How to ensure thread-safety

[Olly Betts]

On Thu, Feb 08, 2018 at 04:18:12PM +0100, Kim Walisch wrote:
> But it is still not clear to me how to ensure thread-safety when using
> libxapian (C++ API). Usually when doing multi-threading many threads can
> read the same variable concurrently without locking provided none of the
> threads modifies the variable.

That's true for simple types, but breaks down for classes because they
may have mutable members - e.g. for caching values computed lazily:

class FactorialFactory {
  private:
     mutable int r = -1;
     mutable int n;
  public:
     FactorialFactory() {}

     int calc(int v) const {
         if (r < 0 || n != v) {
	     r = n;
	     n = v;
	     for (int i = n - 1; i > 1; --i) {
	         r *= i;
	     }
	 }
	 return r;
     }
};

It's not safe to concurrently call f.calc() from different threads, even
though conceptually calc() is a read-only method.

Cheers,
    Olly