error while using class method as callback in cpp

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Hi, i'm actually trying to use my own class method as callback of a topic. I do it like this

First i created a publisher :

    ros::Publisher topicDirection = n.advertise<std_msgs::Float64>("commandeDir", 1);

then i subscribe to this topic (commandeDir) with :

Servo pwmDir();
ros::Subscriber topicDir = n.subscribe("commandeDir", 1, &Servo::commande(const std_msgs::Float64& msg), pwmDir);

and my function looks like this :

void Servo::commande(const std_msgs::Float64& msg)
{
     //.......
}

it throw me this error :

/opt/ros/indigo/include/ros/message_traits.h:138:31: error: ‘__s_getDataType’ is not a member of ‘int’
 return M::__s_getDataType().c_str();

i don't get what i am doing wrong ?