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* [Bug c++/21917] Missing warning when trying to explicitly construct a virtual base
[not found] <bug-21917-4@http.gcc.gnu.org/bugzilla/>
@ 2010-10-28 9:17 ` redi at gcc dot gnu.org
0 siblings, 0 replies; 3+ messages in thread
From: redi at gcc dot gnu.org @ 2010-10-28 9:17 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21917
--- Comment #8 from Jonathan Wakely <redi at gcc dot gnu.org> 2010-10-28 09:17:23 UTC ---
I've relied on exactly this behaviour and wouldn't want a warning.
struct Base : virtual std::ios {
Base() : std::ios(0) { }
std::streambuf buf;
};
struct MyStream : Base, std::iostream {
MyStream() : std::iostream(0) { }
};
If a concrete Base is constructed I want to call ios::ios(0) but if a MyStream
is constructed then I want to call the ios::ios() default ctor and then have
iostream::iostream(0) call ios::init(0)
If this warning is added (which I'm not convinced it should be) then it should
not be given if the most-derived class *explicitly* uses a different ctor for
the virtual base i.e. I could suppress the warning by changing the code above
like so:
MyStream() : std::ios(), std::iostream(0) { }
^^^^^^^^^^^
I also don't think the warning should be given if the unused mem-initializer in
der1 and der2 calls the same constructor as is implicitly called by the
most-derived class, i.e. if der1 and der2 called virt() then that has the same
effect as the implicit initialization done by top.
I don't think this should be P2, and I think a better summary would be "Missing
warning when virtual base is initialized with different constructor than
requested by an ignored mem-initializer"
^ permalink raw reply [flat|nested] 3+ messages in thread
[parent not found: <bug-21917-6594@http.gcc.gnu.org/bugzilla/>]
* [Bug c++/21917] Missing warning when trying to explicitly construct a virtual base
[not found] <bug-21917-6594@http.gcc.gnu.org/bugzilla/>
@ 2007-01-20 16:52 ` manu at gcc dot gnu dot org
2008-06-05 23:07 ` r_q_einstein-gccgnuorg at yahoo dot com
1 sibling, 0 replies; 3+ messages in thread
From: manu at gcc dot gnu dot org @ 2007-01-20 16:52 UTC (permalink / raw)
To: gcc-bugs
------- Comment #6 from manu at gcc dot gnu dot org 2007-01-20 16:52 -------
Not sure if the summary is 100% correct but it is far better than the previous
one.
--
manu at gcc dot gnu dot org changed:
What |Removed |Added
----------------------------------------------------------------------------
Summary|Missing warning? |Missing warning when trying
| |to explicitly construct a
| |virtual base
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21917
^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug c++/21917] Missing warning when trying to explicitly construct a virtual base
[not found] <bug-21917-6594@http.gcc.gnu.org/bugzilla/>
2007-01-20 16:52 ` manu at gcc dot gnu dot org
@ 2008-06-05 23:07 ` r_q_einstein-gccgnuorg at yahoo dot com
1 sibling, 0 replies; 3+ messages in thread
From: r_q_einstein-gccgnuorg at yahoo dot com @ 2008-06-05 23:07 UTC (permalink / raw)
To: gcc-bugs
------- Comment #7 from r_q_einstein-gccgnuorg at yahoo dot com 2008-06-05 23:07 -------
I've run across this, too.
I agree with Ivan's 2005-06-04 suggestion.
It would be nice if the compiler would emit a warning that the derived class's
constructor's call to the virtual base class's non-default constructor is being
ignored and overridden by an implicit call to the virtual base class's default
constructor.
Similarly, if the virtual base class fails to specify a default constructor
(and specifies only a non-default one), the derived class's constructor's call
to the virtual base class's non-default constructor produces only this error
message:
In constructor 'top::top()':
error: no matching function for call to 'virt::virt()'
which leaves the developer scratching his/her head if he doesn't know that the
C++ spec. requires virtual base classes to have default constructors. (True,
s/he should know, but the compile error message could be more helpful.)
Something to the effect of:
"virtual base class lacks default constructor" would be much more helpful.
If this issue is different enough to warrant a separate "bug" report, let me
know and I'll be happy to file it separately.
Thanks.
--
r_q_einstein-gccgnuorg at yahoo dot com changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |r_q_einstein-gccgnuorg at
| |yahoo dot com
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21917
^ permalink raw reply [flat|nested] 3+ messages in thread
end of thread, other threads:[~2010-10-28 9:17 UTC | newest]
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2010-10-28 9:17 ` [Bug c++/21917] Missing warning when trying to explicitly construct a virtual base redi at gcc dot gnu.org
[not found] <bug-21917-6594@http.gcc.gnu.org/bugzilla/>
2007-01-20 16:52 ` manu at gcc dot gnu dot org
2008-06-05 23:07 ` r_q_einstein-gccgnuorg at yahoo dot com
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