void main(){
	// create the client
	auto client = std::make_unique<sf::TcpSocket>();

	// setup listener
	.
	.
	.
	listener.accept(*client);
	createthread(...., thread_func,...., std::move<client));
	// this local client variable is no longer valid now !!!!


	// do more stuff
	.
	.
	.



}

void thread_func(std::unique_ptr<sf::TcpClient> client){
	// use the client variable here
}