[core] Report conversion errors using std::string

char* increases the risk of pointing to a invalid reference. Qt had
to use a static variable as retainer to workaround.

include/mbgl/style/conversion.hpp

@@ -2,6 +2,8 @@
 
 #include <mbgl/util/variant.hpp>
 
+#include <string>
+
 namespace mbgl {
 namespace style {
 namespace conversion {
@@ -53,7 +55,7 @@ namespace conversion {
    them for v8 types.
 */
 
-struct Error { const char * message; };
+struct Error { std::string message; };
 
 template <class T>
 class Result : private variant<T, Error> {

platform/node/src/node_map.cpp

@@ -504,7 +504,7 @@ NAN_METHOD(NodeMap::AddSource) {
 
     Result<std::unique_ptr<Source>> source = convert<std::unique_ptr<Source>>(info[1], *Nan::Utf8String(info[0]));
     if (!source) {
-        Nan::ThrowTypeError(source.error().message);
+        Nan::ThrowTypeError(source.error().message.c_str());
         return;
     }
 
@@ -524,7 +524,7 @@ NAN_METHOD(NodeMap::AddLayer) {
 
     Result<std::unique_ptr<Layer>> layer = convert<std::unique_ptr<Layer>>(info[0]);
     if (!layer) {
-        Nan::ThrowTypeError(layer.error().message);
+        Nan::ThrowTypeError(layer.error().message.c_str());
         return;
     }
 
@@ -557,7 +557,7 @@ NAN_METHOD(NodeMap::SetLayoutProperty) {
 
     mbgl::optional<Error> error = setLayoutProperty(*layer, *Nan::Utf8String(info[1]), info[2]);
     if (error) {
-        return Nan::ThrowTypeError(error->message);
+        return Nan::ThrowTypeError(error->message.c_str());
     }
 
     nodeMap->map->update(mbgl::Update::RecalculateStyle);
@@ -595,7 +595,7 @@ NAN_METHOD(NodeMap::SetPaintProperty) {
 
     mbgl::optional<Error> error = setPaintProperty(*layer, *Nan::Utf8String(info[1]), info[2], klass);
     if (error) {
-        return Nan::ThrowTypeError(error->message);
+        return Nan::ThrowTypeError(error->message.c_str());
     }
 
     nodeMap->map->update(mbgl::Update::RecalculateStyle | mbgl::Update::Classes);
@@ -627,7 +627,7 @@ NAN_METHOD(NodeMap::SetFilter) {
     if (!info[1]->IsNull() && !info[1]->IsUndefined()) {
         Result<Filter> converted = convert<Filter>(info[1]);
         if (!converted) {
-            Nan::ThrowTypeError(converted.error().message);
+            Nan::ThrowTypeError(converted.error().message.c_str());
             return;
         }
         filter = std::move(*converted);

platform/qt/src/qmapboxgl.cpp

@@ -628,7 +628,7 @@ void QMapboxGL::addSource(const QString& sourceID, const QVariant& value)
 
     Result<std::unique_ptr<Source>> source = convert<std::unique_ptr<Source>>(value, sourceID.toStdString());
     if (!source) {
-        qWarning() << "Unable to add source:" << source.error().message;
+        qWarning() << "Unable to add source:" << source.error().message.c_str();
         return;
     }
 
@@ -665,7 +665,7 @@ void QMapboxGL::addLayer(const QVariant& value)
 
     Result<std::unique_ptr<Layer>> layer = convert<std::unique_ptr<Layer>>(value);
     if (!layer) {
-        qWarning() << "Unable to add layer:" << layer.error().message;
+        qWarning() << "Unable to add layer:" << layer.error().message.c_str();
         return;
     }
 
@@ -692,7 +692,7 @@ void QMapboxGL::setFilter(const QString& layer_, const QVariant& filter_)
 
     Result<Filter> converted = convert<Filter>(filter_);
     if (!converted) {
-        qWarning() << "Error parsing filter:" << converted.error().message;
+        qWarning() << "Error parsing filter:" << converted.error().message.c_str();
         return;
     }
     filter = std::move(*converted);

platform/qt/src/qt_geojson.hpp

@@ -29,22 +29,16 @@ Result<GeoJSON> convertGeoJSON(const QVariant& value) {
         d.Parse<0>(value.toByteArray().constData());
     }
 
-    // Needed to keep the error message alive
-    // when we go out of this scope.
-    static std::string error;
-
     if (d.HasParseError()) {
         std::stringstream message;
         message << d.GetErrorOffset() << " - " << rapidjson::GetParseError_En(d.GetParseError());
 
-        error = message.str();
-        return Error { error.c_str() };
+        return Error { message.str() };
     }
 
     conversion::Result<GeoJSON> geoJSON = conversion::convertGeoJSON<JSValue>(d);
     if (!geoJSON) {
-        error = geoJSON.error().message;
-        return Error { error.c_str() };
+        return Error { geoJSON.error().message };
     }
 
     return geoJSON;

src/mbgl/style/sources/geojson_source_impl.cpp

@@ -83,7 +83,7 @@ void GeoJSONSource::Impl::load(FileSource& fileSource) {
 
             conversion::Result<GeoJSON> geoJSON = conversion::convertGeoJSON<JSValue>(d);
             if (!geoJSON) {
-                Log::Error(Event::ParseStyle, "Failed to parse GeoJSON data: %s", geoJSON.error().message);
+                Log::Error(Event::ParseStyle, "Failed to parse GeoJSON data: %s", geoJSON.error().message.c_str());
                 // Create an empty GeoJSON VT object to make sure we're not infinitely waiting for
                 // tiles to load.
                 mapbox::geojson::feature_collection features;