如何解决此错误:跳转到case标签交叉初始化

Question

我的计算器代码中有以下错误，不知道如何纠正它。请给我一些建议。

错误:错误:跳转到大小写标签[-fpermissive]|错误:交叉初始化'int sum'|错误:'exit'未在此范围内声明|

代码:

#include <iostream>
#include <cmath>
using namespace std;         
void display_menu(); 
int get_menu_choice();
void get_two_numbers(int &a, int &b);
int add(int a, int b);
int subtract(int a, int b);

int main()
 {
 int choice;
  do
   {
    display_menu();
    choice = get_menu_choice();
    int x, y;
    switch (choice)
    {
        case 1: get_two_numbers(x, y);
                int sum = add(x, y);
                cout << x << " + " << y << " = " <<  sum << endl;
                break;
        case 2: get_two_numbers(x, y);
                int diff = subtract(x, y);
                cout << x << " - " << y << " = " <<  diff << endl;
                break;
        default:;
    }
     } while (choice != 3);
     cout << "Good bye...now." << endl;
     return 0;
       }

 void display_menu()
  {
   cout << endl;
   cout << "Simple Calculator Menu" << endl;
   cout << "----------------------" << endl;
   cout << " 1. Addition (+) " << endl;
   cout << " 2. Subtraction (-) " << endl;
   cout << " 3. Quit to exit the program" << endl;
   cout << endl;
  }
 int get_menu_choice()
  {
   int choice;
   cout << "Enter your selection (1, 2, or 3): ";
   cin >> choice;
  while(((choice < 1) || (choice > 3)) && (!cin.fail()))
   {
    cout << "Try again (1, 2, or 3): ";
    cin >> choice;
    }
  if (cin.fail())
    {
      cout << "Error: exiting now ... " << endl;
      exit(1);
     }
   return choice;
    }
 void get_two_numbers(int &a, int &b)
  {
    cout << "Enter two integer numbers: ";
    cin >> a >> b;
  }

 int add(int a, int b)
  {
   return (a + b);
  }
 int subtract(int a, int b)
  {
    return (a - b);
  }

Answer 1

在case语句中声明新变量而不创建封闭作用域:

switch (choice)
{
    case 1: get_two_numbers(x, y);
            //* vv here vv *
            int sum = add(x, y);
            //* ^^ here ^^ */
            cout << x << " + " << y << " = " <<  sum << endl;
            break;
    case 2: get_two_numbers(x, y);
            //* vv here vv */
            int diff = subtract(x, y);
            //* ^^ here ^^ */
            cout << x << " - " << y << " = " <<  diff << endl;
            break;
    default:;
}

修复方法:

switch (choice)
{
    case 1:
        {
            get_two_numbers(x, y);
            int sum = add(x, y);
            cout << x << " + " << y << " = " <<  sum << endl;
        }
        break;
    case 2:
        {
            get_two_numbers(x, y);
            int diff = subtract(x, y);
            cout << x << " - " << y << " = " <<  diff << endl;
        }
        break;
    default:
        break;
}

当然，括号和缩进的确切格式取决于您。

Answer 2

开关的"case"不创建作用域，因此，正如错误提示的那样，如果选项不为1，则跳过了"sum"的初始化。

你需要在switch之外声明sum和diff，或者为每种情况创建{}块

Answer 3

应该在switch语句之外而不是在case中声明变量。特别是sum和diff在您的代码中有问题。在switch语句之外初始化这些变量之后，您就可以设置它们的值了。