C ++中的异常和return语句

Question

我是C ++编程的新手，我试图了解C ++中的异常。 我做了一个简单的模型情况，显示了我不理解的东西（我希望我不会把代码弄得太乱）。 我用很少的方法制作了2个基本类（CPerson类基本上是链表）。 我的答案是如何异常停止当前任务。 我可以调用异常，但是任务继续进行，并在程序中造成一些混乱。

#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <iostream>
using namespace std;

class CPerson{
 public:
   CPerson(){
     p_next_person = NULL;
   }
   CPerson* p_next_person; // pointer to next person in linked list
   int Postcode(); // returns postcode of person
   friend ostream& operator<<(ostream& stream, const CPerson& pers){
      cout << pers.ID << pers.postcode;
      return stream;
   }
   char* ID;
   int   postcode;
};
//---------------------------------------------------------------
class CPeople{
 public:
   CPeople(){
     first_person = NULL;
   }
   CPerson Person( const char*  personID); // finds person by ID and returns it
   bool NewPerson( const char*  personID, int person_postcode); // add new person
   CPerson* first_person ; // start of linked list

};

//-----------------------------------------------------------------
int CPerson::Postcode(){
    return postcode;
}
//-----------------------------------------------------------------
CPerson CPeople::Person( const char*  personID){
  CPerson* now;
  now = first_person;
  while(now != NULL){
    if(strcmp(now->ID,personID)==0){
      break;
    }
      now = now->p_next_person;
  }
// our person is in now (or now is NULL - if person wasn't found).
    try{
      if(now == NULL ){
        throw 0;
        // I need to stop code here
      }else return *now;
    }
    catch (int e)
    {
      cout << "bla bla " << e << '\n';
    }
  }
//----------------------------------------------------------

int main(){
 CPeople people;
 int i = 0;
 people.NewPerson( "JAck", 100 );
 people.NewPerson( "Josh", 100 );
// Bob is not in people right now.
  i = people.Person("BOB").Postcode();
  cout << i;
 // gives exception, which is nice. but it also changes i to some nonsence .. how do I fix it ?
  cout << people.Person ( "BOB" );
// gives exception, which is nice. but also gives segmentation fault. how do I fix it ?
}

Answer 1

您在'throw周围有try块。 try块应该在调用该函数的位置周围，并且应该被catch 。 因此，您的功能将更改为：

CPerson CPeople::Person( const char*  personID){
  CPerson* now;
  now = first_person;
  while(now != NULL){
    if(strcmp(now->ID,personID)==0){
      break;
    }
    now = now->p_next_person;
  }
  // our person is in now (or now is NULL - if person wasn't found).

  if (now == NULL ){
    throw 0;
    // I need to stop code here
    }
    else return *now;
  }

和main看起来像：

int main(){
  try {
    CPeople people;
   int i = 0;
   people.NewPerson( "JAck", 100 );
   people.NewPerson( "Josh", 100 );
   // Bob is not in people right now.
   i = people.Person("BOB").Postcode();
   cout << i;
   // gives exception, which is nice. but it also changes i to some nonsence .. how do I fix it ?
   cout << people.Person ( "BOB" );
   // gives exception, which is nice. but also gives segmentation fault. how do I fix it ?
   }
   catch (int e)
   {
      cout << "bla bla " << e << '\n';
   }
}

一旦通知书catch遇到，后下面的语句catch会被执行。 这就是为什么你应该catch出函数的定义。

Answer 2

像这样的代码

try{
  if( now == NULL ){
    throw 0;
       // I need to stop code here
  } else return *now;
} catch (int e) {
  cout << "bla bla " << e << '\n';
}

完全错了。 无法耸耸肩（“ bla bla”）继续执行，就好像什么都没发生一样。 您可以确保在捕获中满足所有紧急情况，或者应该在更高级别捕获异常。 此处：函数的返回值没有定义，这会在调用CPeople::Person地方引起麻烦。

您可以使用try - catch包围这些呼叫； 在函数中忽略它们，然后抛出。

不要抛出0 。 使用能够保存某些信息的对象。 按价值投掷，按参考捕获。