remove char from stringstream and append some data

You can seek the stringstream and go back 1 character, using stringstream::seekp. Note that it does not remove the last character, but only moves the write head. This is sufficient in this case, as we overwrite the last character with an }.

douCoh << '{';
for(unsigned int i=0;i<dataSize;i++)
  if(v[i].test) douCoh << i+1 << ',';
douCoh.seekp(-1,douCoh.cur); douCoh << '}';