c++ single-arg ctor is by default implicit

[[c++codingStandard]] point out the many subtle traps a beginner can hit when an implicit conversion is accidentally applied to her data type.

Therefore it's good to spot any implicit conversion feature. For a custom class, there are only 2 ways to convert implicitly and one of them is a single-arg constructor. It's implicit by default. You can disable the conversion capability by force it to be always-explicit