C++ ::  Default Document Folder Path

im trying to write a file to a default document folder..something like...

FILE* file;
file = fopen("%docdir% est.txt", "w");
fputs("Hello", file);
fclose(file);
"%docdir% est.txt" this isnt working for me, i have to write it as
"C:userspublicdocument est.txt"

any method to write directly to default document folder so it will work in most Windows ? for example in windows 7 this is the default folder "C:users publicdocument est.txt" in windows XP its different

i have to detect the windows version first, to write the correct path