[property_tree] Copy constructor and assignment operator in xml_writer_settings class
Hi, What is recommended or required policy regarding copy operations for types which consist of const data members, within Boost? I'm having problem with understanding why the xml_writer_settings class do not have the copying operations disabled. Perhaps it should be patched? Quick chat on the #boost IRC channel confirms it could be improved. I'm looking forward comments. [1] https://svn.boost.org/trac/boost/browser/trunk/boost/property_tree/detail/xm... Best regards, -- Mateusz Loskot, http://mateusz.loskot.net Charter Member of OSGeo, http://osgeo.org Member of ACCU, http://accu.org
Hi, Timorously, I'd like to bump my question. Mat mloskot wrote:
Hi,
What is recommended or required policy regarding copy operations for types which consist of const data members, within Boost?
I'm having problem with understanding why the xml_writer_settings class do not have the copying operations disabled. Perhaps it should be patched?
Quick chat on the #boost IRC channel confirms it could be improved.
I'm looking forward comments.
[1] https://svn.boost.org/trac/boost/browser/trunk/boost/property_tree/detail/xm...
Best regards, -- Mateusz Loskot, http://mateusz.loskot.net Charter Member of OSGeo, http://osgeo.org Member of ACCU, http://accu.org _______________________________________________ Unsubscribe & other changes: http://lists.boost.org/mailman/listinfo.cgi/boost
----- -- Mateusz Loskot, http://mateusz.loskot.net Charter Member of OSGeo, http://osgeo.org Member of ACCU, http://accu.org -- View this message in context: http://boost.2283326.n4.nabble.com/property-tree-Copy-constructor-and-assign... Sent from the Boost - Dev mailing list archive at Nabble.com.
On 18.10.2010 12:22, mloskot wrote:
mloskot wrote:
Hi,
What is recommended or required policy regarding copy operations for types which consist of const data members, within Boost?
I'm having problem with understanding why the xml_writer_settings class do not have the copying operations disabled.
Why would it? There's nothing semantically wrong with copying a settings object. It's just not very useful.
Sebastian
Sebastian Redl wrote:
On 18.10.2010 12:22, mloskot wrote:
mloskot wrote:
Hi,
What is recommended or required policy regarding copy operations for types which consist of const data members, within Boost?
I'm having problem with understanding why the xml_writer_settings class do not have the copying operations disabled.
Why would it? There's nothing semantically wrong with copying a settings object. It's just not very useful.
Looking at the current definition [1], I'm not convinced. According to C++(n3092)/12.8/25, assignment operator for this class is defined as deleted due to "non-static data members of const non-class type". [1] https://svn.boost.org/trac/boost/browser/trunk/boost/property_tree/detail/xm... Best reagards, ----- -- Mateusz Loskot, http://mateusz.loskot.net Charter Member of OSGeo, http://osgeo.org Member of ACCU, http://accu.org -- View this message in context: http://boost.2283326.n4.nabble.com/property-tree-Copy-constructor-and-assign... Sent from the Boost - Dev mailing list archive at Nabble.com.
On 18.10.2010, at 14:22, mloskot wrote:
Sebastian Redl wrote:
On 18.10.2010 12:22, mloskot wrote:
mloskot wrote:
Hi,
What is recommended or required policy regarding copy operations for types which consist of const data members, within Boost?
I'm having problem with understanding why the xml_writer_settings class do not have the copying operations disabled.
Why would it? There's nothing semantically wrong with copying a settings object. It's just not very useful.
Looking at the current definition [1], I'm not convinced. According to C++(n3092)/12.8/25, assignment operator for this class is defined as deleted due to "non-static data members of const non-class type".
I have no idea what your point is. Do you want me to prevent copy construction too, just because copy assignment is not possible? Or do you want me to make the members non-const so that copy assignment is possible? If the former, why? If the latter, I suppose I could do that. I'm not the one who made the members const in the first place. Sebastian
On 18/10/10 14:50, Sebastian Redl wrote:
On 18.10.2010, at 14:22, mloskot wrote:
Sebastian Redl wrote:
On 18.10.2010 12:22, mloskot wrote:
mloskot wrote:
Hi,
What is recommended or required policy regarding copy operations for types which consist of const data members, within Boost?
I'm having problem with understanding why the xml_writer_settings class do not have the copying operations disabled.
Why would it? There's nothing semantically wrong with copying a settings object. It's just not very useful.
Looking at the current definition [1], I'm not convinced. According to C++(n3092)/12.8/25, assignment operator for this class is defined as deleted due to "non-static data members of const non-class type".
I have no idea what your point is.
My point is that the class definition suggests its copying might not be kosher. It is valid to copy-construct it but not assign.
Do you want me to prevent copy construction too, just because copy assignment is not possible? Or do you want me to make the members non-const so that copy assignment is possible?
In my opinion, copy constructor and assignment operator walk together to make a type fully copyable. Thus, IMO, either the definition should change or both copy operations disabled.
If the former, why? If the latter, I suppose I could do that. I'm not the one who made the members const in the first place.
I would suggest to either disable both copy operations (does it make sense to have copy ctor without assignment operator?) or change the definition. I'd also like to point that I have asked generally: What is recommended or required policy regarding copy operations, it is copy ctor and assignment operator. So, I have asked for comments and discussions too, as my understanding of this matter may be incorrect. Best regards, -- Mateusz Loskot, http://mateusz.loskot.net Charter Member of OSGeo, http://osgeo.org Member of ACCU, http://accu.org
At Mon, 18 Oct 2010 15:02:39 +0100, Mateusz Loskot wrote:
Looking at the current definition [1], I'm not convinced. According to C++(n3092)/12.8/25, assignment operator for this class is defined as deleted due to "non-static data members of const non-class type".
I have no idea what your point is.
My point is that the class definition suggests its copying might not be kosher. It is valid to copy-construct it but not assign.
That's because it has members that are, um, const. Assignment is a mutating operation. Construction is not.
Do you want me to prevent copy construction too, just because copy assignment is not possible? Or do you want me to make the members non-const so that copy assignment is possible?
In my opinion, copy constructor and assignment operator walk together to make a type fully copyable.
The proper term here is "regular," I think (see http://www.stepanovpapers.com/DeSt98.pdf). But then you need equality and a bunch of other operations to make it meaningful.
Thus, IMO, either the definition should change or both copy operations disabled.
I disagree. We have movable but noncopyable types. There's nothing wrong in principle with copyable but not-assignable types.
If the former, why? If the latter, I suppose I could do that. I'm not the one who made the members const in the first place.
I would suggest to either disable both copy operations (does it make sense to have copy ctor without assignment operator?) or change the definition.
I'd also like to point that I have asked generally: What is recommended or required policy regarding copy operations, it is copy ctor and assignment operator. So, I have asked for comments and discussions too, as my understanding of this matter may be incorrect.
There's no required policy. As for recommendations, I say strive for regularity, but sometimes it's just not appropriate. -- Dave Abrahams BoostPro Computing http://www.boostpro.com
On 18/10/10 15:27, David Abrahams wrote:
At Mon, 18 Oct 2010 15:02:39 +0100, Mateusz Loskot wrote:
Looking at the current definition [1], I'm not convinced. According to C++(n3092)/12.8/25, assignment operator for this class is defined as deleted due to "non-static data members of const non-class type".
I have no idea what your point is.
My point is that the class definition suggests its copying might not be kosher. It is valid to copy-construct it but not assign.
That's because it has members that are, um, const. Assignment is a mutating operation. Construction is not.
Yes, it's clear to me.
Do you want me to prevent copy construction too, just because copy assignment is not possible? Or do you want me to make the members non-const so that copy assignment is possible?
In my opinion, copy constructor and assignment operator walk together to make a type fully copyable.
The proper term here is "regular," I think (see http://www.stepanovpapers.com/DeSt98.pdf).
Interesting.
But then you need equality and a bunch of other operations to make it meaningful.
I wasn't looking that far.
Thus, IMO, either the definition should change or both copy operations disabled.
I disagree. We have movable but noncopyable types. There's nothing wrong in principle with copyable but not-assignable types.
Yes, I forgot and confused CopyConstructible != Assignable So, xml_writer_settings should be considered as CopyConstructible having implicitly declared copy constructor, but not Assignable having implicitly disabled assignment operator. No explicit motions are required here.
If the former, why? If the latter, I suppose I could do that. I'm not the one who made the members const in the first place.
I would suggest to either disable both copy operations (does it make sense to have copy ctor without assignment operator?) or change the definition.
I'd also like to point that I have asked generally: What is recommended or required policy regarding copy operations, it is copy ctor and assignment operator. So, I have asked for comments and discussions too, as my understanding of this matter may be incorrect.
There's no required policy. As for recommendations, I say strive for regularity, but sometimes it's just not appropriate.
Understood. I'm still digesting Stepanov's elements of programming and finding their applications in practice. Thanks David! Sebastian, my apologies for wasting your time. Best regards, -- Mateusz Loskot, http://mateusz.loskot.net Charter Member of OSGeo, http://osgeo.org Member of ACCU, http://accu.org
participants (4)
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David Abrahams -
Mateusz Loskot -
mloskot
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Sebastian Redl