Re: Inconsistent behaviour for certain conditional expressions

"Doug Harrison [MVP]" <>
Wed, 09 Aug 2006 16:09:50 -0500
On 9 Aug 2006 13:12:35 -0700, "redblue" <> wrote:

The example I concocted was for illustration only.

The place where using this kind of construct is useful is when doing a
type of RAiI (Resource Aquisition is Initialization):

class Lock
Resource& r;
operator bool();

class Resource
void Release();
Lock Acquire();

Now, normally you would write code like this:

Resource r;
const Lock l = r.Aquire();
if (l)

The problem is that the Lock l variable is in the scope outside the
conditional block. This means that you can't make the automatic
destructor call free the resource at the end of the conditional block.

Now observe this code:
if (const Lock l = r.Aquire())


Not only is the code smaller, the lock only exists in the scope of the
conditional block.

What I had in my older code was a expression that looked like:
if (const Lock l(r))....

which I think is better looking and avoids the problematic style of
using the the assignment operator in a conditional expression (the
classic error of writing a=b, instead of a==b)

If somebody can suggest a more elegant way of doing RAiI than the one I
illustrated above, I am all ears.

Most of the lock usage I've seen (not to mention my own usage) is
unconditional, so it looks like this:

 Lock lk(mx); // mx is a mutex

The failure mode is to throw an exception. Conditional usage is done like

 Lock lk(mx, false);
 if (lk.IsLocked())

Note that MFC gets this backwards and defaults the ctor's "lock now"
parameter to false. I've always wondered how many people inadvertently do
the following:

 CSingleLock lk(mx);
 ... proceed as if locked

Doug Harrison
Visual C++ MVP

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