C++ Explicit Constructor

Question

I have a class with 2 constructors.

   explicit MyClass(size_t num);
   template<class T> MyClass(T myObj);

And I want that whenever I make

MyClass obj( 30 );

The first constructor will be called,

And on implicit constructors and

MyClass obj = 30;

The second ctor will be called.

How can I make it happen?

Drill

AFAIK that should already be happening. Did you try it and it didn't work? – Expound

yes I did try...it works only wheb i do like (size_t)30 – Drill

What happened? What was the error? – Expound

no error, but when I debug I can see that the second ctor is being called. – Drill

What happens in the following case MyClass obj( 30u );? – Hypostasis

calls to the first ctor! :-) but I would like it to work even when not writing 30u. – Drill

Remove explicit then. Explicit means the type must match exactly. – Caducous

@Caducous no, that's not what explicit means – Inaccurate

Answer 1

30 is a signed integer value, so it doesn't exactly fit the signature of your first constructor (therefore, the template gets instantiated).

You can either change the signature of the explicit constructor to accept an int, and than Myclass obj( 30 ); will call the explicit constructor, or call it with 30u so that you match the explicit signature.

Fistic

Is there a way to make it work with like short, long long etc..? – Drill

@TaruStolovich What do you mean? Have one explicit constructor that accepts all such literals? You can't, you'd have to overload the explicit constructor. – Fistic

@DimaRudnik you can use std::enable_if with std::is_integral. – Phantasm

Answer 2

Regarding the first object

MyClass obj (30);

This is a direct initialization, thus the constructor should be called if the argument has a correct type of the parameter. In this case the parameter is incorrect so just to be more accurate in this case i would change the size_t to unsigned int and then pass 30u to this object. In this case the first constructor would be called. Regrding the second object

MyClass obj = 30;

This is an initialization by copy, thus i would change the second constructor to a copy constructor like this:

template<class T> MyClass(const T& myObj);

In my opinion in this case its even better to change your data members to ints. Nevertheless the first constructor should be called and then the second as wanted.

Geddes