[core] Named members for diff before/after

src/mbgl/renderer/render_style.cpp

@@ -134,7 +134,7 @@ void RenderStyle::update(const UpdateParameters& parameters) {
 
     // Update changed images.
     for (const auto& entry : imageDiff.changed) {
-        spriteAtlas->updateImage(entry.second[1]);
+        spriteAtlas->updateImage(entry.second.after);
     }
 
     if (parameters.spriteLoaded && !spriteAtlas->isLoaded()) {
@@ -157,7 +157,7 @@ void RenderStyle::update(const UpdateParameters& parameters) {
 
     // Update render layers for changed layers.
     for (const auto& entry : layerDiff.changed) {
-        renderLayers.at(entry.first)->setImpl(entry.second[1]);
+        renderLayers.at(entry.first)->setImpl(entry.second.after);
     }
 
     // Update layers for class and zoom changes.

src/mbgl/renderer/style_diff.cpp

@@ -33,7 +33,7 @@ StyleDifference<T> diff(const Immutable<std::vector<T>>& a,
             bIt++;
         } else {
             if (aIt->get() != bIt->get()) {
-                result.changed.emplace((*bIt)->id, std::array<T, 2> {{ *aIt, *bIt }});
+                result.changed.emplace((*bIt)->id, StyleChange<T> { *aIt, *bIt });
             }
             aIt++;
             bIt++;
@@ -73,7 +73,7 @@ bool hasLayoutDifference(const LayerDifference& layerDiff, const std::string& la
     const auto it = layerDiff.changed.find(layerID);
     if (it == layerDiff.changed.end())
         return false;
-    return it->second[0]->hasLayoutDifference(*it->second[1]);
+    return it->second.before->hasLayoutDifference(*it->second.after);
 }
 
 } // namespace mbgl

src/mbgl/renderer/style_diff.hpp

@@ -10,12 +10,19 @@
 
 namespace mbgl {
 
+template <class T>
+class StyleChange {
+public:
+    T before;
+    T after;
+};
+
 template <class T>
 class StyleDifference {
 public:
     std::unordered_map<std::string, T> added;
     std::unordered_map<std::string, T> removed;
-    std::unordered_map<std::string, std::array<T, 2>> changed;
+    std::unordered_map<std::string, StyleChange<T>> changed;
 };
 
 using ImmutableImage = Immutable<style::Image::Impl>;