Re: Template conversion and default copy constructor
Stephan Tolksdorf wrote:
All the compilers I tested produced from the following code
executables that printed "2". Shouldn't the implicitly defined
copy constructor be called?
--
#include <iostream>
class Test{
public:
Test() : data(0) {}
template <typename T> Test(T& test) : data(2) { }
int data;
};
int main(int argc, char* argv[]) {
Test t1;
t1.data = 3;
Test t2(t1);
std::cout << t2.data;
}
That's a good question. I don't see any context where the copy
constructor is implicitly called for. In "Test t2(t1);" (or for
that matter, in "Test t2 = t1;", since t1 and t2 have the same
type), the standard says simply that "The applicable
constructors are enumerated, and the best one is chosen through
oeverload resolution." The implicit copy constructor is there,
of course, and is considered in overload resolution, so it is a
question of which is the better match. (If they are otherwise
equal, preference goes to the non-template version.) In this
case, I think that binding the non-const object to a non-const
reference wins out, by the rule in ?13.3.3.2/2, point 5: "S1 and
S2 are reference bindings, and the types to which the reference
refers are the same type except for top-level cv-qualifiers, and
the type to which the reference initialized by S2 refers is more
cv-qualifed than the type to which the reference initialized by
S1 refers." (There is, presumably, an "S1 is a better
conversion sequence than S2" somewhere before the list of
points, but I can't find it.) Basically, this is the same rule
which makes the compiler prefer non-const functions to const, or
in general a non-const reference to a const reference, when the
call would be legal for both.
If you change the parameter of the template constructor to T
const&, the compiler generated default should be called.
Similarly, if you explicitly provide a copy constructor which
takes a non-const Test&, it should be provided. But in the case
at hand, t1 is considered a better match for T& (where T is
instantiated as Test) than for the Test const& parameter of the
implicitly generated constructor.
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