Re: [boost] [variant2] Andrzej's review -- design

Subject: Re: [boost] [variant2] Andrzej's review -- design
From: Peter Dimov (pdimov_at_[hidden])
Date: 2019-04-02 17:53:50

Andrzej Krzemienski wrote:

> I have started a thread in this list a while ago, requesting for an 
> example of code that *correctly handles exceptions* (does not stop stack 
> unwinding at random places), where the programmer would make use of the 
> never-empty guarantee , and chose something else than destroying or 
> resetting the variant. And although I received some generic statements, 
> referring to the purity of the design, strength of the invariants, and the 
> easiness of thinking or "correctness", none of the proponents of the 
> never-empty guarantee gave such an example.

template<class T> class monitor
{
private:

    char const* file_;
    int line_;
    T const& v_;
    T old_;

public:

    explicit monitor( char const* file, int line, T const& v ):
        file_( file ), line_( line ), v_( v ), old_( v ) {}

    ~monitor()
    {
        if( v_ != old_ )
        {
            std::clog << file_ << ":" << line_
                << ": monitored value changed from "
                << old_ << " to " << v_ << std::endl;
        }
    }
};

Or in general, in a destroy-only world, you can never read any values in a 
destructor or in a function called from a catch clause. It might be an 
interesting experiment to make Clang warn on such a use and then try it on 
some real codebases.

These reads of a potentially-destroy-only values are invisible to you 
because we don't live in such a world.