c++ - 为什么我会收到此错误？二进制 '==' : no operator found which takes a left-hand operand of type 'std::string'

Question

我不知道为什么会出现此编译错误。我尝试了通常的方法来定义字符串，我也尝试了 std::string 但都没有用。另外我认为我尝试打印函数的方式可能存在问题。

#include "stdafx.h"
#include <iostream>
#include <cmath>
#include <string>

float userInput1() // Defines the inputs from the user
{
using namespace std;
    cout << "Please enter first number" << endl;
    float number1;
    cin >> number1;

    return number1;
}

float userInput2() // Defines the inputs from the user
{
using namespace std;
    cout << "Please enter second number" << endl;
    float number2;
    cin >> number2;

    return number2;
}

std::string getOperation()
{
using namespace std;
    cout<<"Please enter the operator. + - * /" << endl;
    std::string userOperator;
    cin>>userOperator;

    return userOperator;
}

float computeValue(float value1, float value2, std::string operation)
{
using namespace std;

    if(operation == '+')
    {
        cout<< value1 + value2<< endl;
    }else if(operation =='-')
    {
        cout<< value1 - value2<< endl;
    }else if(operation =='*')
    {
        cout<< value1 * value2<< endl;
    }else if(operation == '/')
    {
        cout<< value1 / value2<< endl;
    }else
    {
        cout<< "Please enter: + - * /"<< endl;
    }
    return 0;
}


int main(){
using namespace std;

    computeValue(userInput1(), userInput2(), getOperation());

return 0;
}

Answer 1

您正在使用 == 运算符将 std::string 与 char 值进行比较。标准库没有在这两种类型之间定义相等运算符。有效比较列表在这里:http://www.cplusplus.com/reference/string/string/operators/

最简单的方法是使用 " 而不是 ' 将 char 值转换为 char*，像这样:

 if(operation == '+')

成为

 if(operation == "+")