Re: C++ Threads, what's the status quo?
Le Chaud Lapin skrev:
Jeff Koftinoff wrote:
Here we go again. :)
[snip]
There are two different points of views coming from two different
groups of programmers:
Group 1 says, "f takes a const & argument, and therefore does not
modify it, so technically, it is const, and therefore it should be
thread safe for read operations"
Group 2 says, "Not only do we think that it is presumptive to imply
that C++ should 'honor' 'read-only' access to global variables, we do
not recognize this point of view."
Note that I could take your example, make a large const global object
with 100 data members, all declared mutable, invoke some operations on
it from two threads and say, "Mon Dieu..!!!" my const global variable
with all those mutable members did not withstand simultaneous access by
multiple threads!!!
You've got that part completely wrong. Noone questions the fact that
objects may have mutable members. On the contrary, this is one of the
things the new standard should sort out. In other words: threading
guarantees should be given in the standard regarding e.g. access to
std::string and other containers. There are different options - one is
that only one thread must use a given container-type at a time. Another
is that only one thread may use a given container at a time, and the
last (and what is no doubt going to be the standard) is that a
container may be read simuiltaneously by any number of threads - this
last option is what is often called "as safe as an int".
In another thread you argue that code motion (moving an assignment
outside of a protective region) is simply a question about
optimisation. That statement is meaningless as the standard says
nothing at all about optimisation. Once again, there is a need to
specify what is allowed and what is not. You can't assume e.g. that if
you just disable optimisations for your code then it will perform
correctly when running on a multicore cpu. Such an approach simply is
not good enough for many of us.
/Peter
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