String

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String

Standard data type: 8 bit character string

Syntax

Dim variable As String [ * size]

Description

A String is an array of characters.

A String declared without the size parameter is dynamically resized depending on the length of the string. The length can range from 0 bytes to 2 gigabytes. A descriptor contains a pointer to the actual string, the length of the string, and the amount of space allocated for it. VarPtr will return a pointer to the descriptor, while StrPtr will point to the actual string.

Because of the hidden descriptor with a String, manual allocation of space, for example using the memory allocation function CAllocate (preferentially), for a String is not encouraged. The common way to ensure a certain amount of space is reserved for a String, to prevent unnecessary allocations inside a loop for instance, is to use the Space or String functions.

Nevertheless if necessary, dynamic allocation may be carefully used by means of the memory allocation functions Allocate, CAllocate, Reallocate (see precautions for use) and string pointer (which is a pointer to a string descriptor, not string data). When memory is allocated to hold string descriptors, the string must always be destroyed (setting to "``") before deallocate each string descriptor (allowing to deallocate the memory taken up by the string data), otherwise, it is not possible to deallocate it later, and it may induce memory leak in the program continuation.

A simpler and safer method for dynamic allocation /deallocation is to use the advanced New / Delete keyword (which itself also calls the constructor / destructor of the string object).

Despite the use of the descriptor, an implicit NULL character (Chr(0)) is added to the end of the string, to allow passing them to functions in external libraries without making slow copies. FreeBASIC's internal functions will ignore this character, and not treat it as part of the string.

A String declared with a fixed size (numeric constant, or expression that can be evaluated at compile time) is a QB-style fixed length string. Unused characters are set to space, regardless of what "-lang" compiler option is used. It has no descriptor and it is not resized to fit its contents. As in QB, if data overflows the size of the string, it is truncated on the right side.
Fixed length strings are not terminated with a NULL character, to prevent the redundant character complicating data layout in user-defined Types. Note: Dereferencing a pointer to a fixed length string can lead to an extension of the string by some pollution, or even to an outright crash (because there is no NULL terminal character).

String variable names need not end in a dollar sign $ as in other dialects of BASIC. In lang fb variable suffixes, including the dollar sign, are disallowed entirely.

Remarks

Differences from QB

• In QB the strings were limited to 32767 characters. In QB, the unused characters of a fixed-length string were initialized with 32 (space, or " ", in ASCII).
  In QB static or fixed-size strings were often used in records to represent a number of bytes of data; for example, a string of 1 length to represent 1 byte in a UDT read from a file.

See also

• String (Function)
• Space
• ZString
• WString
• Str
• StrPtr
• VarPtr
• Standard Data Type Limits
• Operator [] (String Index)
• String Operators

Examples

'' Variable length

Dim a As String

a = "Hello"

Print a

a += ", world!"

Print a

Dim As String b = "Welcome to FreeBASIC"

Print b + "! " + a

'' QB-like $ suffixes

#lang "qb"

'' DIM based on $ suffix

Dim a$

a$ = "Hello"

'' Implicit declaration based on $ suffix

b$ = ", world!"

Print a$ + b$

'' Variable-length strings as buffers

'' Reserving space for a string,

'' using Space() to produce lots of space characters (ASCII 32)

Dim As String mybigstring = Space(1024)

Print "buffer address: &h" & Hex( StrPtr( mybigstring ), 8 ) & ", length: " & Len( mybigstring )

'' Explicitly destroying a string

mybigstring = ""

Print "buffer address: &h" & Hex( StrPtr( mybigstring ), 8 ) & ", length: " & Len( mybigstring )

'' Variable-length string as Const parameter

'' Const qualifier preventing string from being modified

Sub silly_print( ByRef printme As Const String )

    Print ".o0( " & printme & " )0o."

    'next line will cause error if uncommented

    'printme = "silly printed"

End Sub

Dim As String status = "OK"

silly_print( "Hello FreeBASIC!" )

silly_print( "Status: " + status )

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Category: Operators
Kind: keyword

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Reproduced from the FreeBASIC Wiki Manual under the GNU Free Documentation License 1.3 — with no Invariant Sections, no Front-Cover Texts and no Back-Cover Texts. Copyright © the FreeBASIC project and wiki contributors. See License.