From: Simonson, Lucanus J
Fernando Cacciola wrote:
Pendantically speaking, there is no such thing as a "safe reinterpret_cast". While this is somewhat permmited in production code where the typical deadline is yesterday, here in boost you need a definite need for using
that. Anyway, can you explain in some detail how doesn't your adaptation pattern avoid copying?
The ability to modify the contents of an object in place without copying was the original intent of the entire exercise in generic programming.
If you have two classes
class A {...}; class B : public A {...};
or by composition
class A {...}; class B { A a; };
You know it is safe to reinterpret_cast A to B and B to A (because they are the same data, and differ in behavior only.)
No. 5.2.10 [expr.reinterpret.cast] p7 of the standard says "A pointer to an object can be explicitly converted to a pointer to an object of different type ... the result of such a pointer conversion is unspecified." Notes: - I have snipped the bit where it says you can sometimes cast to a different pointer and back. - Casts between references are defined in terms of casts between pointers. If A is POD, 9.2 [class.mem] p17 guarantees that you can reinterpret_cast B* to A* (but /not/ visa-versa). If B were extended in such a way as to be non-POD, then you lose even that guarantee. There is /no/ guarantee that you can reinterpret_cast between pointers to base and derived class. (static/dynamic cast is different. But even then you can't cast an A* to B* if the A* doesn't point at the A base object of a B) It may work as you expect on the platforms you have used with the types you have used - but it is not guaranteed by the standard.
If this use of reinterpret_cast to change the behavior of an object is unacceptable for a boost submission that pretty much closes the whole case, since the design pattern depends upon it in a very necessary way.
I don't think it is /absolutely/ unacceptable, but I think you would be fighting an uphill battle. I also think you would need to show that you understood the limitations (both according to the standard, and in practise). -- Martin Bonner Senior Software Engineer/Team Leader PI SHURLOK LTD Telephone: +44 1223 441434 / 203894 (direct) Fax: +44 1223 203999 Email: martin.bonner@pi-shurlok.com www.pi-shurlok.com disclaimer