Re: List container passed as a reference

From: SaltPeter (SaltPeter_at_Jupiter.sys)
Date: 07/27/04

  • Next message: Alwyn: "Re: List container passed as a reference" 
    Date: Tue, 27 Jul 2004 07:17:51 -0400

    "James Dennett" <jdennett@acm.org> wrote in message
    news:HQlNc.2891$mg6.1310@fed1read02...
    > SaltPeter wrote:
    > > "James Dennett" <jdennett@acm.org> wrote in message
    > > news:s31Nc.222$mg6.171@fed1read02...
    > >
    > >>SaltPeter wrote:
    > >>
    > >>>"Alwyn" <dt015a1979@mac.com.invalid> wrote in message
    > >>>news:250720040838569316%dt015a1979@mac.com.invalid...
    > >>>
    > >>>
    > >>>>In article <yEwMc.19430$Fj.936881@news20.bellglobal.com>, SaltPeter
    > >>>><SaltPeter@Jupiter.sys> wrote:
    > >>>>
    > >>>>
    > >>>>
    > >>>>>Thats without mentioning the fact that a constructor is called during
    a
    > >>>>>pass_by_value and the variable within the function is completely
    > >>>
    > >>>unrelated
    > >>>
    > >>>
    > >>>>>with the passed entity(other than common values). Example:
    > >>>>
    > >>>>That's not fair, my Point did not have a constructor!
    > >>>
    > >>>
    > >>>Point can't exist without a constructor, whether compiler generated or
    > >
    > > not.
    > >
    > >>>Thats true for any abstract type. The exact same applies to the
    > >
    > > destructor.
    > >
    > >>>Your suggestion is that an object can be generated without cstors. This
    > >
    > > is
    > >
    > >>>not possible in C++. An instance implies a constructor in C++. The same
    > >
    > > goes
    > >
    > >>>with copy cstors, if you don't supply one, the compiler must supply a
    > >>>default constructor.
    > >>
    > >>struct Point { int x, y; };
    > >>
    > >>int main() {
    > >> Point p = {1, 2};
    > >>}
    > >>
    > >>Which constructor do you think is being used here? I see
    > >>an instance p of class Point, but the only constructors
    > >>Point has are a default and a copy constructor, and neither
    > >>of those is used when p is initialized as above.
    > >>
    > >>-- James
    > >
    > >
    > > Thats a misguided statement.
    >
    > I think not.
    >
    > > Point p;
    > > is itself a call to a constructor,
    >
    > Well, it causes an object of type Point to be created,
    > which uses a default constructor, but close enough.

    yes, my bad, it invokes a constructor, whether that be a default cstor,
    alternate cstor or copy cstor is irrelevent. Its still a cstor.

    >
    > > Point p = {1, 2};
    > > is a cstor with an assignment operator that initializes publicly
    accessible
    > > members x and y.
    >
    > No, it is not. It is initialization, not assignment. No
    > assignment is involved. To illustrate that, for example,
    > make the members const:
    > struct Point { int const x, y; };
    > int main() { Point p = {1, 2}; }

    You can't initialize without

    a) a default cstor which creates an instance with default values (Point p
    could be anything)
    b) an asiignment operator which then initializes p's contents

    Yes, thats initialization. Which would be impossible without a cstor.

    >
    > All quite legal, though assignment to a Point is not
    > permitted.
    >
    > > Which happen to be accessible directly since its a struct
    > > not a class.
    >
    > Point is a class; the fact that it was defined using the
    > struct keyword is irrelevant.

    No, you stated that Point p is neither a class nor a struct, since in your
    opinion a cstor needs not be invoked to generate an instance (initialized or
    not). Point is an abstract type, whether class or struct is irrelevent.
    Thats not the issue here.

    >
    > struct Point { int x, y; };
    >
    > is exactly equivalent, with a conforming compiler, to
    >
    > class Point { public: int x, y; };

    equivalent also in the fact that a cstor must be called, as i said,
    regardless of which cstor actually gets called.

    >
    > > If Point was a class, this initialization of the Point variable
    > > would fail (private members).
    > >
    > > To prove that a constructor indeed needs to be called, try either of the
    2
    > > versions below:
    > >
    > > class Point { int x, y; };
    > >
    > > int main()
    > > {
    > > Point p = {1, 2}; // error, default cstor can't initialize
    privates.
    > > }
    >
    > That proves no such thing -- you can't prove a falsehood
    > in a consistent logical system. All this shows is that
    > there are limits on which classes can be initialized as
    > aggregates.

    agregates can't initialize a non-existant object.

    >
    > (Incidentally, the conventional abbreviation for "constructor"
    > is "ctor", and similarly "destructor" is abbreviated to "dtor";
    > using the accepted abbreviations will make your posts easier
    > for others to read.)
    >
    > > Note that 2 constructors are called in next example.
    >
    > I'll note no such thing! I believe it's true that the copy
    > constructor should indeed be used for pass by value here,
    > though if modifyPoint were passed a temporary then a
    > compiler could legally elide the copy, so the "copy cstor"
    > comment is somewhat misplaced, and would be more accurate
    > at the call site. (But then again modifyPoint would be
    > better known as "doNothing" :-)

    This proves the fact that 2 entities exist, although the function's Point is
    nothing more than a temporary. If you were to define a d~stor in Point,
    leaving the compiler to generate the cstors, you'ld still see 2 d~stors
    called.

    Explain this:
    #include <iostream>

    struct Point
    {
     int x;
     int y;
     ~Point() { std::cout << "d~stor invoked"; }
    };

    int main()
    {
     Point p = {1,2};

     return 0;
    }

    So, now explain to me why the d~stor is invoked when the above compiles?
    Since you're convinced that initialization doesn't require a cstor. Frankly,
    that baffles me.

    >
    > > Thats why the value in
    > > main doesn't get modified.
    > >
    > > #include <iostream>
    > >
    > > struct Point
    > > {
    > > int x;
    > > int y;
    > > };
    > >
    > > void modifyPoint(Point Pt) // copy cstor
    > > {
    > > Pt.x += 10;
    > > Pt.y += 10;
    > > }
    > >
    > > int main()
    > > {
    > > Point p = {1,2}; // cstor + assignment operator
    >
    > No; this is initialization, but it does not use constructors
    > or assignment operators. (Quick test: if you think it uses
    > an assignment operator, from which expression (of which type)
    > does it assign?

    the very same used to write:
    int x = 5;
    or int x[] = {1,2};

    Initialization it is. You can't initialize a non-existant entity. Hence, you
    can't initialize anything without a cstor.

    >
    > >
    > > modifyPoint(p);
    >
    > The copy constructor will be used to pass p to the
    > function.
    >
    > -- James

  • Next message: Alwyn: "Re: List container passed as a reference"

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