release underlying memory (if any was previously allocated at all). Empty strings are*not'' stored as described above. | |
| 40 | 41 | ||
| 41 | 42 | ## see also | |
| 42 | 43 | ||
| 43 | 44 | - [[Character and string types]] | |
| 44 | 45 | - [Ansistrings](https://www.freepascal.org/docs-html/current/ref/refsu9.html#x32-370003.2.4) in the reference guide | |
| 45 | 46 | - [Ansistrings](https://www.freepascal.org/docs-html/current/prog/progsu161.html#x205-2160008.2.7) in the programmer’s guide | |
| 46 | 47 | ||
| 47 | 48 | --- | |
| 48 | 49 | *From the Free Pascal wiki, page "AnsiString", as it stood on 2023-12-22 after a change by Alextpp, 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).* | |
| 49 | 50 | Category: Data types |