C++ - Exception in Constructor

Question

I have a problem. I must throw an exception in the constructor One() but do not know how do I suppose to catch it. Can someone suggest something? I have tried this method: Throwing exceptions from constructors , What happens if a constructor throws an exception?

My code:

class One
{
    int a, b;

public:

    One()
    {
        a = 7;
        b = 0;
        if (b == 0)
        {
            throw "except";
        }       
    }

};
int main()
{
    One j;
    try 
    {
        cout << "good"; 
    }
    catch(const char *str)
    {
        cout << str;
    }
}

You should put `One j;` into the `try-catch` block to catch `One`'s exceptions. — vahancho, Feb 26 '20 at 13:39
The operation that throws the exception needs to go inside the `try` block. It's you telling the compiler, "Hey I'm going to try something. Here's a list of bad things that might happen (various catches). If they do happen, do X, Y, Z..." — JohnFilleau, Feb 26 '20 at 13:40
@NutCracker - What? You have no other way to indicate construction failure. The bad idea is throwing in a **destructor**. — StoryTeller - Unslander Monica, Feb 26 '20 at 13:40
@NutCracker why bad idea? If construction fails, I wouldnt know how to signal that if not with an exception. — 463035818_is_not_an_ai, Feb 26 '20 at 13:40
@StoryTeller-UnslanderMonica there are other ways but all that come to my mind are really bad ideas ;) (like eg using a `isProperlyConstructed` flag) — 463035818_is_not_an_ai, Feb 26 '20 at 13:42
@idclev463035818 - That's not a way, that's a weaseling around a lie. Construction ran to completion, the object is done. If it can't be created in a valid state, it shouldn't exist. — StoryTeller - Unslander Monica, Feb 26 '20 at 13:44

score 6 · Answer 1 · answered Feb 26 '20 at 13:41

6

Place the variable definition inside the try block:

try 
{
    One j;
    std::cout << "good"; 
}
catch(const char *str)
{
    std::cout << str;
}

answered Feb 26 '20 at 13:41

Erlkoenig

2,664
1
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18

score 2 · Answer 2 · answered Feb 26 '20 at 13:44

First of all, don't throw non exception. 2. If you call constructor inside the try block, you can catch it then.

#include <iostream>
#include <stdexcept>

class One
{
    int a, b;
public:
    One():
     a(7),
     b(0) 
   {
        if (b == 0) {
            throw std::runtime_error("except");
        }       
   }

};

...

try { 
   One j; 
   std::cout << "good" << std::endl; 
} catch(std::exception& e) { 
   std::cerr << e.what() << std::endl; 
}

score 0 · Answer 3 · answered Feb 26 '20 at 13:49

Another solution if you don't want to have the whole code inside a try..catch block:

int main()
{
  One* j = nullptr;
  try 
  {
      j = new One;
      cout << "good"; 
  } catch (const char *str)
  {
      cout << str;
      return 0;
  }
  // continue with object in j ...
}

And of course you should a smart pointer in this case:

int main()
{
  std::unique_ptr< One> j;
  try 
  {
      j.reset( new One());   // or use std::make_unique<>()
      cout << "good"; 
  } catch (const char *str)
  {
      cout << str;
      return 0;
  }
  // continue with object in j ...
}

C++ - Exception in Constructor

3 Answers3