CODEWITHSUNDEEP
c++11
Grim Fandango
Grim Fandango

Reputation: 2426

Does template copy constructor override default copy constructor?

I would expect that the existence (or absence) of the line with the default copy-constructor in the following example should make some difference, but the behavior (on Windows) is the same in both cases:

template <class T>
struct Vec {
    T x = T();
    T y = T();

    Vec() = default;

    Vec(const Vec&) = default;   // default copy ctor

    template <class Other>
    Vec(const Other &o)          // templated copy ctor
      : x(o.x), y(o.y) 
    {
        cout << "templated copy ctor";
    }
};

Vec<int> i;
Vec<double> d = i;     // calls templated 'copy ctor'
Vec<double> dd = dd;   // does not call templated copy ctor, whether 'default copy ctor' exists, or not !

It seems that the compiler always generates a default constructor (that is if I do not explicitly delete it, or make it protected), and therefore the templated copy ctor never matches.

If the compiler always generates a default constructor, why would I ever want to write this line? Why not always omit it instead?

Vec(const Vec&) = default;   // default copy ctor

What would be a wise coding standard ?

Upvotes: 0

Views: 57

Answers (1)

max66
max66

Reputation: 66230

It seems that the compiler always generates a default constructor (that is if I do not explicitly delete it, or make it protected), and therefore the templated copy ctor never matches.

Yes: the copy constructor isn't template so is a best match for a Vec const &.

As Some programmer dude observed in a comment, the template one isn't (to be exact) a copy-constructor.

If the compiler always generates a default constructor, why would I ever want to write this line?

Not always.

The rules regarding deleted/defaulted constructors and deleted/defaulted assignment operators are complicated so, for example, the copy-constructor is implicitly deleted when there is a user declared move constructor or a user declared move assignment.

In that case, a = default can re-enable the default copy constructor.

Upvotes: 2

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