Re: Slicing with ?: operator - expected?

From:
Bart van Ingen Schenau <bart@ingen.ddns.info>
Newsgroups:
comp.lang.c++
Date:
Wed, 06 Jul 2011 16:41:19 +0200
Message-ID:
<iv1s6f$pde$1@localhost.localdomain>
Victor Bazarov Wrote:

Hello All,

Here is the code:

    class Foo {
    public:
       virtual void bar() const = 0;
    };

    #include <ostream>
    #include <iostream>

    class Foo1 : public Foo {
       void bar() const { std::cout << "Foo1::bar\n"; }
    };

    class Foo2 : public Foo {
       void bar() const { std::cout << "Foo2::bar\n"; }
    };

    void foo(const Foo* pFoo)
    {
       (pFoo ? *pFoo : Foo1()).bar(); // line 19 ************
    }

    void blah(const Foo& rFoo)
    {
        foo(&rFoo);
    }

    int main()
    {
       blah(Foo2());
    }

Line 19 is the line in question. Could you please interpret it for me?
  It seems to skip the virtual function dispatch, and attempt to call
the pure function. The common type of *pFoo and Foo1() is 'class
Foo', and the compiler seems to resolve the 'bar' statically, without
the use of virtual function mechanism, as if there is an instance of
class Foo /sliced/ from both original objects. Is that supposed to
happen?


Yes, that is supposed to happen.
The conditional operator (?:) is defined to only yield rvalues, and never a
reference.
This means that objects of class-type indeed get sliced by the ?: operator.

Standard chapter and verse would be helpful.


The behaviour of the conditional operator is described in section 5.16
[expr.cond], where especially the last paragraph is of relevance here.

Thanks!

V


Bart v Ingen Schenau

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