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Why does the BGL interface use friend functions (or free functions) instead of member functions?

April 26, 2017BGL Free friend functions interface

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Why does the BGL interface use friend functions (or free functions) instead of member functions?

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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, we had 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 as a graph due to the overloads in vector_as_graph.hpp. Of course, there is a 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 convenien