8.2 Function calls

Function calls are part of expressions (although, using extended syntax, they can be statements too). They are constructed as follows:

_________________________________________________________________________________________________________ Function calls
-- --function call-||---function identi er-------|------------------------
|----method designator----| -actual parameter list-
|quali ed method designator
----variable reference -----

-- --actual parameter list-(|--------------)--------------------------
-|expression ---
6---,------
___________________________________________________________________

The variable reference must be a procedural type variable reference. A method designator can only be used inside the method of an object. A qualied method designator can be used outside object methods too. The function that will get called is the function with a declared parameter list that matches the actual parameter list. This means that

The number of actual parameters must equal the number of declared parameters (unless default parameter values are used).
The types of the parameters must be compatible. For variable reference parameters, the parameter types must be exactly the same.

If no matching function is found, then the compiler will generate an error. Depending on the fact of the function is overloaded (i.e. multiple functions with the same name, but dierent parameter lists) the error will be dierent. There are cases when the compiler will not execute the function call in an expression. This is the case when assigning a value to a procedural type variable, as in the following example:

Type
  FuncType = Function: Integer;
Var A : Integer;
Function AddOne : Integer;
begin
  A := A+1;
  AddOne := A;
end;
Var F : FuncType;
    N : Integer;
begin
  A := 0;
  F := AddOne; { Assign AddOne to F, Don't call AddOne}
  N := AddOne; { N := 1 !!}
end.

In the above listing, the assigment to F will not cause the function AddOne to be called. The assignment to N, however, will call AddOne. A problem with this syntax is the following construction:

If F = AddOne Then
DoSomethingHorrible;

Should the compiler compare the addresses of F and AddOne, or should it call both functions, and compare the result ? Free Pascal solves this by deciding that a procedural variable is equivalent to a pointer. Thus the compiler will give a type mismatch error, since AddOne is considered a call to a function with integer result, and F is a pointer, Hence a type mismatch occurs. How then, should one compare whether F points to the function AddOne ? To do this, one should use the address operator @:

If F = @AddOne Then
WriteLn ('Functions are equal');

The left hand side of the boolean expression is an address. The right hand side also, and so the compiler compares 2 addresses. How to compare the values that both functions return ? By adding an empty parameter list:

If F()=Addone then
WriteLn ('Functions return same values ');

Remark that this behaviour is not compatible with Delphi syntax.

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