Re: [Boost-users] BGL: std::map<Edge, int> needs the < operator

From: Dmitry Bufistov (dmitry_at_[hidden])
Date: 2006-04-21 08:13:24

Ovanes,
In your implementation you need to change edge_descriptor class. Is it 
possible to do the same without such modification? I was trying define 
such global operator

bool	operator<(const edge_descriptor_t& __x, const edge_descriptor_t&__y)
{
        return	__x.get_property() < __y.get_property();
}

  but it doesn't work for me (when I try to instantinate some std::map 
for edge I have compilation error:
1>C:\Program Files\Microsoft Visual Studio 8\VC\include\functional(143) 
: error C2678: binary '<' : no operator found which takes a left-hand 
operand of type 'const edge_descriptor_t' (or there is no acceptable 
conversion)

Thank you,
--dmitry
Ovanes Markarian wrote:
> 
>  Another way would be to implement a global less than operator overload. You can also implement
>  this operator within a class, so that std::less_than is able to call it.
> 
> 
>  //implementig the comaprison operator in the class
>  // if you don't want this operator to be accessible from other classes,
>  // declare the std::less_than<Edge> as friend and make this operator protected
>  class Edge
>  {
>  	friend class std::less_than<Edge>;
> 
>  	protected:
>  		inline bool operator <(const Edge& e)
>  		{
>  			return (prop1 < e.prop1) && (prop2 < e.prop2) && ...;
>  		}
> 
>  	//your class implementation
>  };
> 
> 

> Ovanes Markarian