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Subject: Re: [boost] Variadic append for std::string
From: Christof Donat (cd_at_[hidden])
Date: 2017-01-23 11:32:17


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.

2. concat() in calls to format():

format("%|1$40t|%2%", concat(first_name, " ", last_name),

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():

      my_files | transformed([](const std::filesystem::path& f) -> auto {
                                 return concat(f.filename, ": ",

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?


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