[core] add polyfill for codecvt for STLs that don't have it yet

CMakeLists.txt

@@ -142,6 +142,8 @@ else()
     message(STATUS "Can't find ccache — consider installing ccache to improve recompilation performance")
 endif()
 
+include(cmake/codecvt.cmake)
+
 if(NOT EXISTS ${CMAKE_SOURCE_DIR}/platform/${MBGL_PLATFORM}/config.cmake)
     message(ERROR "Can't find config.cmake file for platform ${MBGL_PLATFORM}")
 endif()

cmake/codecvt.cmake

@@ -0,0 +1,19 @@
+add_library(codecvt INTERFACE)
+
+# Determine if the STL has codecvt
+file(WRITE "${CMAKE_BINARY_DIR}/features/codecvt/main.cpp" "#include <codecvt>\nint main() {}")
+try_compile(STL_SUPPORTS_CODECVT
+    "${CMAKE_BINARY_DIR}/features/codecvt"
+    SOURCES "${CMAKE_BINARY_DIR}/features/codecvt/main.cpp"
+    CMAKE_FLAGS "-DCMAKE_MACOSX_BUNDLE:STRING=YES" "-DCMAKE_XCODE_ATTRIBUTE_CODE_SIGNING_REQUIRED:STRING=NO"
+    COMPILE_DEFINITIONS "-std=c++14" # CXX_STANDARD wasn't added to try_compile until CMake 3.8
+    OUTPUT_VARIABLE CODECVT_TEST_OUTPUT
+)
+
+if (NOT STL_SUPPORTS_CODECVT)
+    if ($ENV{V})
+        message("codecvt support not detected: ${CODECVT_TEST_OUTPUT}")
+    endif()
+    target_include_directories(codecvt INTERFACE platform/default/codecvt)
+    target_add_mason_package(codecvt INTERFACE boost)
+endif()

cmake/core.cmake

@@ -7,6 +7,8 @@ target_include_directories(mbgl-core
     PRIVATE src
 )
 
+target_link_libraries(mbgl-core PRIVATE codecvt)
+
 target_add_mason_package(mbgl-core PUBLIC geometry)
 target_add_mason_package(mbgl-core PUBLIC variant)
 target_add_mason_package(mbgl-core PRIVATE unique_resource)

platform/android/config.cmake

@@ -71,6 +71,7 @@ macro(mbgl_filesource)
     target_add_mason_package(mbgl-filesource PUBLIC jni.hpp)
 
     target_link_libraries(mbgl-filesource
+        PRIVATE codecvt
         PUBLIC sqlite
         PUBLIC -llog
         PUBLIC -landroid
@@ -86,6 +87,7 @@ add_library(mapbox-gl SHARED
 )
 
 target_link_libraries(mapbox-gl
+    PRIVATE codecvt
     PRIVATE mbgl-core
     PRIVATE mbgl-filesource
 )
@@ -106,6 +108,7 @@ macro(mbgl_platform_test)
     )
 
     target_link_libraries(mbgl-test
+        PRIVATE codecvt
         PRIVATE mbgl-core
         PRIVATE mbgl-filesource
     )

platform/default/codecvt/codecvt

@@ -0,0 +1,30 @@
+#pragma once
+
+// This is a minimal polyfill that'll only work exactly for how we use codecvt
+
+#include <string>
+#include <boost/locale/encoding_utf.hpp>
+
+namespace std {
+
+template <typename Codecvt, typename Elem = wchar_t>
+class wstring_convert {
+public:
+    static_assert(std::is_same<Elem, typename Codecvt::Elem>::value, "type mismatch");
+
+    inline std::basic_string<Elem> from_bytes(const string& str) {
+        return boost::locale::conv::utf_to_utf<Elem>(str);
+    }
+
+    inline string to_bytes(const std::basic_string<Elem>& str) {
+        return boost::locale::conv::utf_to_utf<char>(str);
+    }
+};
+
+template <typename E>
+class codecvt_utf8_utf16 {
+public:
+    using Elem = E;
+};
+
+} // namespace std

platform/default/utf.cpp

@@ -1,25 +1,13 @@
 #include <mbgl/util/utf.hpp>
 
-#include <memory>
 #include <locale>
-
-// GCC 4.9 compatibility
-#if !defined(__GNUC__) || __GNUC__ >= 5
 #include <codecvt>
-#else
-#include <boost/locale/encoding_utf.hpp>
-#endif
 
 namespace mbgl {
 namespace util {
 
-std::u16string utf8_to_utf16::convert(std::string const& utf8) {
-#if !defined(__GNUC__) || __GNUC__ >= 5
-    std::wstring_convert<std::codecvt_utf8_utf16<char16_t>, char16_t> converter;
-    return converter.from_bytes(utf8);
-#else
-    return boost::locale::conv::utf_to_utf<char16_t>(utf8.c_str(), utf8.c_str() + utf8.size());
-#endif
+std::u16string utf8_to_utf16::convert(const std::string& utf8) {
+    return std::wstring_convert<std::codecvt_utf8_utf16<char16_t>, char16_t>().from_bytes(utf8);
 }
 
 } // namespace util