qx.Class.define("...",
    {

        extend:qx.ui.core.Widget,

        construct:function (data) {
            this.base(arguments);
            ...
            this._createChildControl("gcontainer");
            this._setLayout(new qx.ui.layout.HBox(1));

        },

        members:{
            _createChildControlImpl:function (id, hash) {
                var control;
                switch (id)  {
                    case 'gcontainer':

                        control = new qx.ui.container.SlideBar().set({
                            backgroundColor:"white",
                            orientation: 'horizontal',
                            width: 200
                        });

                        control.setLayout(new qx.ui.layout.HBox());
                        this._add(control);
                        return control;

                    ...

                    default:
                        return this.base(arguments, id);

                }

            }
}