Re: [Boost-users] Another lambda question

From: Maximilian Wilson (wilson.max_at_[hidden])
Date: 2004-09-01 18:23:39

On Wed, 1 Sep 2004 22:11:28 +0300, Peter Dimov <pdimov_at_[hidden]> wrote:
> Maximilian Wilson wrote:
> > I am getting another compiler error when trying to use the lambda
> > library. Current experiment is string extraction. I'm trying to create
> > a lambda(x) that pushes any char passed to it onto a particular
> > string.
> >
> >      string s2;
> >      void (std::string::*ft)(char) = &std::string::push_back;
> >      // extract function pointer because push_back is overloaded
> >      char c = 'n';
> >      bind<void>(ft, var(s2), _1, 1)(c);
> 
>     bind( ft, var(s2), _1 )( c );
> 
> ft has two arguments, 'this' and a char. No need for the trailing '1'.
> 
> Try
> 
>     (s2 += _1)( c );
> 
> too.

That's true about the two arguments--I used the wrong function
signature, so the extraneous 1 will be ignored. That still can't be
the whole solution, though, because that would be a runtime error and
not a compiler error in lambda. You're actually right about
(s+=_1)--thank you.

After I read your message, I went back and looked more closely at the
types I was using. It turns out to be an elementary C++ error
unrelated to lambda. I thought push_back had the interface of append,
and I switched the params (size_t, char) for (char, int). No wonder
the compiler was confused. This works fine:

string&(string::*ft)(std::allocate<char>::size_type,char) = &string::append;
bind(ft, var(s2), 1, _1);

So there is nothing fundamentally wrong with my understanding of the
lambda library. Good. Now I'm ready to start using it.

Thanks muchly,
Max

-- 
Sometimes I think decisions through, weigh the consequences, decide
it is not a good idea, but blind myself and make it anyway. I do not
think this is a good thing to do, but it might be. Humans are
endowed with both reason and emotion for a reason.
   -Tara Greenwood