Quick Q: Does the default constructor initialize built-in types?
Quick A: No.
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Does the default constructor initialize built-in types?
Implicitly defined (by the compiler) default constructor of a class does not initialize members of built-in types.
However, you have to keep in mind that in some cases the initialization of a instance of the class can be performed by other means. Not by default constructor, nor by constructor at all...

ACCU’s Overload journal of April 2019 is out. It contains the following C++ related articles.