DRTVWR-558: Add unit test for VAPPLY().

Add to apply_test.cpp a collect() function that incrementally accumulates an
arbitrary number of arguments into a std::vector<std::string>. Construct a
std::array<std::string> to pass it, using VAPPLY().

Clarify in header comments that LL::apply() can't call a variadic function
with arguments of dynamic size: std::vector or LLSD. The compiler can deduce
how many arguments to pass to a function with a fixed argument list; it can
deduce how many arguments to pass to a variadic function with a fixed number
of arguments. But it can't compile a call to a variadic function with an
arguments data structure whose size can vary at runtime.

(cherry picked from commit ceed33396266b123896f7cfb9b90abdf240e1eec)

1 files changed, 5 insertions, 1 deletions

diff --git a/indra/llcommon/apply.h b/indra/llcommon/apply.h
index 26753f5017..9f4c268895 100644
--- a/indra/llcommon/apply.h
+++ b/indra/llcommon/apply.h
@@ -108,7 +108,11 @@ auto apply_impl(CALLABLE&& func, const std::vector<T>& args, std::index_sequence
                  std::make_tuple(args[I]...));
 }
 
-// this goes beyond C++17 std::apply()
+/**
+ * apply(function, std::vector) goes beyond C++17 std::apply(). For this case
+ * @a function @emph cannot be variadic: the compiler must know at compile
+ * time how many arguments to pass. This isn't Python.
+ */
 template <typename CALLABLE, typename T>
 auto apply(CALLABLE&& func, const std::vector<T>& args)
 {