CppTools: Apply 'static' pattern to StringTable

This also fixes a crash on loading (some?) projects introduced
in 577bf7c08a.

Change-Id: Ie35d466fa3b84b183118fe93f55393a4c59755de
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@qt.io>

src/plugins/cpptools/stringtable.cpp

@@ -28,8 +28,11 @@
 #include <utils/qtcassert.h>
 
 #include <QDebug>
+#include <QMutex>
+#include <QSet>
 #include <QThreadPool>
 #include <QTime>
+#include <QTimer>
 
 using namespace CppTools::Internal;
 
@@ -37,9 +40,37 @@ enum {
     GCTimeOut = 10 * 1000 // 10 seconds
 };
 
-StringTable::StringTable()
+enum {
+    DebugStringTable = 0
+};
+
+class StringTablePrivate : public QObject
+{
+public:
+    StringTablePrivate();
+
+    QString insert(const QString &string);
+    void startGC() { QThreadPool::globalInstance()->start(&m_gcRunner); }
+    void GC();
+
+    class GCRunner: public QRunnable {
+        StringTablePrivate &m_stringTable;
+
+    public:
+        GCRunner(StringTablePrivate &stringTable): m_stringTable(stringTable) {}
+        virtual void run() { m_stringTable.GC(); }
+    } m_gcRunner;
+
+    mutable QMutex m_lock;
+    QAtomicInt m_stopGCRequested{false};
+    QSet<QString> m_strings;
+    QTimer m_gcCountDown;
+};
+
+static StringTablePrivate *m_instance = nullptr;
+
+StringTablePrivate::StringTablePrivate()
     : m_gcRunner(*this)
-    , m_stopGCRequested(false)
 {
     m_strings.reserve(1000);
 
@@ -48,10 +79,15 @@ StringTable::StringTable()
     m_gcCountDown.setObjectName(QLatin1String("StringTable::m_gcCountDown"));
     m_gcCountDown.setSingleShot(true);
     m_gcCountDown.setInterval(GCTimeOut);
-    connect(&m_gcCountDown, &QTimer::timeout, this, &StringTable::startGC);
+    connect(&m_gcCountDown, &QTimer::timeout, this, &StringTablePrivate::startGC);
 }
 
 QString StringTable::insert(const QString &string)
+{
+    return m_instance->insert(string);
+}
+
+QString StringTablePrivate::insert(const QString &string)
 {
     if (string.isEmpty())
         return string;
@@ -70,17 +106,19 @@ QString StringTable::insert(const QString &string)
 
 void StringTable::scheduleGC()
 {
-    QMetaObject::invokeMethod(&m_gcCountDown, "start", Qt::QueuedConnection);
+    QMetaObject::invokeMethod(&m_instance->m_gcCountDown, "start", Qt::QueuedConnection);
 }
 
-void StringTable::startGC()
+void StringTable::initialize()
 {
-    QThreadPool::globalInstance()->start(&m_gcRunner);
+    m_instance = new StringTablePrivate;
 }
 
-enum {
-    DebugStringTable = 0
-};
+void StringTable::destroy()
+{
+    delete m_instance;
+    m_instance = nullptr;
+}
 
 static inline bool isQStringInUse(const QString &string)
 {
@@ -88,7 +126,7 @@ static inline bool isQStringInUse(const QString &string)
     return data_ptr->ref.isShared() || data_ptr->ref.isStatic();
 }
 
-void StringTable::GC()
+void StringTablePrivate::GC()
 {
     QMutexLocker locker(&m_lock);