Replace the default copy constructor with a template copy constructor - c++

Question

I have a template class C. I want such a template copy constructor that it will resize the data array according to the size of the other class.
The following is a simple example. It works fine. However, notice that Constructor 2 and Constructor 3 are so similar, I am wondering whether it is possible to merge them as one?
Also, if I simple remove Constructor 3, then C<int> c3( c1 ) won't call Constructor 2, but will rather call a default copy constructor added by the compiler. This will result in the memory not being allocated properly.
template<typename T>
class C
{
public:
T* val;
int size;
public:
C( int s = 0 ) {
cout << "Constructor 1" << endl;
size = s;
val = ( size ) ? new T[size] : nullptr;
}
template<class T2>
C( const C<T2>& c2 ) {
cout << "Constructor 2" << endl;
size = c2.size;
val = ( size ) ? new T[size] : nullptr;
}
C( const C<T>& c2 ) {
cout << "Constructor 3" << endl;
size = c2.size;
val = ( size ) ? new T[size] : nullptr;
}
~C() {
cout << "~C()" << endl;
delete[] val;
val = nullptr;
}
};
int main(void)
{
C<int> c1( 5 );
C<float> c2( c1 );
C<int> c3( c1 );
return 0;
}
Output of the above function:
Constructor 1
Constructor 2
Constructor 3
~C()
~C()
~C()

Answer 1

Replace
template<class T2>
C( const C<T2>& c2 ) {
cout << "Constructor 2" << endl;
size = c2.size;
val = ( size ) ? new T[size] : nullptr;
}
C( const C<T>& c2 ) {
cout << "Constructor 3" << endl;
size = c2.size;
val = ( size ) ? new T[size] : nullptr;
}
by
template<class T2>
C( const C<T2>& c2, int dummy) {
cout << "Constructor 2" << endl;
size = c2.size;
val = ( size ) ? new T[size] : nullptr;
}
template<class T2>
C( const C<T2>& c2 ) : C(c2, 0) {}
C( const C<T>& c2 ) : C(c2, 0) {}
Update
You can just use:
C( int s = 0 ) {
cout << "Constructor 1" << endl;
size = s;
val = ( size ) ? new T[size] : nullptr;
}
template<class T2>
C( const C<T2>& c2 ) : C(c2.size) {}
C( const C<T>& c2 ) : C(c2.size) {}
and not need the second constructor.

Answer 2

One would like to implement this with a C++11 delegating constructor. However due to ambiguity in if template paramaters are to the class or to the constructor, this is not possible in a straightforward manner. As a workaround one can factor out the common code into a helper function.
something like:
template<class T2>
C( const C<T2>& c2 ) {
Copy<T2>(c2);
}
C( const C<T>& c2 ) {
Copy<T>(c2);
}
private:
template<class T2>
void Copy( const C<T2>& c2 ) {
cout << "Constructor 2" << endl;
size = c2.size;
val = ( size ) ? new T[size] : nullptr;
}