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digitalmars.D.bugs - [Issue 3960] New: Unused variable warning

reply d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=3960

           Summary: Unused variable warning
           Product: D
           Version: 2.041
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: DMD
        AssignedTo: nobody puremagic.com
        ReportedBy: bearophile_hugs eml.cc


--- Comment #0 from bearophile_hugs eml.cc 2010-03-13 16:48:40 PST ---
This is C program:

#include "stdio.h"
#include "stdlib.h"
int main(int argc, char** argv) {
    int x, y;
    x = (argc > 1) ? atoi(argv[1]) : 10;
    printf("%d\n", x);
    return 0;
}


Compiled with gcc v.4.4.1 with:
gcc -Wall foo.c -o foo

The compiler prints:
foo.c: In function 'main':
foo.c:4: warning: unused variable 'y'


The Intel C/C++ compiler gives a similar error (this is a mockup, but it's
equal or very close to the real one):
foo.c(4): warning #177: variable "y" was declared but never referenced
    int x, y;
           ^
-----------------------

This is C# code:

using System;
public class Foo {
    public static void Main() {
        int x, y;
        x = int.Parse(Console.ReadLine());
        Console.WriteLine(x);
    }
}


With default settings the C# compiler prints:
prog.cs(4,16): warning CS0168: The variable `y' is declared but never used
Compilation succeeded - 1 warning(s)


This explains the warning CS0168:
http://msdn.microsoft.com/en-us/library/tf85t6e4.aspx

-----------------------

In Java all the most important editors/IDEs detect unused local variables:
JDeveloper, Eclipse, JEdit, JBuilder, BlueJ, CodeGuide, NetBeans/Sun Java
Studio Enterprise/Creator, etc.


If the most important compilers/IDEs of the most important and used languages
(C/C++, C#, Java) give warnings for unused variables, then it can be an useful
warning.

By itself an unused variable is not an error, but its presence is often caused
due to oversight (either you declared a variable you didn't need or you
refactored and a variable that was once needed now is no long needed). While
coding in C the GCC compiler has avoided me 2 possible bugs thanks to that
unused variable warning, because I was forgetting some part of the code.

So I believe this warning can be useful in D too.

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--- Comment #1 from bearophile_hugs eml.cc 2010-03-17 05:30:02 PDT ---
A related warning can be see with this C# program:


using System;
namespace Test {
    class Foo {
        int x;
        private void Add(int y) {
            x += y;
        }
        static void Main() {
            System.Console.WriteLine("test");
        }
    }
}



Compiled with mono:
...gmcs test.cs
test.cs(5,22): warning CS0169: The private method `Test.Foo.Add(int)' is never
used
Compilation succeeded - 1 warning(s)


In D private methods can be used by other functions in the same module, so it's
less easy to produce a warning like this.

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--- Comment #2 from bearophile_hugs eml.cc 2010-08-20 09:04:27 PDT ---
There is more than just unused variables, there are also unused last
assignments:


void main() {
    int x;
    x = 10;
}


Here 'x' is not an unused variable, because the code does something with it,
but the code deserves a warning anyway (and one C compiler-like shows this
warning) because the last value assigned to it gets unused; and this is wasted
coding/running effort at best, or sign of a possible latent bug (just like
unused variables).

This warning isn't necessary if the variable address is assigned to a pointer
or similar things.

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prev sibling next sibling parent d-bugmail puremagic.com writes:
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Jonathan M Davis <jmdavisProg gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jmdavisProg gmail.com


--- Comment #3 from Jonathan M Davis <jmdavisProg gmail.com> 2010-08-20
09:54:44 PDT ---
Okay. Giving a warning for an unused variable makes sense. However, giving a
warning for setting it somewhere that is not where it is declared doesn't make
sense. Sure, in this case, it's dumb of the programmer to have done that, but

1. The compiler should be able to optimize out such a simple case.

2. More complex cases much harder to detect, and it's not all that hard for you
to _want_ to disconnect the declaration of the variable and initializing it -
likely because of scoping issues or conditional blocks. The compiler is only
going to be able to detect the simplest cases, and as I said in #1, such simple
cases will likely get optimized away. Cases too complex to optimize away are
going to be much harder for the compiler to detect, and if you start going that
far, you're getting to the point where you just about should have done what
Java did and force initialization of variables rather than default initialize
them.

Not initializing a variable yourself, and the setting it later with a value
that you could have set it to at its declaration is not a coding error. It may
not be best practice, but it's not going to result in an error. So, I don't
think that it makes any sense to make it a warning.

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--- Comment #4 from bearophile_hugs eml.cc 2010-08-20 14:48:59 PDT ---
Sorry, my mistake, I have lumped two different warnings into this enhancement
request, so please ignore comment 2

I have moved the second warning to bug 4694

Look at bug 4694 for an answer to comment 3 regarding the second warning.

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--- Comment #5 from bearophile_hugs eml.cc 2010-09-30 18:16:17 PDT ---
See also "Using Redundancies to Find Errors", by Yichen Xie and Dawson Engler,
2002:
www.stanford.edu/~engler/p401-xie.pdf

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--- Comment #6 from bearophile_hugs eml.cc 2011-03-27 08:15:13 PDT ---
Two new good related flags in GCC 4.6:


New -Wunused-but-set-variable and -Wunused-but-set-parameter warnings were
added for C, C++, Objective-C and Objective-C++. These warnings diagnose
variables respective parameters which are only set in the code and never
otherwise used. Usually such variables are useless and often even the value
assigned to them is computed needlessly, sometimes expensively. The
-Wunused-but-set-variable warning is enabled by default by -Wall flag and
-Wunused-but-set-parameter by -Wall -Wextra flags.<




Some discussions in this thread:
http://www.digitalmars.com/webnews/newsgroups.php?art_group=digitalmars.D&article_id=132909

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yebblies <yebblies gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |peng2cheng2 yahoo.com


--- Comment #7 from yebblies <yebblies gmail.com> 2011-06-10 11:14:55 PDT ---
*** Issue 3163 has been marked as a duplicate of this issue. ***

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Stewart Gordon <smjg iname.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|diagnostic                  |accepts-invalid
            Summary|Unused variable warning     |Unused local variables not
                   |                            |reported
           Severity|enhancement                 |normal


--- Comment #8 from Stewart Gordon <smjg iname.com> 2011-06-10 12:00:38 PDT ---
This is accepts-invalid, not an enhancement request.

http://www.digitalmars.com/d/1.0/function.html
"It is an error to declare a local variable that is never referred to."

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--- Comment #9 from Jonathan M Davis <jmdavisProg gmx.com> 2011-06-10 12:13:16
PDT ---
I would point out that if it is made an error to have unused local variables,
function parameters must _not_ be included in it. That would be a major problem
in cases where overridden functions don't use all of their parameters, but they
have to have them because the base class' function does.

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--- Comment #10 from bearophile_hugs eml.cc 2011-06-10 14:04:58 PDT ---
(In reply to comment #9)

 I would point out that if it is made an error to have unused local variables,
 function parameters must _not_ be included in it. That would be a major problem
 in cases where overridden functions don't use all of their parameters, but they
 have to have them because the base class' function does.



Don't worry, warnings against "unused local variables" and "unused argument
values" are meant to be two distinct features. Each of them needs a separate
discussion and has to stand for its own.

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