prefix calculator having trouble compilng - c++

Question

I am having some trouble with a program that is supposed to take a command line expression and interpret it as a normal mathematical expression.This is what I am getting as an error:
driver.cpp: In function ‘int main(int, char**)’:
driver.cpp:17:57: error: no matching function for call to‘PrefixCalculator::eval(std::istringstream)’
driver.cpp:17:57: note: candidate is:
PrefixCalculator.h:33:3: note: T PrefixCalculator::eval(std::istringstream&) [with T = int, std::istringstream = std::basic_istringstream]
PrefixCalculator.h:33:3: note: no known conversion for argument 1 from ‘std::istringstream {aka std::basic_istringstream}’ to ‘std::istringstream& {aka std::basic_istringstream&}’
I can't understand what the error is trying to suggest to me.
Any suggestions for fixing that? I'm going to add exceptions later, so they are commented out for now.
This is the code:
PrefixCalculator.cpp
#pragma once
#include <sstream>
using namespace std;
template<class T>
class PrefixCalculator {
public:
PrefixCalculator(void){
numOperator = 0;
numOperand = 0;
};
~PrefixCalculator(void){};
T eval(istringstream&);
int getNumOperator() {
return numOperator;
};
int getNumOperand() {
return numOperand;
};
private:
//if you feel you need private helper functions and/or helper data
int numOperator;
int numOperand;
};
template<class T>
T PrefixCalculator<T>::eval(istringstream& input) {
//this function needs to throw an exception if there's a problem with the expression or operators
char nextChar = input.peek();
//this while loop skips over the spaces in the expression, if there are any
while(nextChar == ' ') {
input.get(); //move past this space
nextChar = input.peek(); //check the next character
}
if(nextChar == '+') {
input.get(); //moves past the +
numOperator++;
return eval(input) + eval(input); //recursively calculates the first expression, and adds it to the second expression, returning the result
}
/***** more operators here ******/
if(nextChar == '-') {
input.get();
numOperator++;
return eval(input) - eval(input);
}
if(nextChar == '*') {
input.get();
numOperator++;
return eval(input) * eval(input);
}
if(nextChar == '/') {
input.get();
numOperator++;
return eval(input) / eval(input);
}
/****** BASE CASE HERE *******/
//it's not an operator, and it's not a space, so you must be reading an actual value (like '3' in "+ 3 6". Use the >> operator of istringstream to pull in a T value!
input>>nextChar;
T digit = nextChar - '0';
numOperand++;
return digit;
//OR...there's bad input, in which case the reading would fail and you should throw an exception
}
driver.cpp
#include <sstream>
#include <string>
#include <iostream>
#include "PrefixCalculator.h"
using namespace std;
int main(int argc, char** argv) {
PrefixCalculator<int> calc;
string expression;
cout << "Give a prefix expression to evaluate, or q to quit." << endl;
getline(cin,expression);
while(expression[0] != 'q') {
//try {
int result = calc.eval(istringstream(expression));
cout << result << endl;
//}
//catch { //will not compile, you have to finish this!
//
//}
cout << "Give a prefix expression to evaluate or q to quit." << endl;
getline(cin,expression);
}
return 0;
}

Answer 1

calc.eval(istringstream(expression)); passes a temporary instance to eval(), you'll need an lvalue.
Provide an extra variable for the stream
istringstream iss(expression);
and pass that one
int result = calc.eval(iss);