异常处理Exception Handling

异常是指程序运行期间发生的不正常情况，如new无法获得所需内存、数组下标越界、运算溢出、除数为0、无效函数参数以及打开不存在的文件等。异常处理就是指对程序执行过程中发生的异常进行适当的处理，避免程序出现丢失数据活破坏系统运行等灾难性的后果。

其实异常处理可以用 if语句代替，但是会使代码难以维护。

举个例子

使用 if语句的代码：

// if statement
#include <iostream>
using namespace std;
int main()
{
    // Read two intergers
    cout << "Enter two integers: ";
    int number1, number2;
    cin >> number1 >> number2;
    if (number2 != 0)
    {
        cout << number1 << " / " << number2 << " is ";
        cout << (number1 / number2) << endl;
    }
    else
    {
        cout << "Divisor cannot be zero" << endl;
    }
    system("pause");
    return 0;
}

使用异常处理的代码：

// Exception Handling
#include <iostream>
using namespace std;

int main()
{
    // Read two intergers
    cout << "Enter two integers: ";
    int number1, number2;
    cin >> number1 >> number2;
    try
    {
        if (number2 == 0)
            throw number1;

        cout << number1 << " / " << number2 << " is " << (number1 / number2) << endl;
    }
    catch (int e)
    {
        cout << "Exception: an integer " << e << " cannot be divided by zero" << endl;
    }
    cout << "Execution continues ..." << endl;
    system("pause");
    return 0;
}

 异常处理的模版：

try{
 ……                                           //try程序块
     if  err1  throw xx1
     ……
     if  err2  throw xx2
     ……
     if  errn  throw xxn
}
catch(type1  arg){……}            //异常类型1错误处理
catch(type2  arg){……}              //异常类型2错误处理
catch(typem  arg){……}          //异常类型m错误处理

举个例子：

#include <iostream>
using namespace std;
void temperature(int t)
{
  if (t == 100)
    throw "feidian";
  else if (t == 0)
    throw "bingdian";
  else
    cout << "the temperature is ..." << t << endl;
}
int main()
{
  try
  {
    temperature(10);
    temperature(50);
    temperature(100);
  }
  catch (const char *s)
  {
    cout << s << endl;
  }
  system("pause");
  return 0;
}