c++ - Why is Boost Graph Library's `source()` a global function? -

i understand in generic programming, algorithms decoupled containers. make no sense implement generic algorithm instance method (the same algorithm should work on multiple concrete classes; don't want make them inherit 1 abc since exponentially increase number of classes).

but in case of source() function in boost graph library, don't understand why global function , not instance method of graph class.

as far tell reading bgl source code, source(e, g) needs know implementation details of graph , edge objects passed it; it's not enough know interfaces.

so source() not generic algorithm. in other words, needs know concrete class of graph instance. why not put in same class instance method? wouldn't lot cleaner / less confusing making global function needs customized each class it's called upon?

update

the relevant source code:

  // dwa 09/25/00 - needed more explicit reverse_graph work.   template <class directed, class vertex,       class outedgelists,       class vertexlists,       class directeds,       class vertexproperty,       class edgeproperty,       class graphproperty, class edgelists>   inline vertex   source(const detail::edge_base<directed,vertex>& e,          const adjacency_list<outedgelists, vertexlists, directeds,                  vertexproperty, edgeproperty, graphproperty, edgelists>&)   {     return e.m_source;   }   namespace boost {    namespace  detail {      template <typename directed, typename vertex>     struct edge_base     {       inline edge_base() {}        inline edge_base(vertex s, vertex d)         : m_source(s), m_target(d) { }       vertex m_source;       vertex m_target;     };   } } 

the source(e, g) not generic algorithm. part of interface, called concept in c++. reason being non-member function can implemented non-intrusively.

say, instance, wanted std::multimap implement incidencegraph concept. if graph library required source() member function out of luck, since std::multimap doesn't provide one.