Add drain_buffer class

fix #457

test/Jamfile

@@ -27,6 +27,7 @@ unit-test core-tests :
     core/consuming_buffers.cpp
     core/doc_core_samples.cpp
     core/doc_snippets.cpp
+    core/drain_buffer.cpp
     core/error.cpp
     core/flat_buffer.cpp
     core/handler_alloc.cpp

test/core/CMakeLists.txt

@@ -22,6 +22,7 @@ add_executable (core-tests
     consuming_buffers.cpp
     doc_core_samples.cpp
     doc_snippets.cpp
+    drain_buffer.cpp
     error.cpp
     flat_buffer.cpp
     handler_alloc.cpp

test/core/drain_buffer.cpp

@@ -0,0 +1,51 @@
+//
+// Copyright (c) 2013-2017 Vinnie Falco (vinnie dot falco at gmail dot com)
+//
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+//
+
+// Test that header file is self-contained.
+#include <beast/core/drain_buffer.hpp>
+
+#include <beast/core/type_traits.hpp>
+#include <beast/unit_test/suite.hpp>
+
+namespace beast {
+
+static_assert(is_dynamic_buffer<drain_buffer>::value,
+    "DynamicBuffer requirements not met");
+
+class drain_buffer_test : public beast::unit_test::suite
+{
+public:
+    void
+    run() override
+    {
+        using boost::asio::buffer_size;
+        drain_buffer b;
+        BEAST_EXPECT(buffer_size(b.prepare(0)) == 0);
+        BEAST_EXPECT(buffer_size(b.prepare(100)) == 100);
+        try
+        {
+            b.prepare(b.max_size() + 1);
+            fail("", __FILE__, __LINE__);
+        }
+        catch(std::length_error const&)
+        {
+            pass();
+        }
+        b.prepare(10);
+        BEAST_EXPECT(b.size() == 0);
+        b.commit(10);
+        BEAST_EXPECT(b.size() == 0);
+        b.consume(10);
+        BEAST_EXPECT(b.size() == 0);
+        b.consume(1000);
+        BEAST_EXPECT(b.size() == 0);
+    }
+};
+
+BEAST_DEFINE_TESTSUITE(drain_buffer,core,beast);
+
+} // beast