How to enable stiffness using C++

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Hello,

I want to enable/disable the stiffness using C++ API. But I don't know how to instantiate a service variable. I am not sure about what should I put between "<>". The code is attached bellow. Anyone can help me?

  ros::init(argc, argv, "controller");
  ros::NodeHandle n;
  ros::Publisher controller_pub = n.advertise<geometry_msgs::Twist>("/cmd_vel", 1000);
  ros::Rate loop_rate(10);
  int count = 0;

  ros::ServiceClient client = n.serviceClient<??>("/body_stiffness/enable");
  while (ros::ok())
  {
      geometry_msgs::Twist control;
      control.linear.x = 1;
      control.linear.y = 0;
      control.angular.z = 0;
      controller_pub.publish(control);  
      ros::spinOnce();

      loop_rate.sleep();
      ++count;
  }