unary plus

I have rewritten bjarne stroustrup's calculator program to accept input from a file and parse it and evaluate it. Everything works fine except for the instance where the plus sign acts as a signed value. For example 3*+2.5 equals 7.5 but the program expects a parenthesis. How can I implement the change?

Code:

                  }
	  case MINUS :			// unary minus
	        return -prim(true);
	  case LP :
	  {
	        double e = expr(true);
	        if (curr_tok != RP) return error("')' expected");
	        get_token();			//eat ')'
	        return e;
	  }

This is what is applied when a unary minus is used but I cannot add a unary plus statement since it creates an error that plus is already used.

Once you read an operator, look to see if the next character is another operator (well, one of + -). If its a "+", ignore it. If its a "-", then multiply the operand it follows by -1 then continue as normal.

For example "1++2". Read the left operand "1". Read the operator "+" and check the next character for "+" or "-". Since its a "+", ignore it and continue normally. This is true because: 1 + 2 = 1 + (+2). If the input was "1+-2", the left operand again is "1", read the operator "+", and see that the following character is one of "+" or "-". So multiply the right operand by -1: (-1)*2=-2, and continue normally. So 1+-2 = 1 + (-1)*(2) = 1 + (-2) = 1 - 2.