HOWTO: Get Warning on implicit type casts that may lead to data loss

[elliot.li.tech]

Hello everyone,

I'm migrating a C++ program from 32-bit systems to 64-bit systems. The
old programmers used some annoying assignments like assigning "long"
to "int", which may lead to data loss.

Is it possible to let the compiler (specifically, GCC) spew some
warnings on this kind of implicit type casts?

Or you suggest use some other static source codes analysis tools?

Thanks in advance!

[=?UTF-8?B?RXJpayBXaWtzdHLDtm0=?=]

On 2007-11-11 15:39, el************@ gmail.com wrote:

Hello everyone,

I'm migrating a C++ program from 32-bit systems to 64-bit systems. The
old programmers used some annoying assignments like assigning "long"
to "int", which may lead to data loss.

Is it possible to let the compiler (specifically, GCC) spew some
warnings on this kind of implicit type casts?

Or you suggest use some other static source codes analysis tools?

Any good compiler should warn when there is a risk of data loss, have
you turned up the warning levels? Good flags to always include when
compiling with gcc are -Wall and -std=c++98, there are probably more but
those are the ones I can remember right now.

--
Erik Wikström

[Rolf Magnus]

Erik Wikström wrote:

On 2007-11-11 15:39, el************@ gmail.com wrote:

>Hello everyone,

I'm migrating a C++ program from 32-bit systems to 64-bit systems. The
old programmers used some annoying assignments like assigning "long"
to "int", which may lead to data loss.

Is it possible to let the compiler (specifically, GCC) spew some
warnings on this kind of implicit type casts?

There is no such thing as an implicit type cast. It's an implicit
conversion.

>Or you suggest use some other static source codes analysis tools?

Any good compiler should warn when there is a risk of data loss, have
you turned up the warning levels? Good flags to always include when
compiling with gcc are -Wall and -std=c++98, there are probably more but
those are the ones I can remember right now.

I would suggest to also add -pedantic and -Wextra.

[elliot.li.tech]

Thanks Erik and Rolf!

On Nov 12, 1:45 am, Rolf Magnus <[e-mail]:

There is no such thing as an implicit type cast. It's an implicit
conversion.

OK, thanks!

Any good compiler should warn when there is a risk of data loss, have
you turned up the warning levels? Good flags to always include when
compiling with gcc are -Wall and -std=c++98, there are probably more but
those are the ones I can remember right now.

I would suggest to also add -pedantic and -Wextra.

I've tried "-pedantic -Wall -Wextra -std=c++98" but see no warning
about such kind of error. Just tried a little program like this:

#include <iostream>

using namespace std;
int main ()
{
long l;
int i;

cout << "input a long number: ";
cin >l;
i = l;
cout << "the int number is: " << i << endl;

return 0;
}

[elliot.li.tech]

I've tried options "-m64 -pedantic -Wall -Wextra -std=c++98" on both
x86-64 and PPC64, neither issued a single waring on assigning a "long"
to "int."

[Rolf Magnus]

el************@ gmail.com wrote:

I've tried options "-m64 -pedantic -Wall -Wextra -std=c++98" on both
x86-64 and PPC64, neither issued a single waring on assigning a "long"
to "int."

You are right. I could have sworn that there is a warning for that, but I
looked into the manual and didn't find any option that gets me such a
warning. You could ask in gnu.g++.help, but it seems to me there is no
warning for that.

[James Kanze]

On Nov 12, 5:53 am, Rolf Magnus <[e-mail]:

elliot.li.t...@ gmail.com wrote:

I've tried options "-m64 -pedantic -Wall -Wextra -std=c++98" on both
x86-64 and PPC64, neither issued a single waring on assigning a "long"
to "int."

You are right. I could have sworn that there is a warning for that, but I
looked into the manual and didn't find any option that gets me such a
warning. You could ask in gnu.g++.help, but it seems to me there is no
warning for that.

The problem is that it would also warn on things like:

char* p ;
int ch = getchar() ;
if ( ch != EOF ) {
*p ++ = ch ;
// ...
}

or even:

short a, b, c ;
a = b + c ;

The first is an extremely common idiom in C, and is probably
used in quite a lot of C++ code as well. The second will occur
anytime arithmetic is done on short or character types, because
of integral promotion.

The utility of the warning may be less that you might think,
too. An expression like:

int a, b, c ;
a = b + c ;

is probably even more common, and has the same risks.

--
James Kanze (GABI Software) email:ja******* **@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientier ter Datenverarbeitu ng
9 place Sémard, 78210 St.-Cyr-l'École, France, [phone]

[karpov2007]

On Nov 11, 5:39 pm, elliot.li.t...@ gmail.com wrote:

Hello everyone,

I'm migrating a C++ program from 32-bit systems to 64-bit systems. The
old programmers used some annoying assignments like assigning "long"
to "int", which may lead to data loss.

Is it possible to let the compiler (specifically, GCC) spew some
warnings on this kind of implicit type casts?

Or you suggest use some other static source codes analysis tools?

Thanks in advance!

Viva64 tool can find dangerous implicit and explicit type conversion.
Viva64 is the lint-like tool for searching of errors and possible
problems in C/C++ code while porting the application to the 64-bit
platform.
http://www.viva64.com/viva64.php

[elliot.li.tech]

On Nov 12, 12:53 pm, Rolf Magnus <[e-mail]:

elliot.li.t...@ gmail.com wrote:

I've tried options "-m64 -pedantic -Wall -Wextra -std=c++98" on both
x86-64 and PPC64, neither issued a single waring on assigning a "long"
to "int."

You are right. I could have sworn that there is a warning for that, but I
looked into the manual and didn't find any option that gets me such a
warning. You could ask in gnu.g++.help, but it seems to me there is no
warning for that.

Thanks, I'll ask this question over there. I've read through GCC's
man page and info docs, but found nothing regards this neither.

[elliot.li.tech]

On Nov 12, 6:27 pm, "karpov2...@gma il.com" <karpov2...@gma il.com>
wrote:

Or you suggest use some other static source codes analysis tools?

Viva64 tool can find dangerous implicit and explicit type conversion.
Viva64 is the lint-like tool for searching of errors and possible
problems in C/C++ code while porting the application to the 64-bit
platform.http://www.viva64.com/viva64.php

Thanks Karpov! It seems a good tool. But it's hard for us to adopt non-
FLOSS software in our development cycle currently. Do you have any
other suggestions?