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

Subject: Re: [boost] Variadic append for std::string
From: Christof Donat (cd_at_[hidden])
Date: 2017-01-03 09:16:27

Hin

Am 03.01.2017 14:20, schrieb Olaf van der Spek:
> On Tue, Jan 3, 2017 at 2:19 PM, Christof Donat <cd_at_[hidden]> wrote:
>> Am 01.01.2017 00:21, schrieb Andrey Semashev:
>>> 
>>>   throw std::runtime_error(format(std::string()) << "Error " << 47);
>> 
>> 
>> How would that differ from
>> 
>>   throw std::runtime_error((std::ostringstream{} << "Error " << 
>> 47).str());
> 
> Simpler syntax, better performance

I see the chances for better performance, but for the syntax I don't 
really see
any remarkable improvements.

If performance matters, I'd try with boost::spirit::karma. The syntax 
will be
less concise, but I am not aware of a faster generic solution.

   auto message = std::string{10}; // <- preallocate enough memory for 
the message
   if( !karma::generate(std::begin(message), ascii::space, "Error " << 
karma::uint_, 47) ) {
       // formating the error message failed. throw something else.
   }
   // since this is the exit of the function, the compiler might apply 
copy elision.
   throw std::runtime_error(message);

If you have multiple places like that in your code, I guess, you'd like 
to wrap it
into a generic function and you have a similar API to the "append()" 
proposal. Now
I see, how it might be useful, thanks. I think, append() should rely on 
karma
generators then, instead of yet another int to string implementation, 
because we
only already have five dozens.

Christof