Question

我遇到一个应该采用命令行表达式并将其解释为正常数学表达式的程序有些麻烦。这就是我得到的错误：

driver.cpp：在函数'int main（int，char **）'中： driver.cpp：17：57：错误：没有匹配函数来调用'PrefixCalculator :: eval（std :: istringstream）'

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

PrefixCalculator.h:33：3：注意：参数1从'std :: istringstream {aka std :: basic_istringstream}'到'std :: istringstream＆amp; {aka std :: basic_istringstream＆amp;}'

我无法理解错误试图向我建议的内容。有任何修复建议吗？我稍后会添加例外，所以现在它们已被注释掉了。

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));将临时实例传递给eval()，您需要一个左值。
为流提供额外的变量

 istringstream iss(expression);

并传递那个

 int result = calc.eval(iss);

前缀计算器编译有问题

1 个答案: