Re: Generic C++ templates/library for FreeBSD base

On 14 Mar 2025, at 14:36, John Baldwin <jhb@FreeBSD.org> wrote:
> 
> On 3/14/25 00:19, Gleb Popov wrote:
>> On Thu, Mar 13, 2025 at 11:53 PM John Baldwin <jhb@freebsd.org> wrote:
>>> 
>>> One is a stringf() function that accepts printf() style format string and
>>> arguments and returns a std::string.  I know C++23 adds <format>, but we
>>> can't assume that yet, and this function is probably more useful when
>>> adapting existing C code.  Compared to some other solutions, I chose to
>>> wrap asprintf() and do an extra copy at the end into a std::string rather
>>> than calling vsnprintf() twice.  It seems less ugly than the vsnprintf()
>>> solutions also:
>>> 
>>> https://github.com/freebsd/freebsd-src/commit/01bd3d89ddf9ccbf884e52fe7289e8a9278e2d63
>> I wonder why std::string always copies the data passed into the constructor.
>> It is possible to avoid extra alloc by returning a std::string_view,
>> but this would force the caller into freeing the memory manually.
>> Maybe derive from it and extend the destructor, but this just brings
>> me to the initial question.
> 
> What you would want is something where you could do std::move() of the pointer
> returned by asprintf(), into the std::string object, but that assumes that
> new/delete are always using malloc/free (which is probably true in practice).

The ideal solution would be to define an "inserter" that can be called
from the guts of __vfprintf() (or wherever the "real" implementation of
printf lives), whenever it wants to emit a character into the output
buffer. And there you would simply do string::push_back(), which is
amortized constant time.

-Dimitry