Revision [16485]

This is an old revision of KeyPgType made by CountingPine on 2012-12-01 13:01:40.

 

TYPE


Declares a user-defined type.

Syntax:
Type typename
fieldname1 KeyPgAs As DataType DataType
fieldname2 KeyPgAs As DataType DataType
KeyPgAs As DataType DataType fieldname3, fieldname4
...
End Type

Type typename [KeyPgExtends Extends base_typename] [KeyPgField Field = alignment]
End Type

Description:
Type is used to declare custom data types containing one or more data fields, including integer types, floating point types, fixed-size arrays, strings, bitfields, and other user-defined types or unions.

Types support various functionality related to object-oriented programming:

Memory layout
Types lay out their fields consecutively in memory, following the native alignment and padding rules (described on the KeyPgField Field page). Special care must be taken when using Types for file I/O or interacting with other programs or programming languages, in case the alignment and padding rules are different. The optional KeyPgField Field = number specifier can be used to change the behavior on the FreeBASIC side.

Dynamic data
Types can contain dynamic strings, but note that the Type will contain the KeyPgString String descriptor structure, not the string data itself, which afterall is dynamically allocated and can be of variable length, and thus can not be inserted into the Type. Because of that, saving such a Type into a file will write out the string descriptor, not the actual string data. In order to embed strings into Types directly, fixed-length strings must be used. Similarly, when maintaining dynamic data manually through the use of pointers within a Type, it does usually not make sense to save the Type to a file, because the address stored in the pointer field will be written to file, not the actual memory it points to. Addresses are meaningful to a specific process only though, and cannot be shared that way.

Special note on fixed length strings
Currently, fixed-length string fields of KeyPgString String * N type have an extra null terminator at their end, for compatibility with C strings, making them incompatible with QB strings inside Types, because they actually use up N+1 bytes, instead of just N bytes. A possible work-around is to declare the field As KeyPgString String * (N-1), though this will not work in future releases if the null terminator is removed. Another alternative is to use a KeyPgByte Byte array with the proper size.

Examples:
This is an example of a QB-style type, not including procedure definitions
Type clr
    red As UByte
    green As UByte
    blue As UByte
End Type

Dim c As clr
c.red = 255
c.green = 128
c.blue = 64


And this is an example of a type working as an object:
'' Example showing the problems with fixed length string fields in UDTs
'' Suppose we have read a GIF header from a file
''                        signature         width        height
Dim As ZString*(10+1) z => "GIF89a" + MKShort(10) + MKShort(11)

Print "Using fixed-length string"

Type hdr1 Field = 1
   As String*(6-1) sig /' We have to dimension the string with 1 char
                        '  less to avoid misalignments '/
   As UShort wid, hei
End Type

Dim As hdr1 Ptr h1 = CPtr(hdr1 Ptr, @z)
Print h1->sig, h1->wid, h1->hei '' Prints GIF89 (misses a char!)  10  11

'' We can do comparisons only with the 5 visible chars and creating a temporary string with LEFT

If Left(h1->sig, 5) = "GIF89" Then Print "ok" Else Print "error"


'' Using a ubyte array, we need an auxiliary function to convert it to a string
Function ub2str( ub() As UByte ) As String
    Dim As Integer length = UBound(ub) + 1
    Dim As String res = Space(length)
    For i As Integer = 0 To length-1
        res[i] = ub(i): Next
    Function = res
End Function


Print
Print "Using an array of ubytes"

Type hdr2 Field = 1
   sig(0 To 6-1) As UByte '' Dimension 6
   As UShort wid, hei
End Type

Dim As hdr2 Ptr h2 = CPtr(hdr2 Ptr, @z)
'' Viewing and comparing is correct but a conversion to string is required

Print ub2str(h2->sig()), h2->wid, h2->hei '' Prints GIF89a  10  11 (ok)
If ub2str(h2->sig()) = "GIF89a" Then Print "ok" Else Print "error" '' Prints ok


Platform Differences:

Dialect Differences:
Differences from QB:
See also:
Back to User Defined Types
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