Refactor conversion to use extensible trait structs

[bkietz]

Still a little half baked:

• Move all conversion logic to an extensible trait structs
• Generalize container conversion
• Fix conversion from altrep vectors
• Add doc indicating how and when to specialize the trait structs

[bkietz]

Odd: https://github.com/r-lib/cpp11/pull/104/checks?check_run_id=1130882732#step:12:198 what feature macro should I use to disable that test case? Also, why is the install cpp11 and cpp11test step marked successful (the failure is actually raised when trying to run the unbuilt tests)

[jimhester]

You can use #if defined(__APPLE__) && defined(R_VERSION) && R_VERSION >= R_Version(3, 5, 0) to guard R_compact_intrange. ALTREP is only available on R 3.5.0+ and that symbol is hidden on linux and windows, but not on macOS, which is why we guard for apple.

utils::install.packages() the installation function shipped with base R only issues warnings when a package installation fails, not a true error.

d9eea11 should convert those warnings to failures and then fail the build in the cpp11test install step in the future.

[bkietz]

Thanks, I'll rebase

[bkietz]

@jimhester I really expected something here to fail since as_cpp<IntegerContainer>() currently relies on INTEGER(), how is that working with ALTREP?

[jimhester]

The vector is still an altrep when you call INTEGER() on it, but it is then a materialized altrep vector.

The way to test that you are not materializing an alterep vector is to check that R_altrep_data2(x) == R_NilValue

[jimhester]

e.g. after doing this the second test fails, as you were expecting it to.

diff --git a/cpp11test/src/test-as.cpp b/cpp11test/src/test-as.cpp
index 6a3282c..0476ef2 100644
--- a/cpp11test/src/test-as.cpp
+++ b/cpp11test/src/test-as.cpp
@@ -243,9 +243,9 @@ context("as_cpp-C++") {
   test_that("as_cpp<std::vector>(ALTREP_INTSXP)") {
     SEXP r = PROTECT(R_compact_intrange(1, 5));
 
-    expect_true(ALTREP(r));
+    expect_true(R_altrep_data2(r) == R_NilValue);
     auto x1 = cpp11::as_cpp<std::vector<int>>(r);
-    expect_true(ALTREP(r));
+    expect_true(R_altrep_data2(r) == R_NilValue);
 
     expect_true(x1.size() == 5);
     int expected = 1;