What do you mean by calling copy constructors explicitly?

[Wulfinite]

Question says it all

[JLBorges]

Constructors can't be explicitly called.

Constructors do not have names. ...
...
A constructor is used to initialize objects of its class type. Because constructors do not have names,
they are never found during name lookup;

- IS [emphasis added]

However,

... however an explicit type conversion using the functional notation will cause a constructor to be called to initialize an object.
...
A functional notation type conversion can be used to create new objects of its type.
[Note: The syntax looks like an explicit call of the constructor. —end note ]
[Example:

complex zz = complex(1,2.3);
cprint( complex(7.8,1.2) );

—end example ]

An object created in this way is unnamed.

- IS [emphasis added]

[Wulfinite]

Thank you.