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Re: Using swap idiom for exception safety in a constructor

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On Wednesday, April 4, 2012 2:52:52 AM UTC+2, Alex wrote:
> Hi, I want to ensure exception safety inside a constructor by creating a temporary object and then swapping it with *this:
> 
> class Test {
> public:
>     Test(int value) : p(0)
>     {
>         Test tmp;
>         tmp.initialize(value); // In case of exception, tmp is destroyed
>         swap(tmp);
>     }
> 
>     ~Test() 
>     { 
>         if (p) delete p; 
>     };
> 
>     swap(Test& other) {...}
> 
> private:
>     Test() : p(0) {} 
>     
>     void initialize(int value) 
>     { 
>        p = new X(value, ...);
>        // and more members...
>     }
> 
> private:
>     X * p;
>     // And many more members...
> };
> 
> I wanted to know:
> a) Is this a common approach? 
> b) What other options do you recommend when this is not possible (e.g. Test has pure-virtual methods so I can't instantiate the tmp object)?
> 
> Thanks

(If I understand you correctly... I presume you have e.g. Y* p2, Z* p3 etc. as a member of Test)

This is not good, and I would hazard it's not common either. It's bad because you just moved the leak from "this" to tmp.Initialize: e.g. if "new p2" fails, p1 is leaked.

One correct approach is e.g.:

class Test
{
X* p1; Y* p2;
public:
Test(A a, B b) : p1(0), p2(0)
{
// can-throw zone here
  auto_ptr<X> P1(new X(a)); // unique_ptr in C++11?
  auto_ptr<Y> P2(new X(b));
// no-throw zone here
  p1 = P1.release();
  p2 = P2.release();
}
~Test()
{ // look, ma, no if-s!
  delete p1; delete p2;
}
};

In short, use of auto_ptr in the constructor ensures proper cleanup.

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Using swap idiom for exception safety in a constructor Alex <acamposr@gmail.com> - 2012-04-03 17:52 -0700
  Re: Using swap idiom for exception safety in a constructor Howard Hinnant <howard.hinnant@gmail.com> - 2012-04-03 19:52 -0700
  Re: Using swap idiom for exception safety in a constructor goran.pusic@gmail.com - 2012-04-04 05:07 -0700
  Re: Using swap idiom for exception safety in a constructor "Alf P. Steinbach" <alf.p.steinbach+usenet@gmail.com> - 2012-04-04 23:19 +0200

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