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digitalmars.D.bugs - [Issue 4500] New: scoped moves class after calling the constructor

reply d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=4500

           Summary: scoped moves class after calling the constructor
           Product: D
           Version: D2
          Platform: Other
        OS/Version: Linux
            Status: NEW
          Severity: normal
          Priority: P2
         Component: Phobos
        AssignedTo: andrei metalanguage.com
        ReportedBy: kamm-removethis incasoftware.de



23:59:57 PDT ---
As far as I remember a class is not supposed to move.

class A {
  this() { a = this; }
  this(int i) { a = this; }
  A a;
  void check() { writeln(this is a); }
}

void main()
{
    auto a1 = scoped!A;
    a1.check(); // fails

    auto a2 = scoped!A(1);
    a2.check(); // fails

    a1.a = a1;
    a1.check(); // ok now
}

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Jul 23 2010
next sibling parent d-bugmail puremagic.com writes:
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Andrei Alexandrescu <andrei metalanguage.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED



09:48:34 PDT ---
Thanks for the catch! This is a serious problem indeed.

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Jul 25 2010
prev sibling next sibling parent d-bugmail puremagic.com writes:
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Max Samukha <samukha voliacable.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |samukha voliacable.com



PDT ---
I didn't know D structs were allowed to be moved without calling the copy
constructor. Is it really the case?

FWIW, modern C++ implementations would optimize the copy out. For example, the
following C++ test passes with a recent GNU C++:

template <typename T>
class Scoped
{
    char data[sizeof(T)];

public:
    T* payload() { return reinterpret_cast<T*>(data); }

    Scoped(int i) { new(payload()) T(i); }

    ~Scoped() { payload()->~T(); }
};

template <typename T>
Scoped<T> scoped(int i)
{
    Scoped<T> s(i);
    return s;
}

class A {
public:
    A() { a = this; }
    A(int i) { a = this; }
    A *a;
    void check() { std::cout << (this == a ? "true" : "false") << std::endl; }
    ~A() { std::cout << "~A dtor" << std::endl; }
};

int main(int argc, char *argv[])
{
    Scoped<A> a2 = scoped<A>(1);
    a2.payload()->check(); // ok
}

Anyway, it is not reasonable to base a fundamental feature like 'scoped' on RVO
without having the latter in the language specification.

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Jul 26 2010
prev sibling next sibling parent d-bugmail puremagic.com writes:
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03:19:53 PDT ---
The language specifies that returning a stack-allocated object by value from a
function does not invoke the constructor. Currently the compiler is not up to
the specification.

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PDT ---
But does it specify that the object should not be moved? Currently, the struct
is constructed on the callee's frame and then blitted (moved) to the caller's
frame, though the compiler can optimize the blit out (as GNU C++ does). If this
kind of RVO is not specified, obviously 'scoped' cannot rely on it and will
remain broken even if dmd will eventually be able to apply the optimization.

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Jul 26 2010
prev sibling next sibling parent d-bugmail puremagic.com writes:
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Created an attachment (id=933)
Fixed scoped

I tried to fix this issue.
It succeeded, but resulting code includes ugly hack.

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Mar 23 2011
prev sibling next sibling parent d-bugmail puremagic.com writes:
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Kenji Hara <k.hara.pg gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |k.hara.pg gmail.com





 Created an attachment (id=933) [details]
 Fixed scoped
 
 I tried to fix this issue.
 It succeeded, but resulting code includes ugly hack.


I worked on 64bit Windows 7.

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Mar 23 2011
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I posted issue 5777 and attached experimental dmd patch.

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Mar 26 2011
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Kenji Hara <k.hara.pg gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |patch



https://github.com/D-Programming-Language/phobos/pull/148

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Jul 17 2011
prev sibling parent d-bugmail puremagic.com writes:
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Walter Bright <bugzilla digitalmars.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
                 CC|                            |bugzilla digitalmars.com
         Resolution|                            |FIXED


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Sep 07 2011