Re: return type of bind object

From: Faheem Mitha (yg-boost-users_at_[hidden])
Date: 2003-03-18 22:50:06

On Sat, 15 Mar 2003 12:22:34 -0500, Douglas Gregor <gregod_at_[hidden]> wrote:
> On Friday 14 March 2003 09:45 pm, Faheem Mitha wrote:
>> Dear People,
>>
>> I have defined a function as follows.
>>
>> double accept_prob_fn(const double x, const double be, const double
>> 			          theta, const double a, const double
>> 			          b);
>>
>> Now, I define the temporary function object
>>
>> bind(accept_prob_fn, _1, be, theta, a, b);
>>
>> So this function object would return the first argument, x. However, I
>> want to save this function object to a function object called foo,
>> say.
>>
>> So I want to do something like
>>
>> Type foo = bind(accept_prob_fn, _1, be, theta, a, b);
>>
>> However, the problem I have is that I'm not sure what type foo should
>> be. 
[snip]

> The type is "unspecified", may vary from compiler to compiler, and is hideous 
> on every compiler. For these reasons, it's intentional that you can't name 
> the type directly (see 
> http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/papers/2003/n1438.htm, section 
> IIIB).
> 
> However, you can include <boost/function.hpp> and make the Type:
>   boost::function<double(double x)>
> 
> (or, if your compiler isn't up to par, boost::function1<double, double>). See 
> the Boost.Function documentation for more information about the Function 
> library.

Thanks, this is very helpful.  boost::function<double(double x)> works
fine with gcc 3.0. Nice simple syntax; takes a double argument and
returns a double.

A small comment. The paper you quote above
(http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/papers/2003/n1438.htm)
does clearly state that the return type is unspecified, but does not
say anything further (as far as I can see) about how to handle
assignments like above, and it is presumably not part of the official
documentation anyway.

Also, the bind documentation itself (again, as far as I can see) does
not say anything about how to handle assignments. Therefore, would it
not be a good idea to include an example in the bind documentation
showing how assignments should be handled? In any case, more and
varied examples are always a good thing.

Bear in mind I don't have much C++ experience, so the needed syntax
may have been quite clear to a more experienced person.

Thanks again for your help.
                                                      Faheem.