Re: [boost] Plea for addition of stream_cast

From: Kevlin Henney (kevlin_at_[hidden])
Date: 2000-06-29 05:49:58

In message <005d01bfe124$c319bf60$3e116e97_at_[hidden]>, Ed Brey
<brey_at_[hidden]> writes
>From: "Kevlin Henney" <kevlin_at_[hidden]>
>>
>> (3) On a point of implementation rather than interface, I was wondering
>> about reducing the cost of stringstream creation in single-threaded
>> apps.
>[...]
>>                 static std::stringstream stream;
>
>This is OK only if we make a rule saying that iostream overload
>implementations for user-defined types can't use stream_cast.  For example,
>say you have a Point class and an Area class, each of which store two
>integers and read and write them to streams of the form "123,456", but are
>otherwise unrelated.  They could be converted with stream_cast.  If either
>of them, for whatever reason, would happen to use stream_cast in their
>streaming implementations, you'd run into a reentrancy problem.

Hmm, that might too restrictive, and could be an interesting source of
bugs. Given that, I will leave it with the overhead in the name of
convenience and correctness.

>On a separate sub-thread, Jeffrey Paquette wrote:
>
>> Why not go with existing practice and write:
>>         a = interpret_cast<A>(b, std::nothrow);
>> for the non-throwing version?
>
>The nothrow version has been suggested before (several times, I think).  A
>problem is that there is no way to know whether the conversion succeeded or
>failed.  Unlike with a pointer to a new memory allocation, there is no
>special value for a converted type to indicate that the conversion failed.
>The form of the conversion/cast that returns a bool addresses this problem.

I would contend that this leads to three ways of using the cast rather
than two:

(1) Use it so that it throws on failure.
(2) Use it so that it does nothing on failure and defaults the value.
(3) Use it so that it returns a status on failure.

I personally would use (1) if I wanted to know about failure and (2) if
I didn't, which makes (3) seem a little like gold plating.
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