ExtensionsVisitor: use forward declared types instead of void* for format-specific pointers

modules/juce_audio_processors/format_types/juce_AudioUnitPluginFormat.mm

@@ -892,7 +892,7 @@ public:
         {
             explicit Extensions (const AudioUnitPluginInstance* instanceIn) : instance (instanceIn) {}
 
-            void* getAudioUnitHandle() const noexcept override   { return instance->audioUnit; }
+            AudioUnit getAudioUnitHandle() const noexcept override   { return instance->audioUnit; }
 
             const AudioUnitPluginInstance* instance = nullptr;
         };

modules/juce_audio_processors/format_types/juce_VST3PluginFormat.cpp

@@ -2303,7 +2303,7 @@ public:
         {
             explicit Extensions (const VST3PluginInstance* instanceIn) : instance (instanceIn) {}
 
-            void* getIComponentPtr() const noexcept override   { return instance->holder->component; }
+            Steinberg::Vst::IComponent* getIComponentPtr() const noexcept override   { return instance->holder->component; }
 
             MemoryBlock getPreset() const override             { return instance->getStateForPresetFile(); }
 

modules/juce_audio_processors/format_types/juce_VSTPluginFormat.cpp

@@ -41,6 +41,8 @@ JUCE_BEGIN_IGNORE_WARNINGS_MSVC (4996)
 
 namespace Vst2
 {
+struct AEffect;
+
 // If the following files cannot be found then you are probably trying to host
 // VST2 plug-ins. To do this you must have a VST2 SDK in your header search
 // paths or use the "VST (Legacy) SDK Folder" field in the Projucer. The VST2
@@ -1265,7 +1267,7 @@ struct VSTPluginInstance final   : public AudioPluginInstance,
         {
             explicit Extensions (const VSTPluginInstance* instanceIn) : instance (instanceIn) {}
 
-            void* getAEffectPtr() const noexcept override   { return instance->vstEffect; }
+            AEffect* getAEffectPtr() const noexcept override   { return reinterpret_cast<AEffect*> (instance->vstEffect); }
 
             const VSTPluginInstance* instance = nullptr;
         };

modules/juce_audio_processors/utilities/juce_ExtensionsVisitor.h

@@ -23,6 +23,30 @@
   ==============================================================================
 */
 
+// Forward declarations to avoid leaking implementation details.
+namespace Steinberg
+{
+    namespace Vst
+    {
+        class IComponent;
+    }
+}
+
+//==============================================================================
+#if (defined(TARGET_OS_IPHONE) && TARGET_OS_IPHONE) || (defined(AUDIOCOMPONENT_NOCARBONINSTANCES) && AUDIOCOMPONENT_NOCARBONINSTANCES)
+struct OpaqueAudioComponentInstance;
+typedef struct OpaqueAudioComponentInstance* AudioComponentInstance;
+#else
+struct ComponentInstanceRecord;
+typedef struct ComponentInstanceRecord* AudioComponentInstance;
+#endif
+
+typedef AudioComponentInstance AudioUnit;
+
+//==============================================================================
+struct AEffect;
+
+//==============================================================================
 namespace juce
 {
 
@@ -50,7 +74,7 @@ struct ExtensionsVisitor
     struct VST3Client
     {
         virtual ~VST3Client() = default;
-        virtual void* getIComponentPtr() const noexcept = 0;
+        virtual Steinberg::Vst::IComponent* getIComponentPtr() const noexcept = 0;
 
         virtual MemoryBlock getPreset() const = 0;
         virtual bool setPreset (const MemoryBlock&) const = 0;
@@ -60,14 +84,14 @@ struct ExtensionsVisitor
     struct AudioUnitClient
     {
         virtual ~AudioUnitClient() = default;
-        virtual void* getAudioUnitHandle() const noexcept = 0;
+        virtual AudioUnit getAudioUnitHandle() const noexcept = 0;
     };
 
     /** Can be used to retrieve information about a VST that is wrapped by an AudioProcessor. */
     struct VSTClient
     {
         virtual ~VSTClient() = default;
-        virtual void* getAEffectPtr() const noexcept = 0;
+        virtual AEffect* getAEffectPtr() const noexcept = 0;
     };
 
     virtual ~ExtensionsVisitor() = default;