If you declare but don't define a private copy constructor/assignment then you hit vague linker errors elsewhere that will likely send a user on a chase trying

If you declare but don't define a private copy constructor/assignment then you hit vague linker errors elsewhere that will likely send a user on a chase trying to figure out how their include paths don't match their linker paths and which library they forgot to link against. '= delete' makes it exceedingly clear that what they are doing is not allowed, rather than "oh god dammit cmake what did I miss now..." Since '= deleted' was added in C++11, there's really not any good "backwards-incompatible" arguments to be made about avoiding it. C++11 support is extremely broad at this point and has been for many, many years now.