Re: [boost] Variadic append for std::string

Subject: Re: [boost] Variadic append for std::string
From: Olaf van der Spek (ml_at_[hidden])
Date: 2017-01-23 13:26:37

On Mon, Jan 23, 2017 at 5:32 PM, Christof Donat <cd_at_[hidden]> wrote:

> Hi,
>
> Am 23.01.2017 16:32, schrieb Hans Dembinski:
>
>> On 23 Jan 2017, at 11:23, Christof Donat <cd_at_[hidden]> wrote:
>>> auto my_new_str =
>>>    concat("Hello ",
>>>           join(", ",std::begin(my_nums), std::end(my_nums)),
>>>           format(" the file %1% contains %2% bytes", filename,
>>> filesize)).str();
>>>
>>
>> It makes sense to me for "join" to return a string factory, because it
>> is likely to be nested in "concat". But I don't see the practical case
>> of nested "concat" calls, at least it is not going to be a common
>> pattern in the need of optimising.
>>
>
> There is several usecases:
>
> 1. scope for formatting tags:
>
> concat(format::hex<int>, 42, " is hex for ", concat(42)).str();
>
> Here the inner concat will convert the 42 to its decimal representation,
> while the outer one converts the first 42 to its hex representation.
>

Wouldn't concat(hex(42), " is hex for", 42) make more sense?

>
> 2. concat() in calls to format():
>
> format("%|1$40t|%2%", concat(first_name, " ", last_name),
> phone_number).str();
>
> Why not fold the name concat into the format string?

> format().str() will allocate the buffer and ask the concat string factory
> to write into it.
>
> 3. results from concat() in a boost::range that is passed to join():
>
> join(separator("\n"),
>      my_files | transformed([](const std::filesystem::path& f) -> auto {
>                                 return concat(f.filename, ": ",
> std::filesystem::file_size(f));
>                             })).str();
>
> join().str() will ask every concat string factory to render directly into
> the common buffer.
>
> If "concat" is the outer layer anyway, I would return a std::string
>> directly for convenience. It is easy to forget the trailing .str() and
>> it does not look elegant.
>>
>
> Of course better proposals are welcome :-) Would you prefer the implicit
> conversion? If so, why?

Implicit is problematic with auto..

-- 
Olaf