Benefits of header-only libraries

Question

What are the benefits of a header only library and why would you write it that way oppose to putting the implementation into separate file?

Answer 1

The main "benefit" is that it requires you to deliver source code, so you'll end up with error reports on machines and with compilers you've never heard of. When the library is entirely templates, you don't have much choice, but when you have the choice, header only is usually a poor engineering choice. (On the other hand, of course, header only means that you don't have to document any integration procedure.)