You had to include the <sstream> header because you were using 'std::ostringstream gld;' which ostringstream is a class presented thru said header.
Again, its great you want to learn SFML but you really need to step back and get general programming knowledge down first. If you didn't know that you had to include a header to get something from that headers class to work then your programming and/or c++ knowledge is still severely lacking. Before diving deeper into SFML you may want to at the very least write small programs that revolve around different aspects of programming. Such programs could be:
Console based:
1.) Take input from the keyboard, only allow numbers, and then calculate whether that number is prime or not, return true or false, if true return the primes root, if false give a close estimate.
2.) Simple text file I/O
3.) Strings, learn to manipulate strings with user input and input from a text file. Print strings back to the console and print back to another text file.
4.) A program that takes a text file of random numbers and gives the user an option to pick one of X number of 'sorts' to sort the list. Sorts could be quicksort, bubble sort, heap sort, any other kind of sort you want.
5.) a simple console based Tic-Tac-Toe game
6.) console based Othello/Chess/Checkers game
7.) a program that makes use of pointers such as your own Linked List
these are just a few examples of the simple basic things I was taught within my first year of University CS courses. All of these were done with just the C++ standard library on the console. If you can get all of these done then you should have the basic understanding of Control structers, control loops, program flow, user/file input and output, pointers, references and values.
