Re: [Boost-users] Boost.lambda: how to sort this vector with function?

Subject: Re: [Boost-users] Boost.lambda: how to sort this vector with function?
From: Roland Bock (rbock_at_[hidden])
Date: 2009-02-09 01:54:51

Joost Kraaijeveld wrote:
> I want to sort a vector of Edges according to their remaining estimated
> distance to the finish point (an own implementation of a graph
> algorithm).
> 
> How do I sort this vector with edges:
> 
> // Returns the cost between start and finish, given a graph
> int CostEstimate( const Graph& aGraph, const Vertex& start, const Vertex& finish)
> ...
> 
> // Member function of a graph
> std::vector< Edge > v = ...;
> // Sort them according to the cost
> std::sort( v.begin(),
>            v.end(),
>            boost::lambda::bind( CostEstimate( (*this),
>                                 boost::lambda::bind( &Edge::otherSide, &start, boost::lambda::_1),
>                                 finish))
>            <
>            boost::lambda::bind( CostEstimate( (*this),
>                                 boost::lambda::bind( &Edge::otherSide, &start, boost::lambda::_2),
>                                 finish)));
>    
> where otherSide is a const function of the class Edge which returns a
> "const Vertex&" 
> 
> Compiling this results in an error:
> 
> error: invalid initialization of reference of type ‘const Vertex&’ from
> expression of type ‘const
> boost::lambda::lambda_functor<boost::lambda::lambda_functor_base<boost::lambda::action<3, boost::lambda::function_action<3, boost::lambda::detail::unspecified> >, boost::tuples::tuple<const Vertex& (Edge::* const)(const Vertex&)const, const Vertex* const, const boost::lambda::lambda_functor<boost::lambda::placeholder<1> >, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type> > >’
> 
> TIA
> 

Hi,

I am sure there is some way to extend the code you wrote in such a way 
that it compiles and works as it should. My question would be: Are you 
going to still understand it in, say, half a year? Four binds in one 
expression is certainly not unheard of, but personally I would do it 
with one bind and an old-fashioned function, see attached code.

Hope I got your settings right. I was a bit confused by otherSide being 
a function of edge. This would mean that start is an Edge, while finish 
is a Vertex in the sort-code. But start and finish are both Vertexes in 
the CostEstimate function...

To avoid such wondering, it would help if you could provide a small but 
complete program which shows the problem. Nothing more and nothing less.

Regards,

Roland