What is this line supposed to do? restxt[i]=(fulscr[i].width+"x"+fulscr[i].height, schwab, 18);
Edit:Nevermind, I looked at sf::Text reference and I see you are trying to construct an sf::Text. Try restxt[i]=sf::Text(std::to_string(fulscr[i].width)+"x"+std::to_string(fulscr[i].height), schwab, 18);
vector<Text> restxt;
for (unsigned i=0; i<fulscr.size(); i++)
{restxt[i]=(fulscr[i].width+"x"+fulscr[i].height, schwab, 18);}
You defined vector restxt as an empty vector
vector<Text> restxt;
so you may not use the subscript operator because there are no such elements in this vector.
As for the last error then it seems that you are using MS VC++ 2010 that contains a bug in the definition of the family of functions std::to_string that is this function is not overloaded for type int. You should cast the argument of the function for one of the types that listed in the error message.