explicit copy constructors

[Dave]

Hello NG,

Can anybody fathom the purpose of an explicit copy constructor? On page 232
of the Josuttis STL reference, I see a reference to such.

How could you ever need to supress the possibility of an implicit conversion
from type T to type T? Such an implicit conversion could never occur
because you're already of the required type!

Thanks,
Dave

 

[Victor Bazarov]

Can anybody fathom the purpose of an explicit copy constructor? On page 232
of the Josuttis STL reference, I see a reference to such.

How could you ever need to supress the possibility of an implicit conversion
from type T to type T? Such an implicit conversion could never occur
because you're already of the required type!

IIRC, an explicit copy constructor would prevent copying in certain
cases. I strongly urge you to look in the archives about 'explicit'.
I do remember some interesting discussions in c.l.c++.moderated about
'explicit' keyword and its use for non-conversion constructors.

V

 

[Ioannis Vranos]

Hello NG,

Can anybody fathom the purpose of an explicit copy constructor? On page 232
of the Josuttis STL reference, I see a reference to such.

How could you ever need to supress the possibility of an implicit conversion
from type T to type T? Such an implicit conversion could never occur
because you're already of the required type!

When such conversion produces undesirable results, as in the case of
containers.


Check TC++PL 11.7.1, on page 284. A quote from there:


"By default, a single argument constructor also defines an implicit
conversion. For some types, that is ideal. For example:

complex z = 2; // initialize z with complex(2)


In other cases, the implicit conversion is undesirable and error-prone.
For example:

string s= ´a´; // make s a string with int(’a’) elements

It is quite unlikely that this was what the person defining s meant.


Implicit conversion can be suppressed by declaring a constructor
explicit. That is, an explicit constructor will be invoked only
explicitly. In particular, where a copy constructor is in principle
needed (11.3.4), an explicit constructor will not be implicitly invoked."