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delphiparametersdelphi-7

Difference between those declarations

I am learning Delphi since the beginning of this week and I am currently reading the Delphi 7 - Developer's Guide. In this book (Chapter 5-37), the author declares a function as follow:

function FromEuro(const AValue: Double, Factor; FRound: TRoundToRange): Double;

If I had to write this function, I would have written it as follow:

function FromEuro(const AValue, Factor: Double; FRound: TRoundToRange): Double;

My question is : Is there any difference between those declarations ?

Solution

  • What I think you meant to ask is what is the difference between these two:

    function FromEuro(const AValue: Double; Factor: Double): Double; 
function FromEuro(const AValue, Factor: Double): Double;

    The second variant is a contraction that expands to:

    function FromEuro(const AValue: Double; const Factor: Double): Double;

    And this differs from 

    function FromEuro(const AValue: Double; Factor: Double): Double;

    by way of the second parameter being const.

    Note that I removed the final parameter since it is not relevant to the question.

    The documentation says:

    A parameter list is a sequence of parameter declarations separated by semicolons and enclosed in parentheses. Each declaration is a comma-delimited series of parameter names, followed in most cases by a colon and a type identifier, and in some cases by the = symbol and a default value.

    What the documentation doesn't mention is that each of the parameters in a comma-delimited series of parameter names has the same type. I guess this is meant to be obvious, and it does follow the example set by variable declarations. The documentation for variables says:

    The basic syntax for a variable declaration is:

    var identifierList:type;

    where identifierList is a comma-delimited list of valid identifiers and type is any valid type. For example:

    var I: Integer;

    declares a variable I of type Integer, while:

    var X, Y: Real;

    declares two variables - X and Y - of type Real.

    On the other hand, perhaps you know all of this already, and all that we have is a typo in a book! And indeed that seems to be the case because the exact same typo can be found here: http://docwiki.embarcadero.com/RADStudio/en/Using_a_Class_to_Manage_Conversions