BBC BASIC Programmers' Reference

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=====Sharing PRIVATE variables=====

//by Richard Russell, May 2007//\\ \\  The [[http://www.bbcbasic.co.uk/bbcwin/manual/bbcwin7.html#private|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.\\ \\ 
===== Method one =====
\\  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:\\ \\ 
<code bb4w>
        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
</code>
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:\\ \\ 
<code bb4w>
        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.
</code>
\\ 
===== Method two =====
\\  This method relies on creating a **structure** to hold the private data, as follows:\\ \\ 
<code bb4w>
      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
</code>
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:\\ \\ 
<code bb4w>
        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)
        PTR(a()) = PTR(array())
        ENDPROC
</code>