refactor: convert tr_net_init_mgr to singleton (#6914)

libtransmission/utils.cc

@@ -733,36 +733,50 @@ std::string tr_env_get_string(std::string_view key, std::string_view default_val
 
 // ---
 
-tr_net_init_mgr::tr_net_init_mgr()
+namespace
 {
-    // try to init curl with default settings (currently ssl support + win32 sockets)
-    // but if that fails, we need to init win32 sockets as a bare minimum
-    if (curl_global_init(CURL_GLOBAL_ALL) != CURLE_OK)
+namespace tr_net_init_impl
+{
+class tr_net_init_mgr
+{
+private:
+    tr_net_init_mgr()
     {
-        curl_global_init(CURL_GLOBAL_WIN32);
+        // try to init curl with default settings (currently ssl support + win32 sockets)
+        // but if that fails, we need to init win32 sockets as a bare minimum
+        if (curl_global_init(CURL_GLOBAL_ALL) != CURLE_OK)
+        {
+            curl_global_init(CURL_GLOBAL_WIN32);
+        }
     }
-}
 
-tr_net_init_mgr::~tr_net_init_mgr()
-{
-    curl_global_cleanup();
-}
-
-std::unique_ptr<tr_net_init_mgr> tr_net_init_mgr::create()
-{
-    if (!initialised)
+public:
+    tr_net_init_mgr(tr_net_init_mgr const&) = delete;
+    tr_net_init_mgr(tr_net_init_mgr&&) = delete;
+    tr_net_init_mgr& operator=(tr_net_init_mgr const&) = delete;
+    tr_net_init_mgr& operator=(tr_net_init_mgr&&) = delete;
+    ~tr_net_init_mgr()
     {
-        initialised = true;
-        return std::unique_ptr<tr_net_init_mgr>{ new tr_net_init_mgr };
+        curl_global_cleanup();
     }
-    return {};
-}
 
-bool tr_net_init_mgr::initialised = false;
+    static void create()
+    {
+        if (!instance)
+        {
+            instance = std::unique_ptr<tr_net_init_mgr>{ new tr_net_init_mgr };
+        }
+    }
 
-std::unique_ptr<tr_net_init_mgr> tr_lib_init()
+private:
+    static inline std::unique_ptr<tr_net_init_mgr> instance;
+};
+} // namespace tr_net_init_impl
+} // namespace
+
+void tr_lib_init()
 {
-    return tr_net_init_mgr::create();
+    tr_net_init_impl::tr_net_init_mgr::create();
 }
 
 // --- mime-type