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Subject: Re: [boost] [review] string convert
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2011-05-04 13:24:53

Gordon Woodhull wrote:
> On May 4, 2011, at 9:57 AM, Matt Chambers wrote:
> > I definitely could do without the tags to pick an overload.
> > I'd much rather have the try_ prefix. So:
> >
> > int i = convert_cast<int>(s); // might throw
> > int i = convert_cast<int>(s, 17); // won't throw
> > optional<int> i = try_convert_cast<int>(s); // won't throw;
> Yep, not bad.
> > convert<int>::result i = try_convert_cast<int>(s, 17); //
> > won't throw; specialized version of optional is necessary to
> > detect failure
> No, no special optional thing is needed. Please reread my
> 8:35am reply to Vladimir.
> Sorry, it's a pair<bool, int>. Why do people hate pair?

I think I covered that well in reply to Vicente.

> There's a boolean and a value, why is that not a pair? The
> returned value is not optional in this case.

I had forgotten the case of needing an initial value because T() isn't available (default constructor or zero-initialization, depending upon type), but also not wanting an exception. That's what this one is supposed to handle, but I think it can be handled better another way.

   optional<int> o(try_convert_cast<int>(s, 17));

If the conversion succeeds, the optional is set and one extracts the value. If the conversion fails, the optional isn't set. 17 represents a value supplied for a non-DefaultConstructible UDT to create the function local variable needed to store the return value.


1. T convert_cast<T, S>(S)
2. T convert_cast<T, S>(S, T)
3. T convert_cast<T, S>(S, T, nothrow_t)
4. optional<T> try_convert_cast<T, S>(S)
5. optional<T> try_convert_cast<T, S>(S, T)

In 1, the result valid unless there's an exception. It doesn't work for non-DefaultConstructible UDTs.

In 2, the result is always valid since it never throws. It works for non-DefaultConstructible UDTs, but not for types without a suitable fallback value.

In 3, the result is valid unless there's an exception. 2 and 3 could be merged, but keeping them distinct is probably more efficient and should be easier to document.

In 4 and 5, the optional is set iff the conversion succeeds.

4 is useful for built-in types and DefaultConstructible UDTs.

5 is useful for all types.

Arguably, "try_convert_cast" wants a better name.

What's missing, then, is getting an exception when a conversion to a non-DefaultConstructible UDT or a type with no fallback value fails. That is, a variant of 2 that throws on conversion failure:

> > // I'd get rid of these:
> > //int i = convert_cast<int>(s, 17,
> throw_even_though_i_specified_a_failback_); // what's the
> purpose of this overload?
> This is the nondefaultable case where you still want it to
> throw.

Right. My case 3.

> > //optional<int> i = try_convert_cast<int>(s, 17); // this
> doesn't allow checking for failure, so there's not much point
> Again, it's for nondefaultable types. You need to supply a
> value, but you wouldn't actually use it if conversion failed.

Right. My case 5.

> > //pair<bool,int> i = convert_cast<int>(s, 17,
> fallback_and_report_success_); // it's burning my eyes
> Hahaha. Again, this is replaced by your
> try_convert_cast<int>(s, 17) and you could call it something
> else but it's really a pair.

Not needed.

Rob Stewart robert.stewart_at_[hidden]
Software Engineer using std::disclaimer;
Dev Tools & Components
Susquehanna International Group, LLP

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