And what, if anything, can I do to avoid it?

	string[] arr = ["abcd", "efgh", "ijkl"];
	char[4096] buf;
	char[] res;
	writeln(GC.stats);
	res = sformat(buf, "%s", arr);
	assert(res.ptr == buf.ptr);
	writeln(res);
	writeln(GC.stats);
	res = sformat(buf, "%s", arr);
	assert(res.ptr == buf.ptr);
	writeln(res);
	writeln(GC.stats);

	// Results:
	Stats(64, 1048512, 80)
	["abcd", "efgh", "ijkl"]
	Stats(272, 1048304, 272)
	["abcd", "efgh", "ijkl"]
	Stats(464, 1048112, 464)

I get similar behavior trying to use formattedWrite with an appender:

	string[] arr = ["abcd", "efgh", "ijkl"];
	auto formatBuffer = appender!(char[])();
	formatBuffer.reserve(4096);
	formatBuffer.clear();
	writeln(GC.stats);
	formatBuffer.formattedWrite!"%s"(arr);
	writeln(formatBuffer.data);
	writeln(GC.stats);
	formatBuffer.clear();
	formatBuffer.formattedWrite!"%s"(arr);
	writeln(formatBuffer.data);
	writeln(GC.stats);

	// Results:
	Stats(12288, 5230592, 4208)
	["abcd", "efgh", "ijkl"]
	Stats(12432, 5230448, 4352)
	["abcd", "efgh", "ijkl"]
	Stats(12576, 5230304, 4496)

Same thing if I use a format string like "%(%s,%)" rather than just "%s".  I'm guessing it's allocating a string first to write the contents of the array and then inserting that string into the buffer I supplied.  Is there no way to have it skip this step and just write each element (plus the joining punctuation, etc) directly into the buffer?

On Saturday, 31 August 2019 at 12:07:56 UTC, cc wrote:
> And what, if anything, can I do to avoid it?

import core.stdc.stdio : printf;

There are no @nogc versions of the Phobos write* and format functions.

On 31.08.19 14:07, cc wrote:
> I'm guessing it's allocating a string first to write the contents of the array and then inserting that string into the buffer I supplied.  Is there no way to have it skip this step and just write each element (plus the joining punctuation, etc) directly into the buffer?

`formatElement` does something like that. It writes the string into a temporary buffer while looking for invalid Unicode. When it finds some, the temporary is discarded and the whole string is formatted differently. When the string is a-ok, the data is copied over to the actual destination.

I'm not sure if that's the best approach, though. The temporary buffer and the string copy are costly.

There is also a closure being allocated for no reason in `sformat` itself. The compiler isn't smart enough to see that it's not really needed.

I've made a pull request to get rid of those allocations:
https://github.com/dlang/phobos/pull/7163

On Saturday, 31 August 2019 at 21:12:32 UTC, ag0aep6g wrote:
> I've made a pull request to get rid of those allocations:
> https://github.com/dlang/phobos/pull/7163

Merged.

On Saturday, 31 August 2019 at 21:12:32 UTC, ag0aep6g wrote:
> I've made a pull request to get rid of those allocations:
> https://github.com/dlang/phobos/pull/7163

Thanks for the responses, very cool seeing these updates happen so fluidly.

On 8/31/19 5:12 PM, ag0aep6g wrote:
> I've made a pull request to get rid of those allocations:
> https://github.com/dlang/phobos/pull/7163

Wonderful!
For my own learning, why was a unittest to ensure no GC added to sformat instead of a @nogc annotation?

On 03.09.19 16:03, James Blachly wrote:
> For my own learning, why was a unittest to ensure no GC added to sformat instead of a @nogc annotation?

`sformat` uses the GC less now, but it's not @nogc. It can still throw GC-allocated exceptions, e.g. when the arguments don't match the format string.