Re: Fun with member-function pointers

From:
Leigh Johnston <leigh@i42.co.uk>
Newsgroups:
comp.lang.c++
Date:
Thu, 25 Aug 2011 14:08:36 +0100
Message-ID:
<T_-dnc5mQYHD1MvTnZ2dnUVZ7sidnZ2d@giganews.com>
On 25/08/2011 13:57, Leigh Johnston wrote:

On 24/08/2011 21:44, Andrey Tarasevich wrote:

On 8/24/2011 1:08 PM, Paul wrote:

Please provide some concrete facts that support your claim and it will
be investigated.


Marcel has already provided a compact example that causes Microsoft
compiler to exhibit incorrect behavior. I'll reproduce an equivalent
example here in more compact form

#include <iostream>

struct tbase {
int x;
tbase() : x(42) {}
};

struct test : tbase {
virtual ~test() {}
void f() { std::cout << "x=" << x << "\n"; }
};

int main() {
void (tbase::*p)() = static_cast<void (tbase::*)()>(&test::f);

test t;
(t.*p)();
}

This code works incorrectly in MSVC++ 2005 and MSVC++ 2010. Function
`test::f` called indirectly by `(t.*p)()` call receives incorrect value
of `this` pointer, which leads to incorrect value of `x` being printed.
This happens regardless of the compiler settings.

The compiler understands in advance that the code will not work. It
issues a warning

warning C4407: cast between different pointer to member representations,
compiler may generate incorrect code

in response to the above `static_cast`. This is, of course, not an
excuse, since the language standard requires this code to work correctly.

MSVC++ compilers have quite a few blatant standard violations. I collect
them in my "notebook" here


You are wrong to say this happens regardless of the compiler settings;
just use the following pragma at the top of your program:

#pragma pointers_to_members( full_generality, multiple_inheritance )

Or alternatively ensure the base class has a virtual destructor if it is
to be derived from.


Presumably Microsoft chose to have a less general *default* pointer to
member representation for performance reasons.

/Leigh

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