Re: [boost] [review] string convert

Subject: Re: [boost] [review] string convert
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2011-05-05 12:57:20

Vicente BOTET wrote:
> De : "Vladimir Batov"
> > "Stewart, Robert"  wrote
>
> > Maybe to avoid the temptation of pairing up we could simplify
> > (at least from the user perspectives) down to:
> >
> > 1. T convert_cast(S)
> > 2. T convert_cast(S, T, throw_t =nothrow)
>
> I don't see why (2) the user could want to throw when is giving
> a fallback. So the throw_t =nothrow is not needed.

That's the non-DefaultConstructible, no-sentinel use case:

struct example
{
   example(int);
};

To get an exception on conversion failure, one writes:

example const e(convert_cast<example>("1")); // exception

However, without the default_value CP, that fails because example is non-DefaultConstructible.

One can avoid the exception by providing a fallback value:

example const e(convert_cast<example>("1", 3)); // no exception

(I'm assuming the fallback value can be of type U implicitly convertible to T.)

In that case, e == example(3).

But if one wants an exception to indicate conversion failure, an initial value is needed and the desire for an exception must be noted:

example const e(convert_cast<example>("1", 3, throw_)); // exception

Obviously, with your default_value CP, the second argument isn't needed for the last use case:

template <>
default_value<example>
{
   static example
   apply()
   {
      return example(3);
   }
};

example const e(convert_cast<example>("1")); // exception

HTH,

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



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