可变参数模板的递归解析

class DbCommand
{
  friend class DbQuery;
public:
  template<typename ...Args>
  DbCommand(const QString& prepare_sql, Args... args) {
    //qDebug() << prepare_sql;
    SetPrepareSql(prepare_sql);
    parse(args...);
  }

  DbCommand() = default;
  DbCommand(const DbCommand&) = default;

  QString ToString();

private:
  void SetPrepareSql(const QString& sql);
  void AddBindValue(const QVariant& var);

  //递归终止函数
  void parse(){}
  //展开函数
  template <typename T, typename ...Args>
  void parse(T head, Args... rest)
  {
    AddBindValue(head);
    //qDebug() << head;
    parse(rest...);
  }

private:
  QString prepare_sql_;
  QVariantList variant_list_;
};

以下是示例

    DbCommand clear_table("delete from T_TEST");
    auto clear_sqlquery = DbHandler().Connection("test_connection").GetQuery();
    clear_sqlquery.SetCommand(clear_table);
    clear_sqlquery.Exec();

    DbCommand command("select id, value from T_TEST where id = ?", 123);
    auto query = DbHandler().Connection("test_connection").GetQuery();
    query.SetCommand(command);
    if(!query.Exec())
    {
      qDebug().noquote() << command.ToString();
      qDebug() << query.ErrorString();  //ERROR REPORT
    }
    auto result = query.Result();
    while(result.Next())
    {
      qDebug() << "id\t" << "value" ;
      qDebug() << result.Value("id").toInt() << "\t" << result.Value("value").toInt();