🚀 go-pugleaf

Two possible ambiguities about initialization in Ellis & Stroustrup,
"The Annotated C++ Reference Manual".  First, suppose class A is an
aggregate (sect 8.4.1) containing one data member, an array of V's,
which are also aggregates.
    struct V { float data[2]; }
    struct A { V array[2]; }
    A wrong = { {1, 2}, {3, 4} };	//{3,4} inits 2nd data member
					//which does not exist
    V v0 = {1, 2};
    V v1 = {3, 4};
    A ambig = { { v0, v1 } };		//What does v0 init?
		//Could be ambig.array[0], could be ambig.array[0].data[0]
When an aggregate is initialized from a list of aggregates, and when one
of these matches a sub-aggregate, is it copied or does it (disastrously)
initialize members of the sub-aggregate?  The book can be read either way.

What I really wanted was the effect shown above as "wrong", just as a
traditional "C" array would be initialized.

Second ambiguity: a derived class has a constructor from a reference to its
base class.  Is this a copy constructor?  Specifically:
    struct Base {
	Base(args);		//No explicit copy constructor here
    };
    struct Derv : public Base {
	Derv(const Base& b);	//Pseudo-copy constructor
    };
    Derv& invert(const Base& b) {
	Derv result(b);
	/* thrash about */
	return result;		//Using which copy constructor?
    }
Reading in sect. 12.6.1 "Explicit Initialization" p. 288: "...function
return is equivalent to Derv dest = result".  p. 284: "This value is
used as the argument to a copy constructor".  Sect. 5.3.3 "New" p. 61:
"Access and  ambiguity control are done for both operator new() and the
constructor". It all implies very strongly that a constructor should be
sought that will swallow a Derv, which the provided constructor will
do.  Nonetheless my compiler demanded an exact match on argument type,
chucked my constructor, and generated a bitwise copy (whereupon on
destruction a free pointer was freed again, the heap was smeared,
MS-DOS was smeared, my disc was smeared, and matters went downhill from
there.  Fortunately I make backups.)

An authoritative statement would be appreciated in the evolving standard
of whether a copy constructor can have non-exact matching arguments.

If you post a reply, I would appreciate a mailed copy since my news machine
has a very short expiration time.

James F. Carter        (213) 825-2897
UCLA-Mathnet;  6221 MSA; 405 Hilgard Ave.; Los Angeles, CA, USA  90024-1555
Internet: jimc@math.ucla.edu            BITNET: jimc%math.ucla.edu@INTERBIT
UUCP:...!{ucsd,ames,ncar,gatech,purdue,rutgers,decvax,uunet}!math.ucla.edu!jimc
James F. Carter        (213) 825-2897
UCLA-Mathnet;  6221 MSA; 405 Hilgard Ave.; Los Angeles, CA, USA  90024-1555
Internet: jimc@math.ucla.edu            BITNET: jimc%math.ucla.edu@INTERBIT
UUCP:...!{ucsd,ames,ncar,gatech,purdue,rutgers,decvax,uunet}!math.ucla.edu!jimc