[ruby-core:103369] [Ruby master Feature#17790] Have a way to clear a String without resetting its capacity

Issue #17790 has been updated by Eregon (Benoit Daloze).


byroot (Jean Boussier) wrote in #note-6:
> Maybe `String#capacity` and `String#capacity=` would make sense? But then there's the question of the behavior if you set the capacity to lower than the `size`. Should it truncate? (this could corrupt UTF-8 for instance) or should it raise?

I would say definitely not truncate.
So either raise an error, or do nothing, since the capacity is kind of a hint.

My feeling is handling the capacity in Ruby code feels wrong and like C++ code.
The example snippet above can just allocate a new String per call to `build_next_packet` and I doubt that would affect performance much:
```ruby
10.times do
  buffer = build_next_packet # build_next_packet can use String.new(capacity: 1024), it probably knows better about it anyway
  udp_socket.send(buffer)
end
```
A bit more GC pressure, but I think anyway there would be many other allocations in `build_next_packet` that this wouldn't matter much.

IMHO we should really have a separate Buffer class and String class here.
For instance if the String class uses a rope representation (e.g. on TruffleRuby) a capacity doesn't make much sense.

----------------------------------------
Feature #17790: Have a way to clear a String without resetting its capacity
https://bugs.ruby-lang.org/issues/17790#change-91462

* Author: byroot (Jean Boussier)
* Status: Open
* Priority: Normal
----------------------------------------
In some tight loop it can be useful to re-use a buffer string. For instance:

```ruby
buffer = String.new(encoding: Encoding::BINARY, capacity: 1024)

10.times do
  build_next_packet(buffer)
  udp_socket.send(buffer)
  buffer.clear
end
```

Currently `Array#clear` preserve the Array capacity, but `String#clear` doesn't:

```ruby
>> puts ObjectSpace.dump(Array.new(20).clear)
{"address":"0x7fd3260a1558", "type":"ARRAY", "class":"0x7fd3230972e0", "length":0, "memsize":200, "flags":{"wb_protected":true}}
>> puts ObjectSpace.dump(String.new(encoding: Encoding::BINARY, capacity: 1024).clear)
{"address":"0x7fd322a8a320", "type":"STRING", "class":"0x7fd3230b75b8", "embedded":true, "bytesize":0, "value":"", "memsize":40, "flags":{"wb_protected":true}}
```

It would be useful if `String#clear` wouldn't free allocated memory, but if it's a backward compatibility concern to change it, then maybe another method could make sense?




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