C++: "Type Checking"

[Web Developer]

Hi,

I come across the term "type checking" very often in my readings on C++, and
have never heard it in Java. Besides the simplistic answer that it checks
the "type", what more does it mean?


WD



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[John Harrison]

Re: "Type Checking"


"Web Developer" <nospam@hotmail .com> wrote in message
news:3f373851_1 @news.iprimus.c om.au...[color=blue]
> Hi,
>
> I come across the term "type checking" very often in my readings on C++,[/color]
and[color=blue]
> have never heard it in Java. Besides the simplistic answer that it checks
> the "type", what more does it mean?
>
>
> WD
>[/color]

I think its a question of timing. All languages check types, you aren't
allowed to multiply two strings in any language (AFAIK). But some languages
delay type checking until the program runs. C++ does most of its type
checking at compile type. This results in faster code, smaller data and
quicker detection of type errors, but it also means a more complex language.

We say C++ is a strongly typed language. In general I would say strong
typing is a good thing but there are occasions where you want to relax it
and check types at run time. That's where virtual functions and RTTI come
in.

john

[Alf P. Steinbach]

Re: &quot;Type Checking&quot;

On Mon, 11 Aug 2003 05:20:48 -0400, "ghl" <glabowitz@comc ast.net> wrote:
[color=blue]
>"John Harrison" <john_andronicu s@hotmail.com> wrote in message
>news:bh7dn6$tv ruf$1@ID-196037.news.uni-berlin.de...[color=green]
>>
>> "Web Developer" <nospam@hotmail .com> wrote in message
>> news:3f373851_1 @news.iprimus.c om.au...[color=darkred]
>> > Hi,
>> >
>> > I come across the term "type checking" very often in my readings on C++,[/color]
>> and[color=darkred]
>> > have never heard it in Java. Besides the simplistic answer that it[/color][/color]
>checks[color=green][color=darkred]
>> > the "type", what more does it mean?
>> >
>> >
>> > WD
>> >[/color]
>>
>> I think its a question of timing. All languages check types, you aren't
>> allowed to multiply two strings in any language (AFAIK). But some[/color]
>languages[color=green]
>> delay type checking until the program runs. C++ does most of its type
>> checking at compile type. This results in faster code, smaller data and
>> quicker detection of type errors, but it also means a more complex[/color]
>language.[color=green]
>>
>> We say C++ is a strongly typed language. In general I would say strong
>> typing is a good thing but there are occasions where you want to relax it
>> and check types at run time. That's where virtual functions and RTTI come
>> in.[/color]
>
>Java is also a strongly typed language and checks for compatible types for
>method arguments and return values at compile time.[/color]

Nope. Consider classes A, B and C in separate files, where A uses B uses
C, and A is the "main" program. Compile all. Change C to be incompatible
with B. Compile class A using Sun's compiler: no errors reported. Run.
Runtime error.

That could not happen in C++.

C++ has _global_ static type checking; Java has runtime type checking.

[Alf P. Steinbach]

Re: &quot;Type Checking&quot;

On Mon, 11 Aug 2003 13:34:20 GMT, tom_usenet@hotm ail.com (tom_usenet) wrote:
[color=blue]
>On Mon, 11 Aug 2003 10:23:27 GMT, alfps@start.no (Alf P. Steinbach)
>wrote:
>[color=green]
>>Nope. Consider classes A, B and C in separate files, where A uses B uses
>>C, and A is the "main" program. Compile all. Change C to be incompatible
>>with B. Compile class A using Sun's compiler: no errors reported. Run.
>>Runtime error.
>>
>>That could not happen in C++.
>>
>>C++ has _global_ static type checking; Java has runtime type checking.[/color]
>
>The Java language has stronger static type checking than C++. e.g.
>implicit conversions from double to int, etc. don't exist. C++ however
>has better idioms for static type checking, thanks to using generics
>rather than an Object base-of-all class. Java encourages use of the
>evil instanceof operator.
>
>I think you're talking about the link time safety that C++ has thanks
>to name mangling. But C++ will just crash if you replace a DLL with
>another one with incompatible types, so I don't get your point.[/color]

DLLs are not part of C++.

[color=blue]
>Java will detect the problem if you recompile[/color]

Nope. Java will only detect the problem if you recompile class B.
You have an executable system without recompiling class B (and this is
not theoretical, it has been a major problem in some large Java projects).

[color=blue]
>just as happens with C++.[/color]

Nope. With C++ you have global type-checking. You cannot obtain an
executable with static type inconsistencies .

[ghl]

Re: &quot;Type Checking&quot;

"Alf P. Steinbach" <alfps@start.no > wrote in message
news:3f376df1.4 31596921@News.C IS.DFN.DE...[color=blue]
> On Mon, 11 Aug 2003 05:20:48 -0400, "ghl" <glabowitz@comc ast.net> wrote:
>[color=green]
> >"John Harrison" <john_andronicu s@hotmail.com> wrote in message
> >news:bh7dn6$tv ruf$1@ID-196037.news.uni-berlin.de...[color=darkred]
> >>
> >> "Web Developer" <nospam@hotmail .com> wrote in message
> >> news:3f373851_1 @news.iprimus.c om.au...
> >> > Hi,
> >> >
> >> > I come across the term "type checking" very often in my readings on[/color][/color][/color]
C++,[color=blue][color=green][color=darkred]
> >> and
> >> > have never heard it in Java. Besides the simplistic answer that it[/color]
> >checks[color=darkred]
> >> > the "type", what more does it mean?
> >> >
> >> >
> >> > WD
> >> >
> >>
> >> I think its a question of timing. All languages check types, you aren't
> >> allowed to multiply two strings in any language (AFAIK). But some[/color]
> >languages[color=darkred]
> >> delay type checking until the program runs. C++ does most of its type
> >> checking at compile type. This results in faster code, smaller data and
> >> quicker detection of type errors, but it also means a more complex[/color]
> >language.[color=darkred]
> >>
> >> We say C++ is a strongly typed language. In general I would say strong
> >> typing is a good thing but there are occasions where you want to relax[/color][/color][/color]
it[color=blue][color=green][color=darkred]
> >> and check types at run time. That's where virtual functions and RTTI[/color][/color][/color]
come[color=blue][color=green][color=darkred]
> >> in.[/color]
> >
> >Java is also a strongly typed language and checks for compatible types[/color][/color]
for[color=blue][color=green]
> >method arguments and return values at compile time.[/color]
>
> Nope. Consider classes A, B and C in separate files, where A uses B uses
> C, and A is the "main" program. Compile all. Change C to be incompatible
> with B. Compile class A using Sun's compiler: no errors reported. Run.
> Runtime error.[/color]

Wrong. Been there. Tried that.
Created classes as above, all returning String type reference variable.
Changed return type of C to int. Message on recompiling A gives:
..\C.java:3: incompatible types
found : java.lang.Strin g
required: int
public int info = "I'm c";
^
..\C.java:13: incompatible types
found : int
required: java.lang.Strin g
return info;
^
2 errors
--
Gary

[ghl]

Re: &quot;Type Checking&quot;

"Alf P. Steinbach" <alfps@start.no > wrote in message
news:3f37cc95.4 55824625@News.C IS.DFN.DE...[color=blue]
> On Mon, 11 Aug 2003 13:06:42 -0400, "ghl" <glabowitz@comc ast.net> wrote:
>[color=green]
> >"Alf P. Steinbach" <alfps@start.no > wrote in message
> >news:3f376df1. 431596921@News. CIS.DFN.DE...[/color][/color]
<<large snip>>[color=blue]
> "Change" means: change the source code of class C, compile class C.
>
> Then compile class A, or not (it makes no difference), and run:
>
>
> Exception in thread "main" java.lang.NoSuc hMethodError:[/color]
C.result()Ljava/lang/String;[color=blue]
> at B.result(B.java :5)
> at A.main(A.java:5 )
>
>
> Got it?[/color]

Ah, I get what you're driving at. Yes, you get that. Does the same thing
happen in C++ with the linker detecting the change?
The Java error at runtime is detected because there is no linker and the
type checking can only occur when class C is actually loaded and checked.
I get your point.
--
Gary

[Alf P. Steinbach]

Re: &quot;Type Checking&quot;

On Mon, 11 Aug 2003 13:36:19 -0400, "ghl" <glabowitz@comc ast.net> wrote:
[color=blue]
>"Alf P. Steinbach" <alfps@start.no > wrote in message
>news:3f37cc95. 455824625@News. CIS.DFN.DE...[color=green]
>> On Mon, 11 Aug 2003 13:06:42 -0400, "ghl" <glabowitz@comc ast.net> wrote:
>>[color=darkred]
>> >"Alf P. Steinbach" <alfps@start.no > wrote in message
>> >news:3f376df1. 431596921@News. CIS.DFN.DE...[/color][/color]
><<large snip>>[color=green]
>> "Change" means: change the source code of class C, compile class C.
>>
>> Then compile class A, or not (it makes no difference), and run:
>>
>>
>> Exception in thread "main" java.lang.NoSuc hMethodError:[/color]
>C.result()Ljav a/lang/String;[color=green]
>> at B.result(B.java :5)
>> at A.main(A.java:5 )
>>
>>
>> Got it?[/color]
>
>Ah, I get what you're driving at. Yes, you get that. Does the same thing
>happen in C++ with the linker detecting the change?[/color]

In practice the linker detects the change, yes.

In theory there need not be a linker in a C++ implementation, but the
_language_ (standard) guarantees global static type-checking whatever
execution system is actually used.

There's no runtime error because you don't get an executable.