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From: Martin Sebor <msebor@gmail.com>
To: Volker Reichelt <v.reichelt@netcologne.de>, gcc-patches@gcc.gnu.org
Subject: Re: {PATCH] New C++ warning -Wcatch-value
Date: Wed, 03 May 2017 03:34:00 -0000	[thread overview]
Message-ID: <297400e4-5e33-08aa-8048-cffd3626740a@gmail.com> (raw)
In-Reply-To: <tkrat.e779dc1a05db1c81@netcologne.de>

On 05/01/2017 02:38 AM, Volker Reichelt wrote:
> Hi,
>
> catching exceptions by value is a bad thing, as it may cause slicing, i.e.
> a) a superfluous copy
> b) which is only partial.
> See also https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#e15-catch-exceptions-from-a-hierarchy-by-reference
>
> To warn the user about catch handlers of non-reference type,
> the following patch adds a new C++/ObjC++ warning option "-Wcatch-value".

I think the problems related to catching exceptions by value
apply to (a subset of) class types but not so much to fundamental
types.  I would expect indiscriminately warning on every type to
be overly restrictive.

The Enforcement section of the C++ guideline suggests to

   Flag by-value exceptions if their types are part of a hierarchy
   (could require whole-program analysis to be perfect).

The corresponding CERT C++ Coding Standard guideline offers
a similar suggestion here:

   https://www.securecoding.cert.org/confluence/x/TAD5CQ

so I would suggest to model the warning on that approach (within
limits of a single translation unit, of course).   I.e., warn only
for catching by value objects of non-trivial types, or perhaps even
only polymorphic types?

Martin

>
> Bootstrapped and regtested on x86_64-pc-linux-gnu.
> OK for trunk?
>
> Regards,
> Volker
>
>
> 2017-05-01  Volker Reichelt  <v.reichelt@netcologne.de>
>
> 	* doc/invoke.texi (-Wcatch-value): Document new warning option.
>
> Index: gcc/doc/invoke.texi
> ===================================================================
> --- gcc/doc/invoke.texi	(revision 247416)
> +++ gcc/doc/invoke.texi	(working copy)
> @@ -265,7 +265,7 @@
>  -Wno-builtin-declaration-mismatch @gol
>  -Wno-builtin-macro-redefined  -Wc90-c99-compat  -Wc99-c11-compat @gol
>  -Wc++-compat  -Wc++11-compat  -Wc++14-compat  -Wcast-align  -Wcast-qual  @gol
> --Wchar-subscripts -Wchkp  -Wclobbered  -Wcomment  @gol
> +-Wchar-subscripts  -Wchkp  -Wcatch-value  -Wclobbered  -Wcomment  @gol
>  -Wconditionally-supported  @gol
>  -Wconversion  -Wcoverage-mismatch  -Wno-cpp  -Wdangling-else  -Wdate-time @gol
>  -Wdelete-incomplete @gol
> @@ -5827,6 +5827,11 @@
>  literals to @code{char *}.  This warning is enabled by default for C++
>  programs.
>
> +@item -Wcatch-value @r{(C++ and Objective-C++ only)}
> +@opindex Wcatch-value
> +@opindex Wno-catch-value
> +Warn about catch handler of non-reference type.
> +
>  @item -Wclobbered
>  @opindex Wclobbered
>  @opindex Wno-clobbered
> ===================================================================
>
> 2017-05-01  Volker Reichelt  <v.reichelt@netcologne.de>
>
> 	* c.opt (Wcatch-value): New C++ warning flag.
>
> Index: gcc/c-family/c.opt
> ===================================================================
> --- gcc/c-family/c.opt	(revision 247416)
> +++ gcc/c-family/c.opt	(working copy)
> @@ -388,6 +388,10 @@
>  C ObjC C++ ObjC++ Var(warn_cast_qual) Warning
>  Warn about casts which discard qualifiers.
>
> +Wcatch-value
> +C++ ObjC++ Var(warn_catch_value) Warning
> +Warn about catch handlers of non-reference type.
> +
>  Wchar-subscripts
>  C ObjC C++ ObjC++ Var(warn_char_subscripts) Warning LangEnabledBy(C ObjC C++ ObjC++,Wall)
>  Warn about subscripts whose type is \"char\".
> ===================================================================
>
> 2017-05-01  Volker Reichelt  <v.reichelt@netcologne.de>
>
> 	* semantics.c (finish_handler_parms): Warn about non-reference type
> 	catch handlers.
>
> Index: gcc/cp/semantics.c
> ===================================================================
> --- gcc/cp/semantics.c	(revision 247416)
> +++ gcc/cp/semantics.c	(working copy)
> @@ -1321,7 +1321,15 @@
>  	}
>      }
>    else
> -    type = expand_start_catch_block (decl);
> +    {
> +      type = expand_start_catch_block (decl);
> +      if (warn_catch_value
> +	  && type != NULL_TREE
> +	  && type != error_mark_node
> +	  && TREE_CODE (TREE_TYPE (decl)) != REFERENCE_TYPE)
> +	warning (OPT_Wcatch_value,
> +		 "catching non-reference type %qT", TREE_TYPE (decl));
> +    }
>    HANDLER_TYPE (handler) = type;
>  }
>
> ===================================================================
>
> 2017-05-01  Volker Reichelt  <v.reichelt@netcologne.de>
>
> 	* g++.dg/warn/Wcatch-value-1.C: New test.
>
> Index: gcc/testsuite/g++.dg/warn/Wcatch-value-1.C
> ===================================================================
> --- gcc/testsuite/g++.dg/warn/Wcatch-value-1.C	2017-05-01
> +++ gcc/testsuite/g++.dg/warn/Wcatch-value-1.C	2017-05-01
> @@ -0,0 +1,45 @@
> +// { dg-options "-Wcatch-value" }
> +
> +struct A {};
> +struct B {};
> +struct C {};
> +
> +void foo()
> +{
> +  try {}
> +  catch (int)      {}  // { dg-warning "catching non-reference type" }
> +  catch (double*)  {}  // { dg-warning "catching non-reference type" }
> +  catch (float&)   {}
> +  catch (A)        {}  // { dg-warning "catching non-reference type" }
> +  catch (A[2])     {}  // { dg-warning "catching non-reference type" }
> +  catch (B*)       {}  // { dg-warning "catching non-reference type" }
> +  catch (B&)       {}
> +  catch (const C&) {}
> +}
> +
> +template<typename T> void foo1()
> +{
> +  try {}
> +  catch (T) {}
> +}
> +
> +void bar1()
> +{
> +  foo1<int&>();
> +  foo1<const A&>();
> +}
> +
> +template<typename T> void foo2()
> +{
> +  try {}
> +  catch (T) {}  // { dg-warning "catching non-reference type" }
> +
> +  try {}
> +  catch (T&) {}
> +}
> +
> +void bar2()
> +{
> +  foo2<int*>();      // { dg-message "required" }
> +  foo2<A>();         // { dg-message "required" }
> +}
> ===================================================================
>

  reply	other threads:[~2017-05-03  3:25 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-05-01  8:38 Volker Reichelt
2017-05-03  3:34 ` Martin Sebor [this message]
2017-05-07 20:28   ` Volker Reichelt
2017-05-08  3:18     ` Martin Sebor
2017-05-08 13:14       ` Jason Merrill via gcc-patches
2017-05-14 15:30       ` Volker Reichelt
2017-05-15 20:03         ` Martin Sebor
2017-05-24 20:13           ` Jason Merrill
2017-05-30  6:22             ` Volker Reichelt
2017-05-31 20:55               ` Jason Merrill
2017-06-05 10:55           ` Volker Reichelt
2017-06-05 19:08             ` Jason Merrill

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