C5_PROCEDURE

PROCEDURE
 Declare a procedure name and formal parameters
------------------------------------------------------------------------------
 Syntax

     [STATIC] PROCEDURE <idProcedure> [(<idParam list>)]
        [FIELD <idField list> [IN <idAlias>]
        [LOCAL <identifier> [[:= <initializer>], ... ]]
        [MEMVAR <identifier list>]
        [STATIC <identifier> [[:= <initializer>], ... ]]
        .
        . <executable statements>
        .
        [RETURN]

 Arguments

     <idProcedure> is the name of the procedure to be declared.
     Procedure names can be any length, but only the first 10 characters are
     significant.  Names can contain any combination of characters, numbers,
     or underscores, but leading underscores are reserved.

     <idParam list> is the declaration of one or more parameter
     variables.  Variables specified in this list are declared local.

     STATIC PROCEDURE declares a procedure that can be called only by
     procedures and user-defined functions declared in the same program
     (.prg) file.

     FIELD declares a list of identifiers, <idField list>, to use as
     field names whenever encountered.  If the IN clause is specified,
     referring to the declared name, <idAlias> is a reference to the
     appropriate work area of the specified database.

     LOCAL declares and optionally initializes a list of variables or
     arrays whose visibility and lifetime is the current procedure.

     <identifier>, <identifier list> is a label or labels used as
     variable or array names.  If the <identifier> is followed by square
     brackets ([ ]), it is created as an array.  If the <identifier> is an
     array, the syntax for specifying the number of elements for each
     dimension can be array[<nElements>, <nElements2>,...] or
     array[<nElements>][<nElements2>]...  The maximum number of elements per
     dimension is 4096.  The maximum number of dimensions per array is
     limited only by available memory.

     <initializer> is the value to which an optional inline assignment
     sets the <identifier> variable--essentially, the assignment operator,
     (:=) --followed by any valid CA-Clipper expression, including a literal
     array.  If no <initializer> is specified, variables are initialized to
     NIL.  In the case of arrays, all element are initialized to NIL.

     MEMVAR declares a list of identifiers, <identifier list>, to use as
     private or public memory variables or arrays whenever encountered.

     STATIC declares and, optionally, initializes a list of variables or
     arrays whose visibility is the current procedure and whose lifetime is
     the duration of the program.

     RETURN passes control back to the calling procedure or user-defined
     function.  If a RETURN is not specified, control passes back to the
     calling routine when the procedure definitions ends.  In all cases, the
     compiler terminates the procedure definition when it encounters another
     PROCEDURE statement, FUNCTION statement, or end of file character.

 Description

     The PROCEDURE statement declares a procedure and an optional list of
     local variables to receive parameters passed from a calling routine.  A
     procedure is a subprogram comprised of a set of declarations and
     statements executed whenever you refer to <idProcedure>, followed by an
     open and close parentheses pair or with the DO statement.  A procedure
     definition begins with a PROCEDURE statement and ends with the next
     PROCEDURE statement, FUNCTION statement, or end of file.

     Procedures that encapsulate computational blocks of code provide
     readability and modularity, isolate change, and help manage complexity.

     A procedure in CA-Clipper is the same as a user-defined function, with
     the exception that it always returns NIL.  Each procedure must begin
     with a PROCEDURE statement and may, optionally, contain a RETURN
     statement to return control to the calling procedure or user-defined
     function.  A RETURN statement, however, is not required.  Procedure
     declarations cannot be nested within other procedure definitions.

     The visibility of procedure names falls into two classes.  Procedures
     that are visible anywhere in a program are referred to as public
     procedures and declared with a PROCEDURE statement.  Procedures that are
     visible only within the current program (.prg) file are referred to as
     static procedures and declared with a STATIC PROCEDURE statement.
     Static procedures have filewide scope.

     Static procedures are quite useful for a number of reasons.  First, they
     limit visibility of a procedure name thereby restricting access to the
     procedure.  Because of this, subsystems defined within a single program
     (.prg) file can provide an access protocol with a series of public
     procedures and conceal the implementation details of the subsystem
     within static procedures and functions.  Second, since the static
     procedure references are resolved at compile time, they preempt
     references to public procedures and functions which are resolved at link
     time.  This ensures that, within a program file, a reference to a static
     procedure executes that procedure if there is a name conflict with a
     public procedure or function.

     For more information on procedures, variable declarations, and parameter
     passing, refer to the "Basic Concepts" chapter in the Programming and
     Utilities Guide.

 Notes

     .  Calling a procedure:  There are two ways to call a procedure
        in CA-Clipper.  The first and preferred way is the function-calling
        convention.  Here you call the procedure as you would a CA-Clipper
        function on a line by itself:

        <idProcedure>([<argument list>])

        The second and obsolete way is the command-calling convention using
        the DO...WITH command.  The two methods of calling procedures differ
        only in the default method of passing parameters.  The function-
        calling convention passes variables by value as a default, whereas
        the command-calling convention passes them by reference as a default.

        A procedure can also be called as an aliased expression if it is
        prefaced with an alias and invoked using the function-calling
        convention, like this:

        <idAlias> ->(<idProcedure>(<argument list>))

        When called as an aliased expression, the work area associated with
        <idAlias> is selected, the procedure is executed, and then the
        original work area is reselected.  Like an expression or function, an
        aliased procedure can be specified on a line by itself.

        A procedure in CA-Clipper may call itself recursively.  This means
        you can call a procedure in the same procedure definition.

     .  Parameters:  Procedures like user-defined functions can
        receive parameters passed from a calling procedure, user-defined
        function, or the DOS command line.  A parameter is a place for a
        value or reference.  In CA-Clipper there are two ways to receive
        parameters:  a list of local variable names can be declared as a part
        of the PROCEDURE declaration (referred to as formal parameters), or a
        list of private variables can be specified in a separate PARAMETERS
        statement.  Note that you cannot mix a declaration of formal
        parameters with a PARAMETERS statement.  Attempting this will cause a
        fatal compiler error.

        Procedures receive parameters in the order passed.  In CA-Clipper the
        number of parameters need not match the number of arguments passed.
        Arguments can be skipped or left off the end of the argument list.  A
        parameter not receiving a value or reference is initialized to NIL.
        If arguments are specified, PCOUNT() returns the position of the last
        argument passed.

        Parameters specified in a procedure can receive arguments passed by
        value or by reference.  The default method for expressions and
        variables depends on the calling convention.  With the
        function-calling convention, the default passing method for
        expressions and variables is by value.  This includes variables
        containing references to arrays and objects.  With the command-
        calling convention, the default method for passing variables is by
        reference except for field variables, which are always passed by
        value.  Whenever a field variable is passed, it must be specified
        enclosed in parentheses unless declared with the FIELD statement.
        Failure to do so will generate a runtime error.

 Examples

     .  This example shows a skeleton of a typical CA-Clipper
        procedure that uses lexical variables:

        PROCEDURE Skeleton( cName, cClassRoom, nBones, ;
                                nJoints )
           LOCAL nCrossBones, aOnHand := {"skull", ;
                                         "metacarpals"}
           STATIC nCounter := 0
           .
           . <executable statements>
           .
           RETURN

     .  This example determines whether an argument was skipped by
        comparing the parameter to NIL:

        PROCEDURE MyProc( param1, param2, param3 )
           IF param2 != NIL
              param2 := "default value"
           ENDIF
           .
           . <statements>
           .
           RETURN

     .  This example invokes the procedure, UpdateAmount(), as an
        aliased expression:

        USE Invoices NEW
        USE Customer NEW
        Invoices->(UpdateAmount(Amount + Amount * nInterest))

See Also: FUNCTION LOCAL PARAMETERS PCOUNT() RETURN STATIC



4 responses to “C5_PROCEDURE

  1. Pingback: C5 Flow Control | Viva Clipper !

  2. Pingback: C5 Statements | Viva Clipper !

  3. Pingback: C5_STATIC | Viva Clipper !

  4. Pingback: C5_RETURN | Viva Clipper !

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