Conversion by constructors

Question

Class X -> converted to Y by two ways 1) constructors, and 2) by conversion functions.
I understood the single argument constructor is used for conversion.

In the specification:

An implicitly-declared copy constructor is not an explicit constructor; it may be called for implicit type conversion.

Question:

So, that means not only single argument constructor is used for the conversion, but also copy constructor?. If so, which scenario it is used?. any snippet of sample code?

Kindly bear with me if the question is very basis.

Answer 1

Copy constructor is not an explicit constructor, so it will be used wherever possible. Copy constructor will "convert" only from the same type, so it is not a conversion in the full sense. However, for the sake of generality it is handy to call it one.

Read this paper: http://www.keithschwarz.com/cs106l/winter20072008/handouts/180_Conversion_Constructors.pdf if you want more details on conversion constructors.

Answer 2

It basically means that you can do:

struct A {};
A a;
A b = a;

If the copy constructor was marked explicit that would fail to compile. You can test it by adding: explicit A( A const & ) {} to the struct and recompiling the program.