Re: Why does tr1::function throw?

From:

Peter Dimov <pdimov@gmail.com>

Newsgroups:

comp.lang.c++.moderated

Date:

Wed, 2 Dec 2009 15:53:16 CST

Message-ID:

<64e4bb03-3874-4e27-96cb-bb76396d52f9@b15g2000yqd.googlegroups.com>

On Dec 2, 7:18 am, Sebastian Hauer <ha...@psicode.com> wrote:

Hi,
Recently I had the need for an efficient polymorphic function wrapper
(aka delegate), so I looked at a few possible options such as
boost::function and FastDelegate. I also came across tr1::function
which I was blissfully unaware of until now, considering that the
original proposal was from late 2002 (see:http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/papers/2002/n1402.html).

While reading the proposal I noticed that a tr1::function can throw a
bad_function_call exception if no target was assigned.

I was just wondering for the rationale behind this design decision.
In my opinion the fact that it throws goes against C++'s notion
of "you only pay for what you use".

I'm interested in hearing what your thoughts are on this.

This behavior doesn't need to impose a cost. An std::function (it's
officially in C++0x) that hasn't been assigned a target may internally
store a target function that throws bad_function_call. It need not
test on every call (even though the test wouldn't impose much overhead
compared to the indirect call that follows.)

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