May volatile be in user defined types to help writing thread-safe code

Question

I know, it has been made quite clear in a couple of questions/answers before, that volatile is related to the visible state of the c++ memory model and not to m

Answer 1

This catches some kinds of thread-unsafe code (concurrent access), but misses others (deadlocks due to locking inversion). Neither is especially easy to test for, so it's a modest partial win. In practice, remembering to enforce a constraint that a particular private member is accessed only under some specified lock, hasn't been a big problem for me.

Two answers to this question have demonstrated that you're correct to say that confusion is a significant disadvantage - maintainers may have been so strongly conditioned to understand that volatile's memory-access semantics have nothing to do with thread-safety, that they will not even read the rest of the code/article before declaring it incorrect.

I think the other big disadvantage, outlined by Alexandrescu in the article, is that it doesn't work with non-class types. This might be a difficult restriction to remember. If you think that marking your data members volatile stops you using them without locking, and then expect the compiler to tell you when to lock, then you might accidentally apply that to an int, or to a member of template-parameter-dependent type. The resulting incorrect code will compile fine, but you may have stopped examining your code for errors of this kind. Imagine the errors which would occur, especially in template code, if it was possible to assign to a const int, but programmers nevertheless expected the compiler would check const-correctness for them...

I think the risk that the data member's type actually has any volatile member functions should be noted and then discounted, although it might bite somebody someday.

I wonder if there's anything to be said for compilers providing additional const-style type modifiers via attributes. Stroustrup says, "The recommendation is to use attributes to only control things that do not affect the meaning of a program but might help detect errors". If you could replace all mentions of volatile in the code with [[__typemodifier(needslocking)]] then I think it would be better. It would then be impossible to use the object without a const_cast, and hopefully you wouldn't write a const_cast without thinking about what it is you're discarding.