[Bug c++/61655] New: Copy constructor not called on CRTP schema

[Bug c++/61655] New: Copy constructor not called on CRTP schema

[fabien.picarougne@univ-nantes.fr]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61655

            Bug ID: 61655
           Summary: Copy constructor not called on CRTP schema
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: critical
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: fabien.picarougne@univ-nantes.fr

Created attachment 33033
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=33033&action=edit
Illustration of the problem

When using a CRTP pattern on a class and returning an object of this class in a
function, the copy constructor is not called.

See test attached file that illustrate the problem. Output on gcc 4.9.0 is
CBase::CBase()
CDerived::CDerived()
CBase::CBase()
CDerived::CDerived()

before test:
operator*
CBase::CBase()
CDerived::CDerived()
end operator*
CBase::Display()
after test:

CDerived::~CDerived()
CBase::~CBase()
CDerived::~CDerived()
CBase::~CBase()
CDerived::~CDerived()
CBase::~CBase()

but the expected outpup is
CBase::CBase()
CDerived::CDerived()
CBase::CBase()
CDerived::CDerived()

before test:
operator*
CBase::CBase()
CDerived::CDerived()
end operator*
CBase::CBase(const CBase &obj)
CDerived::CDerived(const CDerived &obj)
CDerived::~CDerived()
CBase::~CBase()
CBase::Display()
after test:

CDerived::~CDerived()
CBase::~CBase()
CDerived::~CDerived()
CBase::~CBase()
CDerived::~CDerived()
CBase::~CBase()

This code works fine on MSVC2013 C++ compiler

[Bug c++/61655] Copy constructor not called on CRTP schema

[glisse at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61655

--- Comment #1 from Marc Glisse <glisse at gcc dot gnu.org> ---
Try -fno-elide-constructors. Why would you want a copy? Is it because of the
parentheses in the return statement that you expect elision not to be
performed?

[Bug c++/61655] Copy constructor not called on CRTP schema

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61655

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |INVALID

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
The copy constructor does not need to be called if there is no copying going
on. There is two optimizations going on here. The first is the named return
value optimization. The second is related to that in the intilization of the
new variable is handled inside the function itself.

[Bug c++/61655] Copy constructor not called on CRTP schema

[fabien.picarougne@univ-nantes.fr]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61655

--- Comment #3 from Fabien Picarougne <fabien.picarougne@univ-nantes.fr> ---
This code is minimal and is just here to illustrate the problem, I use a more
complex one to handle generic matrix.
But The copy constructor here is not neutral, there is an output on stdout. So
I think, the optimization should not take place here, isn't it?

[Bug c++/61655] Copy constructor not called on CRTP schema

[glisse at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61655

--- Comment #4 from Marc Glisse <glisse at gcc dot gnu.org> ---
Look up "copy elision" (the wikipedia article will do).