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, 6 insertions, 0 deletions

diff --git a/indra/llcommon/llsdutil.h b/indra/llcommon/llsdutil.h
index eaf8825791..eddaa64bd2 100644
--- a/indra/llcommon/llsdutil.h
+++ b/indra/llcommon/llsdutil.h
@@ -31,6 +31,7 @@
 
 #include "llsd.h"
 #include <boost/functional/hash.hpp>
+#include <boost/type_traits.hpp>
 #include <cassert>
 
 // U32
@@ -601,6 +602,11 @@ auto apply_impl(CALLABLE&& func, const LLSD& array, std::index_sequence<I...>)
     return std::forward<CALLABLE>(func)(LLSDParam<LLSD>(array[I])...);
 }
 
+/**
+ * apply(function, LLSD) 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>
 auto apply(CALLABLE&& func, const LLSD& args)
 {