Procedural type and overloaded functions: why am I getting a compile-
time error?
lazarus / 62-General

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EganSolo · 2021-03-12 05:10

Check the silly little program below: If you try to compile it, you'll
get the error indicated in the comment.

I've checked the section of the Free Pascal Reference Guide on
Procedural Type https://freepascal.org/docs-html/ref/refse17.html and
section 14.9.11 on the overload modifier https://freepascal.org/docs-
html/ref/refsu81.html#x201-22300014.9.11 , and neither mention a
conflict between overloading functions and procedural types. Is that
mentioned somewhere else in the manual?

Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i Program
Project1; Type  MyFuncType = function(const S : String) :
Boolean; Function A(const S: String; const N: integer): Boolean;
overload;begin   Result := (N = 0) and (S <> 'honk');End; Function
A(const S: String): Boolean; overload;begin   Result := S <>
'honk';End; Procedure DoSomething(const S : String; const aFunc :
MyFuncType);begin  If aFunc(S)  then Writeln('I'' not honking')  else
Writeln('I''m honking');end; Begin  DoSomething('honk',@A);  //<--:
Compile Error: Got "<address of function(Const AnsiString; const
LongInt): Boolean; Register>" +                          //<--:      
             expected "<procedure variable type of
function(AnsiString): Boolean; Register>"  Readln;End.

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balazsszekely · 2021-03-12 05:42

@EganSolo
QuoteCheck the silly little program below: If you try to compile it,
you'll get the error indicated in the comment.{$mode ObjFPC} is more
restrictive. Please try this:
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i Program
Project1; Type  MyFuncType = function(const S : String) :
Boolean; Function A(const S: String; const N: integer): Boolean;
overload;begin  Result := (N = 0) and (S <> 'honk');End; Function
A(const S: String): Boolean; overload;begin  Result := S <>
'honk';End; Procedure DoSomething(const S : String; const aFunc :
MyFuncType);begin  If aFunc(S)  then Writeln('I'' not honking')  else
Writeln('I''m honking');end; var  MyFunc: MyFuncType;Begin  MyFunc :=
@A;  DoSomething('honk', MyFunc);  Readln;End.
If you switch to {$mode delphi}, your code will work fine. As a side
note your forgot "const S" from  MyFuncType = function(S : String) :
Boolean.

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EganSolo · 2021-03-12 06:38

Got it. Thanks for the quick response GetMem. Would it make sense to
amend the page in the doc about mod ObjFPC to indicate this? I'm
referring to this page: https://www.freepascal.org/docs-
html/prog/progse72.html#x287-303000D.1.

Also, I've fixed my type (missing Const) in the initial code.

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balazsszekely · 2021-03-12 07:03

@EganSolo
QuoteWould it make sense to amend the page in the doc about mod ObjFPC
to indicate this? I'm referring to this page:
https://www.freepascal.org/docs-
html/prog/progse72.html#x287-303000D.1.I'm not sure what exactly we
should amend. Mode ObjFPC differs a lot from mode delphi.

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EganSolo · 2021-03-12 07:22

I mean, would it make sense to list the differences that programmers
are likely to encounter? Perhaps the top ten or fifteen? Just a
thought.

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dseligo · 2021-03-12 07:33

Quote from: EganSolo on March 12, 2021, 08:22:12 amI mean, would it
make sense to list the differences that programmers are likely to
encounter? Perhaps the top ten or fifteen? Just a thought. 
Check this link:
https://wiki.freepascal.org/Code_Conversion_Guide#Syntax_differences

And some more about modes: https://wiki.freepascal.org/Compiler_Mode

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howardpc · 2021-03-12 08:31

It is worth noting that {$Mode fpc} is the compiler default, and is
neither {$Mode objfpc} nor {$Mode Delphi}.
In almost every program understanding and specifying the mode is
critical.