Re: Challenging GotW 66's moral
George2 <george4academic@yahoo.com> wrote:
Hello everyone,
In GotW #66, one of the moral is the exception handler of constructor
should not do any like resource free task. I do not agree. Here is the
quoated moral and my code to prove this moral will have memory leak.
Anything wrong with my analysis?
Your idea is covered in the article:
"--But wait!" I hear someone interrupting from the middle of the
room. "I don't agree with Moral #1. I can think of another possible
use for constructor function-try-blocks, namely to free resources
allocated in the initializer list or in the constructor body!"
Sorry, nope. After all, remember that once you get into your
constructor try-block's handler, any local variables in the
constructor body are also already out of scope, and you are
guaranteed that no base subobjects or member objects exist any more,
period. You can't even refer to their names.
Maybe the output to the following will help:
class B {
public:
B() { cout << "B()\n"; }
~B() { cout << "~B()\n"; }
void foo() { cout << "B::foo()\n"; }
};
class A
{
private:
B b;
public:
A() try: b()
{
throw -1;
}
catch (...)
{
b.foo();
}
};
int main() {
try {
A a;
}
catch ( ... ) { }
}
Note that B::foo() is called *after b's destructor has already been
called.* Thus invoking undefined behavior.
http://www.gotw.ca/gotw/066.htm
Moral #1: Constructor function-try-block handlers have only one
purpose -- to translate an exception. (And maybe to do logging or some
other side effects.) They are not useful for any other purpose.
[Code]
class A
{
private:
int* p;
public:
A()
try
{
p = new int[10];
// there are some other exceptions here
}
catch (bad_alloc)
{
// do not delete since bad_alloc means memory pointed by p is
not allocated
}
catch (...)
{
// if we do not delete p, there will be memory leak
// at this point, we are conflicting with Gotw 66's moral 1
if (p) delete[] p;
'p' doesn't exist once you get in the catch block. Yes, in your example
it happens to still point to the right place, but there is no guarantee
that this is true. Frankly, I'm surprised the code even compiled.
}
}
[/Code]
thanks in advance,
George