PUBLIC STRUCT Identifier

  Field 1 [ Embedded array declaration ] AS [ Datatype ]
  Field 2 [ Embedded array declaration ] AS [ Datatype ]
    .
    .
    .
  Field n [ Embedded array declaration ] AS [ Datatype ]

END STRUCT

This keyword declares a structure.

A structure is exactly like a class that would have only public variables.

If you declare the same structure in two different classes, they must be have exactly the same fields, otherwise the interpreter will raise an error.

Fields Alignment

Structures are never packed, i.e. fields are aligned to a memory address that is a multiple of its memory length:

• A Boolean or a Byte can be stored at any address.
• A Short is stored at an even address.
• An Integer is stored at an address that is a multiple of four.
• ...and so on.

There is a problem then: when compiling the C source code, the C compiler may reorder structure fields. And, as far as I know, that process is not standardized nor documented. There may be a solution in the future with the libffi library used by Gambas. Apparently, that library can send a structure to a C function by taking that problem into account. But that library is far less documented by Gambas, so you see the difficulty!

Embedded Structures

A structure can be embedded inside a normal class or another structure, by declaring a variable with the following syntax:

[ PRIVATE | PUBLIC ] Identifier AS STRUCT Structure name

Example

' Gambas class file

PUBLIC STRUCT Arm
  Length AS Float
  NumberOfFingers AS Integer
  HasGlove AS Boolean
END STRUCT

PUBLIC STRUCT Leg
  Length AS Float
  NumberOfFingers AS Integer
  HasSock AS Boolean
  HasShoe AS Boolean
END STRUCT

PUBLIC STRUCT Man
  FirstName AS String
  LastName AS String
  Age AS Integer
  Eyes AS String
  LeftArm AS STRUCT Arm
  RightArm AS STRUCT Arm
  LeftLeg AS STRUCT Leg
  RightLeg AS STRUCT Leg
END STRUCT

Arrays Of Structure

Identifier [ Dimensions ] AS STRUCT Structure name

The structures are embedded, i.e. their contents is directly allocated inside the array.

Such arrays are not real arrays, they have only a few methods of the original array class:

• They can be accessed by index.
• They can be enumerated.
• You can get information about the array length and array dimensions.

You can create only embedded array of structures.