Re: [boost] Looking for thoughts on a new smart pointer: shared_ptr_nonnull

Subject: Re: [boost] Looking for thoughts on a new smart pointer: shared_ptr_nonnull
From: Ben Pope (benpope81_at_[hidden])
Date: 2013-10-07 21:46:31

On 08/10/13 05:56, Gavin Lambert wrote:
> On 10/7/2013 11:41 PM, Quoth Ben Pope:
>> The point is still that validating at runtime by throwing an exception
>> is not good enough.  If the programmer wants to use a non-null smart
>> pointer, he wants to only ever pass in a non-null pointer.  Detecting a
>> null-pointer inside the construction of the smart pointer is already too
>> late; the programmer thought it was non-null, so in essence, he has no
>> idea what the code is doing... Whatever it *is* doing may not be
>> undefined, but it is most certainly wrong.  How do you recover at
>> runtime from a program that does not do what you thought it did?
> 
> It depends on the program, which is why recovery has to be in the hands
> of the program.

It's more general for a non-null pointer to describe the situation of
non-null, than it is for the smart pointer to describe that situation
and also do validation and potentially throw an arbitrary exception.

> Maybe the pointer was null because an earlier operation transiently
> failed, in which case it could be retried, or the operation abandoned
> and the next operation attempted (think of a web server or similar --
> just because processing of one request got it into a bad state doesn't
> mean that it can't successfully process other requests, since each
> request operates on independent state).

If a previous result can fail, that result should be checked during the
normal flow of the code.  The programmer can then decide to branch or
throw an exception, or abort or whatever he wants to do.

You asked for opinions on non-null shared_ptr; it will be more generic,
more composable and more useful if it doesn't also validate its argument
and throw.

The throwing behaviour can be added with a factory or a new type that
composes this type, but it can't be removed.

Ben