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

On 3/14/25 09:50, Dimitry Andric wrote:
> 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.

That you could do with funopen() where the write method called string::push_back().
This is actually how asprintf (and open_memstream) works internally.  Maybe I will
just fix my implementation to do that.

-- 
John Baldwin