Re: [boost] std::map::find() wrapper

Subject: Re: [boost] std::map::find() wrapper
From: Rob Riggs (rob_at_[hidden])
Date: 2011-04-30 17:18:31

On 04/30/2011 06:55 AM, Olaf van der Spek wrote:
> exceptions are for exceptional cases

That oft-used bromide lacks any useful information about when the use of 
exceptions is appropriate.

Exceptions should be used (thrown) when contracts are violated or 
invariants cannot be maintained.  The only time they should not be used 
in these cases is when checking the invariants would impact performance 
to such a degree that allowing undefined behavior may be preferable.  
And in this case, do what map and vector do: provide a checked and 
unchecked version.

A precondition for at() is that the key exists in the map.  It is 
perfectly legitimate for at() to throw an exception for violating its 
precondition.

And /it is perfectly legitimate to make use of this behavior in one's 
code/.  That is, there is nothing wrong with allowing an exception to be 
thrown if you can afford the stack unwinding.

Two bits of code:

if (foo.good()) {
     do_something_with(foo);
}

and

try {
     do_something_with(foo);
} catch (not_good&) {}

Both are valid.  And sometimes the latter is most appropriate.

Personally, I tend to be more averse to functions that return bare 
pointers.  Given the choice of a function that returns a bare pointer 
which may be NULL or using at() & try/catch, I would probably choose 
at() first.

Then I might try a solution with boost::optional.  (I just would prefer 
that optional::operator*() threw a logic_error for uninitialized values 
rather than declare its behavior as "undefined".)

Rob