tree 5d82de7a9029887b42b6e1e310367bdfa228e79a
parent a331a36c92c01ae459158660a4d1bf4c913f0335
author David 'Digit' Turner <digit@google.com> 1648314524 +0100
committer Commit Bot <commit-bot@chromium.org> 1650036963 +0000

Add ImmutableVector and ImmutableVectorView templates.

These templates allow creating and using heap-allocated but
immutable arrays of items. They will be used by future CLs
in a few key classes to reduce GN RAM usage drastically.

Here's how an ImmutableVector compares to an std::vector<>
in terms of memory usage:

- An empty instance is just an 8-byte null pointer, vs 24 bytes
  for an empty vector.

- Its heap block is sized precisely to fit the instance's size,
  while the std::vector<> will generally over-allocate its
  storage space (calling reserve() does not guarantee minimal
  heap usage).

Note that constructing an non-empty ImmutableVector requires a
source container (std::vector<> or otherwise), or a callable
that can produce a fixed number of items for in-place
construction (see source code for details).

An ImmutableVectorView is a non-owning reference to an
ImmutableVectorView, it is copyable and movable since the
vector is immutable.

Bug: None
Change-Id: I2cb669b7b0fab777bc7067ee7529690b300ea735
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/13621
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: David Turner <digit@google.com>