Removing whitespace from inside the string

Here is my string:
            A B(C)            D            

I wrote a trim function that removes leading and trailing whitespace.
After applying trim, I get a string like this:
A B(C)            D

I have problems deleting whitespace (space and tabs) from that string afterwards.
The resulting string should be:

AB(C)D


string trim(string str)
{
      char const* delims = " \t\r\n";
      int pos;

      // trim leading whitespace
   string::size_type  notwhite = str.find_first_not_of(delims);
   str.erase(0,notwhite);

   // trim trailing whitespace
   notwhite = str.find_last_not_of(delims);
   str.erase(notwhite+1);

   //delete whitespace inside the instruction
   //----I need help here

      while (str.find(" "))
      {
            pos = str.find(" ");
            str=str.substr(pos);
      }

//-------------trim is used in openFile function------

int openFile(string filename)
{
      string line;
      
      ifstream sin(filename.c_str(), ios::in);
      if (!sin) {

      cerr << "Can't open " << filename;
      return 2;
      }
   
      while(getline(sin,line)){

            //Calculate a number of lines
            line_count++;
            
            //remove whitespaces
            line = trim(line);

            //skip empty lines
            if (line.empty())
                  continue;
            cout  << line << endl;
       
      }
      
      cout << "Number of lines: " << line_count << endl;
      return 0;
}