c++ - 减去 std::strings 时出现编译器错误

Question

我是 C++ 的初学者，所以我只是在阅读文章和书籍时弄乱了一些东西。但是我花了 20 分钟一遍又一遍地重新阅读这篇文章，但我不知道它有什么问题。

#include <iostream>
#include <string>

using namespace std;

int main ()
{

    cout << "Hello there, this is your personal simple calculator.";
    cin.get();
    cout << "Type in what you want to do. (Addition, Subtraction, Multiplication, Division)"<< endl;
    string c;
    getline (cin, c);

    if (c == "Addition")
    {
        string a_1;
        string a_2;
        cout << "You chose addition. Press enter" << endl ;
        cin.get();
        cout << "Type in the first value: ";
        getline( cin, a_1);
        cout << "Type in the second value: ";
        getline (cin, a_2);
        cout << a_1 << " + " << a_2 << " = " << a_1 + a_2 << endl;
    }
    else
    {
        cout << "You spelled it wrong.";
        return 0;
    }
    if ( c == "Subtraction")
    {
        string s_1;
        string s_2;
        cout << "You chose subtraction. Press enter" << endl ;
        cin.get();
        cout << "Type in the first value: ";
        getline (cin, s_1);
        cout << "Type in the second value: ";
        getline (cin, s_2);
        cout << s_1 << " - " << s_2 << " = " << s_1 - s_2 << endl;
    }

}

我认为这是唯一的错误

42 83 C:\Users\Jason\Desktop\Lesson - Header Files\LH1.cpp [Error] no match for 'operator-' in 'first_argument - second_argument'

我不明白。加法符号有效，除减法外的所有符号都有效。所以我搞砸了其他事情

cout << first_argument << " - " << second_argument << " = " << first_argument - second_argument << endl;

但是减法部分工作正常。我不明白。请帮忙

Answer 1

string 可以处理文本。当您添加两个字符串时，它们会连接起来("2"+"2"=="22"，而不是 "4")。字符串没有operator-。

要处理 float ，请使用double。要处理整数，请使用 int:

double d1, d2;
//some output
cin  >> d1;
//some output
cin  >> d2;
cout << d1 << " - " << d2 << " = " << (d1-d2) << '\n';