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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    // this local client variable is no longer valid now !!!!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
}