c++ - 计算器.cpp :48:24: error: invalid conversion from 'char' to 'const char*' [-fpermissive]

Question

这是我的披露:我正在做一个学校项目，在该项目中我使用链接堆栈来实现后缀计算器。

我今天的问题是我的代码中出现了一个奇怪的(对我来说)错误，我似乎无法解决。随着时间的推移，我可以弄清楚的逻辑是我需要处理的这些句法问题(而且我认为通过练习会自然而然地出现)。

无论如何，我想知道导致此错误的原因以及原因。根据我的研究，它与字符串和字符比较有关，但这是我知识的限制。

更新:我在代码中出现的错误旁边添加了一些星号

我在学校的编译器上得到的错误 block 如下:

In file included from Calculator.h:35:0,
                 from CalculatorMain.cpp:6:
Calculator.cpp: In member function 'void Calculator::performOp(const string&)':
Calculator.cpp:48:24: error: invalid conversion from 'char' to 'const char*' [-fpermissive]
/opt/csw/lib/gcc/i386-pc-solaris2.10/4.6.3/../../../../include/c++/4.6.3/bits/basic_string.tcc:214:5: error:   initializing argument 1 of 'std::basic_string<_CharT,    aits, _Alloc>::basic_string(const _CharT*, const _Alloc&) [with _CharT = char, _Traits = std::char_traits<char>, _Alloc = std::allocator<char>]' [-fpermissive]
Calculator.cpp:52:30: error: invalid conversion from 'char' to 'const char*' [-fpermissive]
/opt/csw/lib/gcc/i386-pc-solaris2.10/4.6.3/../../../../include/c++/4.6.3/bits/basic_string.tcc:214:5: error:   initializing argument 1 of 'std::basic_string<_CharT,    aits, _Alloc>::basic_string(const _CharT*, const _Alloc&) [with _CharT = char, _Traits = std::char_traits<char>, _Alloc = std::allocator<char>]' [-fpermissive]
Calculator.cpp:53:101: error: invalid conversion from 'char' to 'const char*' [-fpermissive]
/opt/csw/lib/gcc/i386-pc-solaris2.10/4.6.3/../../../../include/c++/4.6.3/bits/basic_string.tcc:214:5: error:   initializing argument 1 of 'std::basic_string<_CharT,    aits, _Alloc>::basic_string(const _CharT*, const _Alloc&) [with _CharT = char, _Traits = std::char_traits<char>, _Alloc = std::allocator<char>]' [-fpermissive]
Calculator.cpp:57:27: error: invalid conversion from 'char' to 'const char*' [-fpermissive]
/opt/csw/lib/gcc/i386-pc-solaris2.10/4.6.3/../../../../include/c++/4.6.3/bits/basic_string.tcc:214:5: error:   initializing argument 1 of 'std::basic_string<_CharT,    aits, _Alloc>::basic_string(const _CharT*, const _Alloc&) [with _CharT = char, _Traits = std::char_traits<char>, _Alloc = std::allocator<char>]' [-fpermissive]
Calculator.cpp:60:23: error: ISO C++ forbids comparison between pointer and integer [-fpermissive]
Calculator.cpp:61:27: error: invalid conversion from 'char' to 'const char*' [-fpermissive]
/opt/csw/lib/gcc/i386-pc-solaris2.10/4.6.3/../../../../include/c++/4.6.3/bits/basic_string.tcc:214:5: error:   initializing argument 1 of 'std::basic_string<_CharT,    aits, _Alloc>::basic_string(const _CharT*, const _Alloc&) [with _CharT = char, _Traits = std::char_traits<char>, _Alloc = std::allocator<char>]' [-fpermissive]
Calculator.cpp:64:23: error: ISO C++ forbids comparison between pointer and integer [-fpermissive]

我的计算器类中的代码(对于 calculator.cpp。头文件是单独的):

** @file Calculator.cpp */

#include <string>
#include <cassert>       
#include "Calculator.h"  

Calculator::Calculator(){
}  // end default constructor

bool Calculator::isOperator(const string& newEntry) {
    string ops[] = {"-", "+", "/", "*"};

    for (int i = 0; i < 4; i++) {
        if (newEntry == ops[i]) {
            return true;
        }
    }
    return false;
}

bool Calculator::isOperand(const string& newEntry) {
    if (!isOperator(newEntry) && newEntry != "(" && newEntry != ")") {
       return true;
    }
    return false;
}

int Calculator::compareOps(const string& op1,const string& op2) {
    if ((op1 == "*" || op1 == "/") && (op2 == "+" || op2 == "-")) {
        return -1; 
    }
    else if ((op1 == "+" || op1 == "-") && (op2 == "*" || op2 == "/")) {
        return 1; 
    }
    return 0;
}

void Calculator::performOp(const string& userExpression) { 
    string temp = userExpression;

    int i = 0;
    cout << "Error log: User expression is now " << temp << endl;
    while(temp[i] > temp.length()) {
        cout << "Error log: begin while loop" << endl;
        **if (isOperand(temp[i])) {**
            postfixString += temp[i];
        }

        else if **(isOperator(temp[i]))** {
            **while (!infixStack.isEmpty() && infixStack.peek() != "(" && compareOps(infixStack.peek(), temp[i]) <= 0) {**
                postfixString += infixStack.peek();
                infixStack.pop();
            }// end while
            infixStack.push(temp[i]);
        }

        **else if (temp[i] == "(") {**
            infixStack.push(temp[i]);
        }

        **else if (temp[i] == ")") {**
            while (!infixStack.isEmpty()) {

                if (infixStack.peek() == "(") {
                    infixStack.pop();
                    break;
                }

                postfixString += infixStack.peek();
                infixStack.pop();
            }//end while
        }//end last else if
        i++;
    }//end while

    while(!infixStack.isEmpty()) {
        postfixString += infixStack.peek();
        infixStack.pop();
    }

    cout << "The Postfix form of your expression is: " << postfixString << endl;
}

这是我测试这个类的文件:

#include <iostream>
#include <string>
#include "Calculator.h"

using namespace std;

int main()
{
    Calculator aCalculator;
    string userString = "1+1";
    cout << "Testing the Link-Based Stack:" << endl;
    aCalculator.performOp(userString);

   return 0;
}  // end main

Answer 1

您的某些函数需要字符串，但您传递的是单个字符。例如，isOperator 应该是:

bool Calculator::isOperator(char newEntry) {
    char ops[] = {'-', '+', '/', '*'};

    for (int i = 0; i < 4; i++) {
        if (newEntry == ops[i]) {
            return true;
        }
    }
    return false;
}

因为您这样调用它:isOperator(temp[i]) 并且 temp[i] 返回一个字符，而不是一个字符串。

还有许多其他地方需要混合使用字符和字符串。