CODEWITHSUNDEEP
c++exception
Vijay
Vijay

Reputation: 67211

exception handling a constructor

This was an interview question of me.

Surprisingly i never thought of this kinda question to myself.

can we have exception handling inside a constructor c++?

in tense and not thinking much i said "yes we could probably do it in a constructor.lets say we are allocating some memory using new operator to a pointer member and it throws a bad alloc exception,in this way there is a possibility of exceptions being raised"

Then later i thought that constructors can never return a value.So how can an exception inside a constructor caught.now i am asking this to myself!

can anybody pls help me to come out of this confusion?

Upvotes: 19

Views: 20643

Answers (4)

Eswaran Pandi
Eswaran Pandi

Reputation: 670

Exception handling and return type are completely different. when the program find the exception in constructor, it throws the exception to nearly by catch block [if used] or thrown to caller (main()). in this case, we have catch block in constructor and exception handled by it. Once exception handled, the remaining statement in the constructor/function will be started executing. see the below example,

class A
{
  public:
   A(){
        printf("Hi Constructor of A\n");        
   try
   {
        throw 10;
   }
   catch(...)
   {
       printf("the String is unexpected one in constructor\n");
   }
   printf("Hi Constructor of A\n");
 }
   ~A(){
   printf("Hi destructor of A\n");
 }
};

int main()
{

try{
    A obj ;
   throw "Bad allocation";
}
catch(int i)
{
    printf("the Exception if Integer is = %d\n", i);
}
 catch(double i)
{
    printf("the Exception if double is = %f\n", i);
}
 catch(A *objE)
{
    printf("the Exception if Object \n");
}
 catch(...)
{
    printf("the Exception if character/string \n");
}
printf("Code ends\n");
return 0;
}

This produce Output:

 Start: Constructor of A
 the String is unexpected one in constructor
 End: Constructor of A
 Hi destructor of A
 the Exception if character/string 
 Code ends

Upvotes: 1

Laas
Laas

Reputation: 6058

C++ has try-catch clauses similar to those of other languages. A tutorial can be found online: http://www.cplusplus.com/doc/tutorial/exceptions/

EDIT: example turned into fully working code

#include <iostream>
using namespace std;

class A
{
public:
  void f(){
    throw 10;
  }

  A(){
    try{
      f();
    }
    catch(int e){
      cout << "Exception caught\n";
    }
  }
};

int main (int argc, const char * argv[])
{

  A a;
  return 0;
}

This produces output:

Exception caught

Upvotes: -2

dubnde
dubnde

Reputation: 4431

See this GOTW Constructor Failures question which addresses your query somewhat and goes on to say it is a waste of time.

Upvotes: 14

helpermethod
helpermethod

Reputation: 62145

Constructors don't have a return type, so it's not possible to use return codes. The best way to signal constructor failure is therefore to throw an exception. If you don't have the option of using exceptions, the "least bad" work-around is to put the object into a "zombie" state by setting an internal status bit so the object acts sort of like it's dead even though it is technically still alive.

You would catch the exception in the calling code, not within the constructor.

See How can I handle a constructor that fails? for further details (actually, I'd suggest reading the whole page about exception handling, truly enlightening).

Upvotes: 2

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