Re: [boost] Preliminary Review Request: boost-memory

From: Scott McMurray (me22.ca+boost_at_[hidden])
Date: 2008-05-06 23:11:06

On Tue, May 6, 2008 at 10:47 PM, shiwei xu <xushiweizh_at_[hidden]> wrote:
> Your question is very interesting. I thought it before. In fact I don't know
>  a perfect solution. If the constructor of an object throws, should the
>  destructor be called?
>
>  class Foo
>  {
>  private:
>     A m_a;
>     B m_b;
>     C m_c;
>
>  public:
>     Foo() {
>         m_a.init();
>         m_b.init();
>         thow std::exception("error");
>         m_c.init();
>     }
>     ~Foo() {
>         ...
>     }
>  };
>
>  Suppose we initialized m_a and m_b. m_c was uninitialized when the exception
>  throws. If the destructor is called, it may cause a crash. If the destructor
>  isn't called, the allocated memory of m_a and m_b will be leaked.
>
>  I choose to call the destructor because I think that a crash is easy to be
>  detected and be solved.
>

This is a non-point.  Go read the rules for which destructors are
called when exceptions are thrown in different places.  If A and B are
written properly, there's no problem.  And in any case, throwing in a
constructor for an automatic variable doesn't call the destructor, so
no matter what you think, you shouldn't call the destructor, since
it'll break expectations.

Here's a simple counter-example:
int *bar() {
    throw 0;
    return new int(42);
}
struct foo {
    int *p;
    foo() : p(bar()) {}
    ~foo() { delete p; }
};

Quite obviously UB to call that destructor.