[boost] Re: Exception safety guarantie

From: David B. Held (dheld_at_[hidden])
Date: 2004-07-14 20:26:06

David B. Held wrote:
> [...]
>     template< typename ContainerT >
>     inline ContainerT to_lower_copy(
>         const ContainerT& Input,
>         const std::locale& Loc=std::locale() )
>     {
>         ContainerT Output;
>         std::transform(
>             string_algo::begin(Input),
>             string_algo::end(Input),
>             std::back_inserter<ContainerT>( Output ),
>             string_algo::detail::to_lowerF(Loc) );
> 
>         return Output;
>     }
> [...]
>  So this function can give the strong guarantee if it has the signature
> 
>    { pure, strong, pure* }
> [...]

And here is where my analysis goes horribly wrong.  First, this is not 
the signature of a strong function.  This is:

    { pure*, strong!, nothrow* }

But std::back_inserter() almost certainly does not give the nothrow 
guarantee, so something is wrong.  Second, in order for to_lower_copy() 
to give the strong guarantee, ContainerT() must be a pure function 
w.r.t. to_lower_copy().  And it is, as long as it does not modify 
non-local data (global state).  Once we add that condition, we get a 
function that is a sequence of pure operations, which *does* give the 
strong guarantee.  So in the notes, you should really say:

    Notes: If SequenceT(void) does not modify global state, then
    the second variant of this function provides the strong
    exception safety guarantee.

Dave