template <typename T>
    T *value(QOpenGLContext *context) {
        QOpenGLContextGroup *group = context->shareGroup();
        // Have to use our own mutex here, not the group's, since
        // m_groups has to be protected too against any concurrent access.
        QMutexLocker locker(&m_mutex);
        T *resource = static_cast<T *>(group->d_func()->m_resources.value(this, 0));
        if (!resource) {
            resource = new T(context);
            insert(context, resource);
        }
        return resource;
    }