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

Implicitly-declared copy constructor cannot use for conversions, since it’s copy-ctor, that has declared as T(const T&) or T(T&).

draft n3337 par 12.8 C++ standard.

8 The implicitly-declared copy constructor for a class X will have the
form X::X(const X&) if — each direct or virtual base class B of X has
a copy constructor whose first parameter is of type const B& or const
volatile B&, and — for all the non-static data members of X that are
of a class type M (or array thereof), each such class type has a copy
constructor whose first parameter is of type const M& or const
volatile M&.119 Otherwise, the implicitly-declared copy constructor
will have the form X::X(X&)

Since copy c-tor is not explicit you can use such code

struct So
{
};

int main()
{
    So s = So();
}

If copy-ctor is explicit you could use only initizaliation like So s((So()));