Utils: Singleton template and migration of the BufferManager

Singletons are not autocreating, the application needs to create
exactly one instance (I want to avoid implicit initialization order
hell)

src/buffer_manager.cc

@@ -10,26 +10,6 @@ namespace Kakoune
 
 struct name_not_unique : logic_error {};
 
-BufferManager* BufferManager::ms_instance = nullptr;
-
-BufferManager& BufferManager::instance()
-{
-    if (not ms_instance)
-        ms_instance = new BufferManager();
-
-    return *ms_instance;
-}
-
-void BufferManager::delete_instance()
-{
-    delete ms_instance;
-    ms_instance = nullptr;
-}
-
-BufferManager::BufferManager()
-{
-}
-
 void BufferManager::register_buffer(Buffer* buffer)
 {
     assert(buffer);

src/buffer_manager.hh

@@ -2,6 +2,7 @@
 #define buffer_manager_hh_INCLUDED
 
 #include "buffer.hh"
+#include "utils.hh"
 
 #include <unordered_map>
 #include <memory>
@@ -9,7 +10,7 @@
 namespace Kakoune
 {
 
-class BufferManager
+class BufferManager : public Singleton<BufferManager>
 {
 public:
     typedef std::unordered_map<std::string, std::unique_ptr<Buffer>> BufferMap;
@@ -32,12 +33,7 @@ public:
 
     Buffer* get_buffer(const std::string& name);
 
-    static BufferManager& instance();
-    static void delete_instance();
-
 private:
-    BufferManager();
-    static BufferManager* ms_instance;
     BufferMap m_buffers;
 };
 

src/main.cc

@@ -329,6 +329,7 @@ void show_buffer(const CommandParameters& params)
 }
 
 CommandManager command_manager;
+BufferManager  buffer_manager;
 
 void do_command()
 {

src/utils.hh

@@ -2,6 +2,7 @@
 #define utils_hh_INCLUDED
 
 #include "exception.hh"
+#include "assert.hh"
 
 #include <memory>
 
@@ -59,6 +60,45 @@ AutoRaii<T, F> auto_raii(T* resource, F cleanup)
     return AutoRaii<T, F>(resource, cleanup);
 }
 
+template<typename T>
+class Singleton
+{
+public:
+    Singleton(const Singleton&) = delete;
+    Singleton& operator=(const Singleton&) = delete;
+
+    static T& instance()
+    {
+        assert (ms_instance);
+        return *ms_instance;
+    }
+
+    static void delete_instance()
+    {
+        if (ms_instance)
+            delete ms_instance;
+    }
+
+protected:
+    Singleton()
+    {
+        assert(not ms_instance);
+        ms_instance = static_cast<T*>(this);
+    }
+
+    ~Singleton()
+    {
+        assert(ms_instance == this);
+        ms_instance = nullptr;
+    }
+
+private:
+    static T* ms_instance;
+};
+
+template<typename T>
+T* Singleton<T>::ms_instance = nullptr;
+
 }
 
 #endif // utils_hh_INCLUDED