BBC BASIC Programmers' Reference

by Richard Russell, May 2007

The PRIVATE statement allows you to create variables (also arrays, structures etc.) which are private to a procedure or function, in the sense that they retain their values from one call to the next, but are not visible to the rest of the program.

Occasionally you might want two or more procedures or functions to share PRIVATE variables, so that they are accessible by those procedures/functions but not by the rest of the program. BBC BASIC has no built-in facility for doing this, but it is possible to achieve an equivalent effect using one or other of the methods described here.

This method relies on providing a common entry point for the procedures and functions, so that the PRIVATE statement may be placed there, as follows:

        DEF PROCone(parameters) : LOCAL type% : type% = 1
        DEF PROCtwo(parameters) : LOCAL type% : type% = 2
        PRIVATE alpha, beta%, gamma$
        CASE type% OF
          WHEN 1:
            REM. Code for PROCone goes here.
          WHEN 2:
            REM. Code for PROCtwo goes here.
        ENDCASE
        ENDPROC

This method can be expanded to as many procedures and functions as are required to share the PRIVATE data. It is even possible to share the data between a procedure and a function, so long as the ENDPROC or = statement is placed within the appropriate WHEN clause:

        DEF PROCone(parameters) : LOCAL type% : type% = 1
        DEF FNtwo(parameters) : LOCAL type% : type% = 2
        PRIVATE alpha, beta%, gamma$
        CASE type% OF
          WHEN 1:
            REM. Code for PROCone goes here.
            ENDPROC
          WHEN 2:
            REM. Code for FNtwo goes here.
            = returnvalue
        ENDCASE
        REM. Execution should never reach here.

This method relies on creating a structure to hold the private data, as follows:

      DEF PROCone(parameters)
      PRIVATE ps{}
      PROCprivate(ps{})
      REM. Code for PROCone goes here.
      REM. Private variables are ps.alpha, ps.beta%, ps.gamma$
      ENDPROC
 
      DEF FNtwo(parameters)
      PRIVATE ps{}
      PROCprivate(ps{})
      REM. Code for FNtwo goes here.
      REM. Private variables are ps.alpha, ps.beta%, ps.gamma$
      = returnvalue
 
      DEF PROCprivate(RETURN t{})
      PRIVATE p{}
      DIM p{alpha, beta%, gamma$}
      DIM t{} = p{}
      PTR (t{}) = PTR(p{})
      ENDPROC

The PROCprivate procedure sets the supplied structure parameter to point to the private structure declared therein, allowing any routine which calls this procedure to access the private data.

You can use the same technique to share PRIVATE arrays:

        DEF PROCone(parameters)
        PRIVATE array()
        PROCshare(array())
        REM. Code for PROCone goes here.
        ENDPROC
 
        DEF FNtwo(parameters)
        PRIVATE array()
        PROCshare(array())
        REM. Code for FNtwo goes here.
        = returnvalue
 
        DEF PROCshare(RETURN a())
        PRIVATE array()
        DIM array(100)
        PROCsetarrayptr(a(), FNgetarrayptr(array()))
        ENDPROC
 
        DEF FNgetarrayptr(RETURN a%%) = a%%
        DEF PROCsetarrayptr(RETURN a%%, p%%) : a%% = p%% : ENDPROC