Audio: use public API of the Extension class.

No need to make friends and use underscores.

src/Magnum/Audio/Context.cpp

@@ -136,7 +136,7 @@ Context::Context(const Configuration& config) {
     /* Add all extensions to a map for faster lookup */
     std::unordered_map<std::string, Extension> extensionMap;
     for(const Extension& extension: Extension::extensions())
-        extensionMap.emplace(extension._string, extension);
+        extensionMap.emplace(extension.string(), extension);
 
     /* Check for presence of extensions */
     const std::vector<std::string> extensions = extensionStrings();
@@ -144,7 +144,7 @@ Context::Context(const Configuration& config) {
         const auto found = extensionMap.find(extension);
         if(found != extensionMap.end()) {
             _supportedExtensions.push_back(found->second);
-            _extensionStatus.set(found->second._index);
+            _extensionStatus.set(found->second.index());
         }
     }
 

src/Magnum/Audio/Context.h

@@ -61,12 +61,13 @@ See also @ref Audio::Extensions namespace, which contain compile-time informatio
 about OpenAL extensions.
 */
 class MAGNUM_AUDIO_EXPORT Extension {
-    friend Context;
-
     public:
         /** @brief All OpenAL extensions */
         static const std::vector<Extension>& extensions();
 
+        /** @brief Internal unique extension index */
+        constexpr std::size_t index() const { return _index; }
+
         /** @brief Extension string */
         constexpr const char* string() const { return _string; }
 
@@ -301,7 +302,7 @@ class MAGNUM_AUDIO_EXPORT Context {
          *      @ref MAGNUM_ASSERT_AUDIO_EXTENSION_SUPPORTED()
          */
         bool isExtensionSupported(const Extension& extension) const {
-            return _extensionStatus[extension._index];
+            return _extensionStatus[extension.index()];
         }
 
     private: