Re: volatile keyword for C++ member functions

James Kanze <>
Fri, 30 Nov 2007 02:55:27 -0800 (PST)
On Nov 30, 7:52 am, Rakesh Kumar <> wrote:

  I am actually trying to get my feet in multi-threaded C++
programming. While I am aware that the C++ standard does not talk
about threads (at least, for now - in C++03) - my question is more
about the language / usage rather than any thread specific question.
Sorry - if posted my mistake.

 I understand that in C++ volatile objects ( those non-primitive type
instances qualified with 'volatile' ) can actually call only those
member functions qualified as volatile .

More precisely: given a volatile qualified lvalue, you can only
call volatile functions on it.

Similar rules affect reference binding and implicit pointer
conversions. A volatile lvalue can only be bound to a reference
to volatile, and a volatile pointer will not convert implicitly
to a non-volatile one.

  But there is also a construct - const_cast< T *> (p) , that can
convert a volatile object pointer to constant pointer that can be used
to invoke non-volatile methods on the class.

The same rules apply as for const. If the actual object is
volatile, and it is accessed through a non-volatile lvalue, the
results are undefined behavior.

Thus, for example:

    Toto volatile t1 ;
    Toto t2 ;

    void f( Toto volatile& t )
        const_cast< Toto& >( t ).someFunction() ;

    // ...
    f( t1 ) ; // causes undefined behavior
    f( t2 ) ; // no problem, according to the standard.

  That being the case - what would be the significance of the
'volatile' keyword qualified for c++ member functions in particular -
if we are able to invoke the non-volatile functions by casting. Am I
missing a design picture here.

What is the signification of const, given that you can invoke
non-const functions by casting? The rules are the same.

More to the point, since you started out by talking about
threading: what is the significance of volatile with regards to
threading? Posix makes no guarantees concerning volatile and
threading; I don't know what Windows "guarantees", but the
current VC++ compiler doesn't generate anything that could
possibly be useful for threading. (Andrei once wrote an article
in which he used volatile to ensure correctly locked accesses,
but his code worked not because it used any of volatiles
semantics, but because it exploited the way volatile works in
the type system.)

James Kanze (GABI Software)
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