Use string_ref in older Boost versions

fix boostorg#543

.travis.yml

@@ -11,8 +11,8 @@ env:
     # to boost's .tar.gz.
     - LCOV_ROOT=$HOME/lcov
     - VALGRIND_ROOT=$HOME/valgrind-install
-    - BOOST_ROOT=$HOME/boost_1_64_0
-    - BOOST_URL='http://sourceforge.net/projects/boost/files/boost/1.64.0/boost_1_64_0.tar.gz'
+    - BOOST_ROOT=$HOME/boost_1_58_0
+    - BOOST_URL='http://sourceforge.net/projects/boost/files/boost/1.58.0/boost_1_58_0.tar.gz'
 
 addons:
   apt:

CHANGELOG.md

@@ -3,6 +3,7 @@ Version 68:
 * Split common tests to a new project
 * Small speed up in fields comparisons
 * Adjust buffer size in fast server
+* Use string_ref in older Boost versions
 
 API Changes:
 

CMakeLists.txt

@@ -48,7 +48,7 @@ endif()
 option (Boost_USE_STATIC_LIBS "Use static libraries for boost" ON)
 
 set (BOOST_COMPONENTS coroutine context filesystem program_options system thread)
-find_package (Boost REQUIRED COMPONENTS ${BOOST_COMPONENTS})
+find_package (Boost 1.58.0 REQUIRED COMPONENTS ${BOOST_COMPONENTS})
 
 link_directories(${Boost_LIBRARY_DIRS})
 

include/beast/core/string.hpp

@@ -8,18 +8,42 @@
 #ifndef BEAST_STRING_HPP
 #define BEAST_STRING_HPP
 
+#include <boost/version.hpp>
+#ifndef BEAST_NO_BOOST_STRING_VIEW
+# if BOOST_VERSION >= 106400
+#  define BEAST_NO_BOOST_STRING_VIEW 0
+# else
+#  define BEAST_NO_BOOST_STRING_VIEW 1
+# endif
+#endif
+
+#if BEAST_NO_BOOST_STRING_VIEW
+#include <boost/utility/string_ref.hpp>
+#else
 #include <boost/utility/string_view.hpp>
+#endif
+
 #include <algorithm>
 
 namespace beast {
 
+#if BEAST_NO_BOOST_STRING_VIEW
+/// The type of string view used by the library
+using string_view = boost::string_ref;
+
+/// The type of basic string view used by the library
+template<class CharT, class Traits>
+using basic_string_view =
+    boost::basic_string_ref<CharT, Traits>;
+#else
 /// The type of string view used by the library
 using string_view = boost::string_view;
 
 /// The type of basic string view used by the library
 template<class CharT, class Traits>
 using basic_string_view =
     boost::basic_string_view<CharT, Traits>;
+#endif
 
 namespace detail {