[TheDong]

The reason Boost is headers only is you have to be a header to be generic. Implementations outside of headers have to friend / otherwise be not truly generic.

[theyoungestgun]

Assuming you mean friend in the standard cpp way, that's not exactly correct.

http://stackoverflow.com/questions/495021/why-can-templates-...

[jgh]

Yes, good point. You're right about that.