前缀计算器编译有问题

Question

我在一个程序中遇到了一些麻烦，该程序应该采用命令行表达式并将其解释为正常的数学表达式。这是我得到的错误:

driver.cpp: In function ' int main(int, char**) ':driver.cpp:17:57:错误:调用' prefixcalculator::eval(std::istringstream) '没有匹配的函数

driver.cpp:17:57:备注:候选人是:PrefixCalculator.h:33:3: note: T PrefixCalculator::eval(std::istringstream&) [with T = int, std::istringstream = std::basic_istringstream]

PrefixCalculator.h:33:3:注意:从' std::istringstream{也称为std::basic_istringstream} '到' std::istringstream&{又名std:: basic_istringstream&} '

我不明白这个错误试图向我暗示什么。有什么建议吗?我稍后会添加异常，所以它们现在被注释掉了。

这是代码:

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));将一个临时实例传递给eval()，您将需要一个左值。
为流

提供一个额外的变量

 istringstream iss(expression); 

并传递

 int result = calc.eval(iss);