FTL: Introduce ftl::Function<F,N> et al.

ftl::Function<F, N> is a container for function object, and can mostly
be used in place of std::function<F>.

Unlike std::function<F>, a ftl::Function<F, N>:

  * Uses a static amount of memory (controlled by N), and never any
    dynamic allocation.
  * Satisfies the std::is_trivially_copyable<> trait.
  * Satisfies the std::is_trivially_destructible<> trait.

However to satisfy those constraints, the contained function object must
also satisfy those constraints, meaning certain types (like
std::unique_ptr's) cannot be part of the contained function object type.

The size of a ftl::Function<F, N> is guaranteed to be:

    sizeof(std::intptr_t) * (N + 2)

If not specified, N defaults to zero, which is big enough to store a lambda
that captures a single pointer (such as "this" for forwarding to a
member function.

By comparison, sizeof(std::function) == sizeof(std::intptr_t) * 4, at
least with on x86-64 with clang 15.

Compile time checks are performed that the constraints are all satisfied,
and that the value of N is large enough to contain the desired function
object type.

ftl::make_function is a helper function to construct a ftl::Function,
and will deduce the template type arguments. In addition to constructing
a ftl::Function for a function object, ftl::make_function has overloads
for creating a ftl::Function which will invoke a member function or a free
(non-member) function.

ftl::no_op is a helper value to construct a ftl::Function<F, N> that
does nothing, except default construct a return value, if one is needed.

A unit test is also included to demonstrate and verify the
implementation, including asserting that function objects which don't
meet the requirements cannot be used. The test also asserts some
non-obvious corner cases for handling argument and return value
conversions to match how std::function behaves.

Bug: 279581095
Test: atest ftl_test
Change-Id: I268facb106a248d0766e931595291036bc606fb7

1 files changed, 1 insertions, 0 deletions

diff --git a/libs/ftl/Android.bp b/libs/ftl/Android.bp
index ea1b5e4998..918680d6a7 100644
--- a/libs/ftl/Android.bp
+++ b/libs/ftl/Android.bp
@@ -17,6 +17,7 @@ cc_test {
         "enum_test.cpp",
         "fake_guard_test.cpp",
         "flags_test.cpp",
+        "function_test.cpp",
         "future_test.cpp",
         "match_test.cpp",
         "mixins_test.cpp",