QFlatMap: add remove_if

The existing API of QFlatMap did not allow efficient removal of
elements:

- std::remove_if does not apply, because it works by moving elements
  back in the range onto those that need to be removed, which doesn't
  work in flat_map's case, because, like for all associative
  containers, the key in value_type is const.

- The node-based erase-loop (over it = cond ? c.erase(it) :
  std::next(it)) works, but, unlike in traditional associative
  containers, is quadratic, because flat_map::erase is a linear
  operation.

According to Stepanov's principle of Efficient Computational Basis
(Elements of Programming, Section 1.4), we're therefore missing API.

Add it.

I couldn't make up my mind about the calling convention for the
predicate and, despite having authored a merged paper about erase_if,
can never remember what the predicate is supposed to take, so be fancy
and accept all: (*it), (it.key(), it.value()), (it.key()). This means
that unary predicates can either not be generic or must be properly
constrained to distinguish between pair<const K, V> and K, but that's
not necessarily a bad thing.

There's no reason to supply a Qt-ified removeIf on top of the standard
name, because this is private API and doubling the names would do
nothing except double the testing overhead.

Fixes: QTBUG-100983
Change-Id: I12545058958fc5d620baa770f92193c8de8b2d26
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

0 files changed, 0 insertions, 0 deletions