Quick A: Because the default assignement operator knows only the type of your object, which it can get be calling the constructor.

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What is the point of calling constructor with implicit conversion instead of assignment operator after an object is initalized?

There is no implicit CBox::operator=(double), so box = 2.0; has to create a temporary CBox object. It's equivalent to box = CBox(2.0);.

Making your constructor explicit disallows the implicit conversion from double to CBox, so no appropriate assignment operator exists, and you get a compile error.