C ++异常try-block

Question

我有一个Fraction类程序。我决定添加异常块try-catch。但是当我写道：

Fraction h;
try
{
  cin >> h;
}
catch(invalid_argument& e)
{
 cout << e.what() << endl;
} 

它不起作用。程序崩溃了。 但如果我改变这行代码：

try
{
  Fraction h(1, 0);
}
catch(invalid_argument& e)
{
  cout << e.what() << endl;
} 

效果很好。
请帮助我理解为什么它以这种方式工作。感谢。

这是我的代码：

Fraction.h

#pragma once
#include "stdafx.h"
#include <iostream>
#include <sstream>
#include <string>
#include <vector>
#include <fstream>

using namespace std;

int GreatestCommonDivisor(int m, int n); //division method to find the 
greatest common divisor

class Fraction;                                            //
ostream& operator<<(ostream& out, const Fraction &fra);    //
istream& operator>>(istream& in, Fraction& fra);    //The three act meet the 
logic compilation and declare the VC6.0

class Fraction {
private:
  int fenzi, fenmu;                            //Molecular, the denominator
public:
  Fraction() {}
  ~Fraction() {}
  Fraction(int fenzi, int fenmu);
  Fraction operator+(const Fraction &fra)const;
  Fraction operator-(const Fraction &fra)const;
  Fraction operator*(const Fraction &fra)const;
  Fraction operator/(const Fraction &fra)const;
  friend ostream& operator<<(ostream& out, const Fraction& fra);//i/O must use a heavy overload
  friend istream& operator>>(istream& in, const Fraction& fra);
  void setValue(int mu, int zi);
  void error_print()
  {
      cout << "The denominator can not be 0!" << endl;
  }
};

Fraction.cpp

#include "stdafx.h"
#include <iostream>
#include <sstream>
#include <string>
#include <vector>
#include <fstream>
#include <stdexcept>
#include "Fraction.h"

using namespace std;

int GreatestCommonDivisor(int m, int n)
{
  int r;
  if (n == 0) throw invalid_argument("Error");
  else
  {

    do
    {
        r = m % n;
        m = n;
        n = r;
    } while (r != 0);
}
//}
return m;
}

Fraction::Fraction(int zi, int mu) :fenzi(zi), fenmu(mu) {

  if (mu == 0)
  {
      throw invalid_argument("Error");
  }


/*try
{
    if (mu == 0) {
        //cout << "The denominator can not be 0!" << endl;
        //exit(0);
    }
}
catch(...)
{
    cout << "Bad";
}*/
}
ostream& operator<<(ostream& out, const Fraction &fra) {
  if (fra.fenmu<0)
     out << "(-" << fra.fenzi << "/" << -fra.fenmu << ")";        //Negative 
  scores
  else if (fra.fenzi<0)
     out << "(-" << -fra.fenzi << "/" << fra.fenmu << ")";
  else if (fra.fenmu == 1)
     out << fra.fenzi;                                    //Integer show
  else
     out << fra.fenzi << "/" << fra.fenmu;                    //Positive 
  scores
 return out;
}

istream& operator>>(istream& in, Fraction& fra) {
   int mu, zi;
   cout << "Please enter the denominator and numerator" << endl;
   in >> mu >> zi;
   fra.setValue(mu, zi);
   return in;
}
void Fraction::setValue(int mu, int zi) {
    fenmu = mu;
    fenzi = zi;
}
Fraction Fraction::operator+(const Fraction &fra)const {
    int a = fenzi, b = fenmu;    
    a = fra.fenmu*fenzi + fra.fenzi*fenmu;
    b *= fra.fenmu;
    int g = GreatestCommonDivisor(a, b);
    if (g != 1)
    {
       a /= g;
       b /= g;
    }
    return Fraction(a, b);
}
Fraction Fraction::operator-(const Fraction &fra)const {
    int a = fenzi, b = fenmu;
    a = fra.fenmu*fenzi - fra.fenzi*fenmu;
    b *= fra.fenmu;
    int g = GreatestCommonDivisor(a, b);
    if (g != 1)
    {
       a /= g;
       b /= g;
    }
    return Fraction(a, b);
}
Fraction Fraction::operator*(const Fraction &fra)const {
    int a = fenzi, b = fenmu;
    a *= fra.fenzi;
    b *= fra.fenmu;
    int g = GreatestCommonDivisor(a, b);
    if (g != 1)
    {
       a /= g;
       b /= g;
    }
    return Fraction(a, b);
}
Fraction Fraction::operator/(const Fraction &fra)const {
    return *this*Fraction(fra.fenmu, fra.fenzi);
}

div.cpp

// Drob.cpp: определяет точку входа для консольного приложения.
 //

  #include "stdafx.h"
  #include "Fraction.h"

  int main()
  {
   /*Fraction a(-1, 24);
   Fraction b(1, 4);
   Fraction c = a + b;
   Fraction d = a - b;
   Fraction e = b - a;
   Fraction f = a*b;
   Fraction g = a / b;
   cout << "a= " << a << endl
    << "b= " << b << endl
    << "a+b=" << c << endl
    << "a-b=" << d << endl
    << "b-a=" << e << endl
    << "a*b=" << f << endl
    << "a/b=" << g << endl;*/

   Fraction h;
   Fraction i;
   try
   {
      //Fraction a(1, 0);
      cin >> h;
      cout << h << endl;
      cin >> i;
      cout << i << endl;
   }
   catch(invalid_argument& e)
   {
      cout << "Erroro" << endl;
      cout << e.what();
   }
   Fraction c = h + i;
   Fraction d = h - i;
   Fraction e = h - i;
   Fraction f = h * i;
   Fraction g = h / i;
   cout << "a= " << h << endl
    << "b= " << i << endl
    << "a+b=" << c << endl
    << "a-b=" << d << endl
    << "b-a=" << e << endl
    << "a*b=" << f << endl
    << "a/b=" << g << endl;
   system("pause");
   return 0;
 }