SFML provides a high level class to send data : sf::packet, you should check the official tutorial (and the others tutorials as well, it'll help you avoiding low level problematics) :
http://www.sfml-dev.org/tutorials/2.3/network-packet.phpIt's easy to use, and you are sure to receive the whole packet at each time (ie: not a part of the packet) :
// on sending side
std::string header = "MyCommandHeaderCode";
double objectToSend = 0.6;
sf::Packet packet;
packet << header << objectToSend;
tcpSocket.send(packet);
//receiving sides
//You are sure to receive the whole packet : not a part, not more
tcpSocket.receive(packet);
std::string headerCode;
double data;
packet >> headerCode >>data;