Add operator[] to placeholders

include/boost/lambda2/lambda2.hpp

@@ -20,6 +20,19 @@ namespace boost
 namespace lambda2
 {
 
+namespace lambda2_detail
+{
+
+struct subscript
+{
+    template<class T1, class T2> decltype(auto) operator()(T1&& t1, T2&& t2) const
+    {
+        return std::forward<T1>(t1)[ std::forward<T2>(t2) ];
+    }
+};
+
+} // namespace lambda2_detail
+
 // placeholders
 
 template<int I> struct lambda2_arg
@@ -28,6 +41,11 @@ template<int I> struct lambda2_arg
     {
         return std::get<std::size_t{I-1}>( std::tuple<A&&...>( std::forward<A>(a)... ) );
     }
+
+    template<class T> auto operator[]( T&& t ) const
+    {
+        return std::bind( lambda2_detail::subscript(), *this, std::forward<T>( t ) );
+    }
 };
 
 #if defined(__cpp_inline_variables) && __cpp_inline_variables >= 201606L

test/Jamfile

@@ -22,3 +22,4 @@ run lookup_problem.cpp ;
 run dereference.cpp ;
 run placeholders.cpp ;
 run increment.cpp ;
+run subscript.cpp ;

test/subscript.cpp

@@ -0,0 +1,27 @@
+// Copyright 2021 Peter Dimov
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/lambda2.hpp>
+#include <boost/core/lightweight_test.hpp>
+#include <functional>
+
+int main()
+{
+    using namespace boost::lambda2;
+
+    int x[] = { 1, 2, 3, 4 };
+
+    BOOST_TEST_EQ( _1[0](x), x[0] );
+    BOOST_TEST_EQ( _1[_2](x, 1), x[1] );
+    BOOST_TEST_EQ( (_1[_2] + _3[_4])(x, 2, x, 3), x[2] + x[3] );
+    BOOST_TEST_EQ( std::bind(_1 + _2, _1[_2], _3[_4])(x, 2, x, 3), x[2] + x[3] );
+
+    _1[0](x) = 7;
+    BOOST_TEST_EQ( x[0], 7 );
+
+    _1[_2](x, 1) = 8;
+    BOOST_TEST_EQ( x[1], 8 );
+
+    return boost::report_errors();
+}