Variable parameter

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A variable parameter is a routine parameter that is a variable. To call a routine with a variable parameter, you need to specify a variable at the proper position. The variable will (temporarily) be in the scope of the routine through the parameter’s name.

Usage

A variable parameter is declared by preceding a formal parameter declaration with the keyword var.

procedure xorSwap(var left, right: integer);
begin
	left := left xor right;
	right := left xor right;
	left := left xor right;
end;

Variable parameters can serve as both input and output, meaning they can be used for passing a value to a routine, and getting a value from it. After procedure xorSwap has been called the variables at the call site will have changed. Assignments to variable parameters have an effect in the scope where the routine is called. Thus, a variable parameter can be considered as an alias for the actual argument given by the calling routine. When a routine changes the value of a variable parameter, it is actually changing the variable in the code that called the routine.

Since a variable parameter may appear on the left hand side of an assignment, only variables may be supplied as arguments when calling the routine, never constants or expressions.

  Note: file and text variables always have to be declared as variable parameters.

With GNU Pascal and Free Pascal, variable parameters may have no associated data type (FPC [internally] calls this “formal type”). Such parameters do not allow any operations on them, but typecasting has to be used. Only the @-address-operator is available:

procedure printAddress(var x);
begin
	write(sysBackTraceStr(@x));
end;

Implementation

The specific implementation of variable parameters is only of concern for those who are programming (pure) assembler routines.

In FPC, variable parameters are implemented by passing a reference to the variable at the call site (call by reference). For this reason, variable parameters are also referred to as reference parameters. If a routine is inlined, the extra level of indirection is eliminated.

  Note: In order to pass an address to the routine, all parameters have to be addressable (by one address).

Although a property may be readable and writable, it does not have a single address in one memory block, thus it cannot be used as a variable parameter.

See also

• § “Variable Parameters” in the FPC Reference Guide
• out designates a parameter as non-readable, only writable
• constRef