Re: copy ctor and 'explicit' qualifier
On Mar 31, 4:28 pm, James Kanze <james.ka...@gmail.com> wrote:
On Mar 31, 11:50 am, "subramanian10...@yahoo.com, India"
<subramanian10...@yahoo.com> wrote:
I have the following understanding: Please let me know if it is
correct:
copy ctor of a class is NOT declared with 'explicit' qualifier
because if it is declared 'explicit', then the class cannot be
used with standard library container. Is this reasoning
correct ?
Only partially. Basically, if the copy constructor is explicit,
then it can only be used in contexts in which the constructor is
considered to have been explicitly called, e.g. explicit
conversions (any of the different cast notations), or direct
initialization (but not copy initialization---which means that
they cannot be passed by value as arguments to a function, nor
returned by value from a function).
Or is there some other reason for NOT declaring the copy ctor
with 'explicit' qualifier ?
I'd say that the principal reason is that it isn't clear what it
means to declare a copy constructor "explicit". At least to me,
it creates confusion as to what was actually desired. It seems
to be a way of saying that an object supports copy, but not
always.
--
James Kanze (GABI Software) email:james.ka...@gmail.com
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I use explicit copy-ctor sometimes to prevent some new programmer (esp
from matlab domain, where cow is the common mode of copying) to copy
class unnecessarily.
class BigClass{
public:
BigClass(){}
explicit BigClass(const BigClass& c){}
};
class BigClassLender{
private:
BigClass bigClass_;
public:
const BigClass& get()const{ return bigClass_;}
};
BigClassLender lender;
BigClass c = lender.get(); /// WOW, he wanted const BigClass& c =
lender.get() actually.
Now with explicit copy ctor, he have to write BigClass
c(lender.get()); if he really want's copy.
so it is just preventive measurement for the guys who usually works
with ref symantica, and suddenly comes to c++ value world!
reference is good as function argument as it silently passes a value
type as reference (on contrary to pointer) , it is bad as return
argument as it silently converts to value from reference type.
thanks
abir