Question

I am just playing around to understand smart pointers and trying to make mine but I come across a situation that I do not fully understand. Here is the code:

#include <iostream>
template <class T>
class Holder
{
private:
        T * obj;
public:
        Holder(T * tt) : obj(tt) 
        {
                std::cout << "ctor : " << tt->dummy << std::endl;
        }
        T * operator -> ()
        {
                return obj;
        }
        operator bool()
        {
                return obj;
        }
        T * const get() const
        {
                return obj;
        }
        void reset() {swap(0);}
        void swap(T * other)
        {
                obj = other;
        }
        Holder & operator = (const Holder& holder)
        {
                obj = holder.get();
                return *this;
        }
        Holder(const Holder & holder) : obj(holder.get()) {} 
};

class A
{
public:
        int dummy;
        A(int a) : dummy(a) {}
};

int main ()
{
        A * a = new A(1);
        Holder<A> holder(a);
        A * b = new A(2);
        holder = b;

        std::cout << holder->dummy << std::endl;

        return 0;
} 

The code compiles and on the line of holder = b; the constructor of Holder class is called. I thought compiler would give an error. It is not the assingment operator but why is it calling constructor?

Answer 1

holder = b attempts to assign from b to Holder. b is of type A*, and holder is of type Holder<A>.

The Holder template defines assignment from another instance of the same Holder type, so the compiler looks for a conversion from A* to Holder<A>. It finds the constructor, and uses that.

Constructors which may take exactly one argument may be used for implicit conversions, unless you tag them with the explicit keyword.