Support return type inferencing for multi-return functions.

README.md

@@ -121,7 +121,7 @@ assert(result == 7);
 
 // Call function that returns multiple values
 int sum, difference;
-std::tie(sum, difference) = state["sum_and_difference"].Call<int, int>(3, 1);
+sel::tie(sum, difference) = state["sum_and_difference"](3, 1);
 assert(sum == 4 && difference == 2);
 
 // Call function in table
@@ -142,8 +142,10 @@ Note that `Call`, unlike
 return. This is because the return types are given to `Call` as a
 template function. Without actually performing a cast, `operator()`
 has no way to know how many values to retrieve and what types to
-retrieve them as. Note that multi-value returns must leverage the
-`Call` syntax because casting to a tuple of LValues is non-trivial.
+retrieve them as. Note that multi-value returns must have `sel::tie`
+on the LHS and not `std::tie`. This will create a `sel::Tuple` as
+opposed to an `std::tuple` which has the `operator=` implemented for
+the selector type.
 
 ### Calling Free-standing C++ functions from Lua
 

include/Selector.h

@@ -4,6 +4,7 @@
 #include "State.h"
 #include <string>
 #include <tuple>
+#include "Tuple.h"
 #include "util.h"
 
 extern "C" {
@@ -106,6 +107,14 @@ class Selector {
     }
 
     void operator=(const char *s) const;
+
+    template <typename... Ret>
+    operator std::tuple<Ret...>() const {
+        _traverse();
+        _get();
+        (*_functor)(sizeof...(Ret));
+        return detail::_pop_n_reset<Ret...>(_state._l);
+    }
     operator bool() const;
     operator int() const;
     operator unsigned int() const;

include/Tuple.h

@@ -0,0 +1,22 @@
+#pragma once
+
+#include <tuple>
+
+namespace sel {
+template <typename... T>
+class Tuple {
+private:
+    std::tuple<T&...> _tuple;
+public:
+    Tuple(T&... args) : _tuple(args...) {}
+
+    void operator=(const sel::Selector &s) {
+        _tuple = std::tuple<typename std::remove_reference<T>::type...>(s);
+    }
+};
+
+template <typename... T>
+Tuple<T&...> tie(T&... args) {
+    return Tuple<T&...>(args...);
+}
+}

test/interop_tests.h

@@ -24,7 +24,7 @@ bool test_add(sel::State &state) {
 bool test_multi_return(sel::State &state) {
     state.Load("../test/test.lua");
     int sum, difference;
-    std::tie(sum, difference) = state["sum_and_difference"].Call<int, int>(3, 1);
+    sel::tie(sum, difference) = state["sum_and_difference"](3, 1);
     return (sum == 4 && difference == 2);
 }
 
@@ -33,7 +33,7 @@ bool test_heterogeneous_return(sel::State &state) {
     int x;
     bool y;
     std::string z;
-    std::tie(x, y, z) = state["bar"].Call<int, bool, std::string>();
+    sel::tie(x, y, z) = state["bar"]();
     return x == 4 && y == true && z == "hi";
 }
 
@@ -114,7 +114,7 @@ bool test_multivalue_c_fun_return(sel::State &state) {
     state.Load("../test/test.lua");
     state["test_fun"] = &my_sum_and_difference;
     int sum, difference;
-    std::tie(sum, difference) = state["test_fun"].Call<int, int>(-2, 2);
+    sel::tie(sum, difference) = state["test_fun"](-2, 2);
     return sum == 0 && difference == -4;
 }