Re: [Boost-Users] Why does BGL use friend instead of member functions?

From: Jeremy Siek (jsiek_at_[hidden])
Date: 2002-08-11 15:06:17

Hi Louis,

On Sun, 11 Aug 2002, Louis Lavery wrote:
Louis>    It appears, to me, that the boost graph classes use friend functions
Louis> and apposed to member functions.

That is correct.

Louis> What are the reasons for this design decision (I like it) and why is the
Louis> graph passed as the last argument?

For the most part, the differences between member functions and free
functions are syntactic, and not very important, though people can get
religious about them. However, I've got one technical reason for favoring
free functions. A programmer can overload a free function for a type
coming from a 3rd party without modifying the source code/definition of
that type. There are several uses of this in the BGL. For example,
Stanford GraphBase and LEDA graphs can both be used in BGL algorithms
because of overloads in stanford_graph.hpp and leda_graph.hpp. One can
even use std::vector<std::list> as a graph due to the overloads in
vector_as_graph.hpp.

Of course, there is way to adapt 3rd party classes into an interface with
member functions. You create an adaptor class. However, the disadvantage
of an adaptor class (compared to overloaded functions) is that one has to
physically wrap and unwrap the objects as they go into/out of BGL
algorithms. So the overloaded function route is more convenient.

Granted, this is not a huge difference, but since there weren't other
strong reasons, it was enough for me to choose member functions.

My religious reason for choosing free functions is to send the message
that BGL is a generic library, and not a traditional object-oriented
library. OO was hip in the 80s and 90s, but its time we moved past it :)

Louis> Thanks,
Louis>
Louis> Louis.
Louis>
Louis> PS. Great graph - great book.

Thanks!
Jeremy

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 Jeremy Siek                          http://php.indiana.edu/~jsiek/
 Ph.D. Student, Indiana Univ. B'ton   email: jsiek_at_[hidden]
 C++ Booster (http://www.boost.org)   office phone: [phone]
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