Re: Structure mapping using reinterpret_cast.

From:
"Bo Persson" <bop@gmb.dk>
Newsgroups:
comp.lang.c++.moderated
Date:
Tue, 23 Jun 2009 14:06:43 CST
Message-ID:
<7abu30F1ul0udU1@mid.individual.net>
Olivier wrote:

Hi all,

I would like to check if a certain technique can be used or whether
there is any entry in the standard that prohibits it explicitly. If
I recall correctly, the standard guarantees that two structures
containing the same starting data members can be mapped on one
another and these common members can be access without yielding
undefined behaviour. That is, given :

struct A { int a; int b; };
struct B { int c; };

the following code is guaranteed to work :

    A a;
    B &b = *reinterpret_cast<B *>(&a);

    b.c = 12;
    assert(a.a == b.c);

And this assert should never fail.


I am not sure. The result of a reinterpret_cast is at best
implementation defined, and at worst undefined.

 From what I can see, the "common initial sequence" rule only applies
to the case where A and B are members of the same union.

In pratice it probably works most of the time, even if they are not.

By extension, if that is true, the following code should be
guaranteed to work :

    #include <iostream>

    // Base class containing the data.
    class A
    {
    public:
        A( int val = 0 ) : val_(val) { }

    protected:
        int get_value( ) const
        { return val_; }

    private:
         int val_;
    };

    // Interface class that uses A's data.
    class B : public A
    {
    public:
        int function( ) const
        { return get_value() / 2; }
    };

    // Another interface class that uses A's data.
    class C : public A
    {
    public:
        int function( ) const
        { return get_value() * 2; }
    };

    int main( )
    {
        A *a = new A(12);

        B *b = reinterpret_cast<B *>(a);
        std::cout << b->function() << "\n";

        C *c = reinterpret_cast<C *>(a);
        std::cout << c->function() << "\n";

        delete a;
    }

If this isn't the case, could anyone quote the entry in the standard
that states this code yields undefined behavior ?


The first section of chapter 10 says that a pointer to a derived class
can be implicitly (no cast needed) converted to a pointer to base. It
doesn't say that is should work the other way round.

Rather, paragraphs 3 & 5 say that they are not required to be layout
compatible. (Even though a Note is not normative, it surely shows the
intent of the standard).

Bo Persson

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