templates copy constructor errors - c++

Question

Here is a minimal code that shows the problem:
template<typename To, typename From> To convert(const From& x);
struct A
{
int value;
template<typename T> A(const T& x) { value = convert<A>(x).value; }
};
struct B : public A { };
int main()
{
B b;
A a = b;
}
It gives me: undefined reference to 'A convert<A, B>(B const&)'
As expected, as I removed the default copy constructor. But if I add this line to A:
A(const A& x) { value = x.value; }
I get the same error. If I try to do this way: (adding a template specialization)
template<> A(const A& x) { value = x.value; }
I get: error: explicit specialization in non-namespace scope 'struct A'.
How to solve it?
My compiler is MinGW (GCC) 4.6.1
EDIT:
The convert functions converts from many types to A and back again. The problem is that don't make sense writing a convert function from B to A because of the inheritance. If I remove the line that calls convert from A it just works. The idea is to call convert for all times that do't inherit from A, for these, the default constructor should be enough.

Answer 1

As for as I understand, when b is passed, as b is not an object of A, copy constructor is not called, instead the template constructor is called.
However, if an object of the derived class is passed, you want the copy constructor of A to be called.
For this, there is one solution using <type_traits> (c++0x):
#include <type_traits>
template<typename To, typename From> To convert(const From& x);
struct A
{
int value;
template<typename T> A(const T& x,
const typename std::enable_if<!std::is_base_of<A,T>::value, bool>::type = false)
{ value = convert<A>(x).value; }
A(){}
};
struct B : public A { };
int main()
{
B b;
A a = b;
}
The template is disabled why an object of a class derived from A is passed, so the only available constructor is the copy constructor.

Answer 2

You can solve it by defining the convert function :
template<typename To, typename From> const To& convert(const From& x)
{
return x;
}

Answer 3

As expected, as I removed the default copy constructor.
No; while you do need to replace the default copy constructor, its omission causes a different sort of problem (and only if you have the sort of calling code that needs it).
The error you report:
undefined reference to 'A convert<A, B>(B const&)
is a linker error. It is telling you that you don't actually have a convert function anywhere.
error: explicit specialization in non-namespace scope 'struct A'
You were right the first time about how to put back the default copy constructor. However, that's still irrelevant to the linker error.

Answer 4

Well, the "undefined reference" should be easily solved by linking in the implementation of the function!
NOTE: as things stand, returning a copy will trigger a stack overflow.
EDIT: IMO, your design is flawed, you have moved the construction logic outside of A into convert; rather than this, you should provide specific conversion constructors in A. It's either that or some boost::enable_if trick to disable the conversion constructor if the passed in type is derived from A (you can use one of the type_traits for that). For example, if you can construct an A from an int, provide the specific constructor in A itself.