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Given the following code snippet we get some unexpected behaviour:
//--------------------------------------------------------------------
#include <iostream>

using namespace std;

struct A
{
A() { cerr << "A()" << endl; }
};

struct C;

struct B : public A
{
B();

explicit
B(const B &rhs);

B(const A&rhs);

C
operator()(int from, int to);
};

struct C: public A
{
B r;

operator const B &() const;
};

B::B() { cerr << "B()" << endl; }

B::B(const B &rhs)
{ cerr << "B(const B &rhs)" << endl; }

B::B(const A &rhs)
{ cerr << "B(const A &rhs)" << endl; }

C
B:perator()(int from, int to) { return C(); }

C:perator const B &() const
{ cerr << "operator const B &() const" << endl;
return r;
}

int main()
{
C c;
B b = c; // conversion constructor called twice here - why??
return 0;
}
//--------------------------------------------------------------------

The conversion constructor B(const A &) is called twice when
initializing object b. We would expect a call of
C:perator const B &() const
and then one call of
B::B(const B &).

Alexander Stippler wrote:
> struct B : public A
> {
> explicit
> B(const B &rhs);
>
> B(const A&rhs);
>
> };

> B b = c; // conversion constructor called twice here - why??

> The conversion constructor B(const A &) is called twice when
> initializing object b. We would expect a call of
> C:perator const B &() const
> and then one call of
> B::B(const B &).

The call to B::B(const B&) is not happening because that copy constructor is explicit.

Regarding calling this constructor twice - See 8.5/14 "Otherwise (i.e. for the remaining...".

Since your copy constructor in B is explicit, so the compiler chooses another path, i.e.
1. "c" is casted to A&
2. a temporary of class B is created from the result via the "B(const A&)" constructor.
3. your "b" is initialised with that temporary.

--

Valentin Samko - http://www.valentinsamko.com

Valentin Samko wrote:
> Alexander Stippler wrote:
> > struct B : public A
> > {
> > explicit
> > B(const B &rhs);
> >
> > B(const A&rhs);
> >
> > };
>
> > B b = c; // conversion constructor called twice here - why??
>
> > The conversion constructor B(const A &) is called twice when
> > initializing object b. We would expect a call of
> > C:perator const B &() const
> > and then one call of
> > B::B(const B &).
>
> The call to B::B(const B&) is not happening because that copy constructor is explicit.
>
> Regarding calling this constructor twice - See 8.5/14 "Otherwise (i.e. for the remaining...".
>
> Since your copy constructor in B is explicit, so the compiler chooses another path, i.e.
> 1. "c" is casted to A&
> 2. a temporary of class B is created from the result via the "B(const A&)" constructor.
> 3. your "b" is initialised with that temporary.

No, there is no temporary. Two user-defined conversions would be one
more than is allowed. Besides, there is no need of a B temporary since
B has a constructor that accepts an A object directly. So b is
constructed from an A class object (namely, c).

Greg

Greg wrote:
> Valentin Samko wrote:
>> Alexander Stippler wrote:
>>> struct B : public A
>>> {
>>> explicit
>>> B(const B &rhs);
>>>
>>> B(const A&rhs);
>>>
>>> };
>>> B b = c; // conversion constructor called twice here - why??
>>> The conversion constructor B(const A &) is called twice when
>>> initializing object b. We would expect a call of
>>> C:perator const B &() const
>>> and then one call of
>>> B::B(const B &).
>> The call to B::B(const B&) is not happening because that copy constructor is explicit.
>>
>> Regarding calling this constructor twice - See 8.5/14 "Otherwise (i.e. for the remaining...".
>>
>> Since your copy constructor in B is explicit, so the compiler chooses another path, i.e.
>> 1. "c" is casted to A&
>> 2. a temporary of class B is created from the result via the "B(const A&)" constructor.
>> 3. your "b" is initialised with that temporary.
>
> No, there is no temporary. Two user-defined conversions would be one
> more than is allowed. Besides, there is no need of a B temporary since
> B has a constructor that accepts an A object directly. So b is
> constructed from an A class object (namely, c).

Please read 8.5/14. The right hand side has to be converted to the destination type (B)
first. Only after the rhs is converted, compiler picks the most appropriate constructor.

Besides, no used-defined conversions happen.

--

Valentin Samko - http://www.valentinsamko.com