Implicit initialization by argument deduction for a templated class array

Question

I have an item class, thats really a wrapper around an int. I have a templated node class. I pass item class to it as template parameter.

I want to create an array of the templated node class, but compiler does not like it. When I create a single object using the same syntax, it works. Why cant it deduce the array elements to initialize array of templated class? What is the correct way of creating an array of objects of a templated class?

Heres item class, which is a wrapper around an int.

using namespace std;

class bItem {
private :     
    int item{INT_MIN};
public:
    bItem() : item(INT_MIN) {}
    bItem(int val) : item(val) {}
    int getItem() { return item; }
    bItem operator==(const bItem& val) {
        return item == val.item;
    }
    bItem operator>(const bItem& val) {
        return item > val.item;
    }
    bItem operator<(const bItem& val) {
        return item < val.item;
    }
    friend ostream& operator<<(ostream& os, const bItem& item) {
        os << "value:" << item.item << endl;
        return os;
    }
};

Heres the templated Node class

#include <iostream>
using namespace std;
template<class itemType>
class bNode {
    itemType item{};
    bNode* prev{};
    bNode* next{};
public:
    bNode() : item(), prev(nullptr), next(nullptr) {}
    bNode(itemType _item) : item(_item), prev(nullptr), next(nullptr) {}
    ~bNode() {
        delete prev;
        delete next;
    }
    
    friend ostream& operator<<(ostream& os, const bNode& node) {
        os << "node " << node.item<< endl;
        return os;
    }
};

and heres the main()

int main()
{    
    bNode<bItem> nodes[5]{ 1,2,3,4,5 }; //this does not compile
    bNode<bItem> node2(6); //this compiles ok

}

and here is the error message

error C2440: 'initializing': cannot convert from 'int' to 'bNode<bItem>'
message : No constructor could take the source type, or constructor overload resolution was ambiguous

songyuanyao · Accepted Answer · 2020-09-01T02:30:33.310

In aggregate initialization of array, the elements are copy-initialized from the corresponding clause of the initializer list. That means the element with type bNode<bItem> needs to be copy-initialized from the int, which requires two user-defined implicit conversions, from int to bItem and from bItem to bNode, which is not allowed.

Each direct public base, (since C++17) array element, or non-static class member, in order of array subscript/appearance in the class definition, is copy-initialized from the corresponding clause of the initializer list.

As the workaround you could impose restrictions on the times of implicit conversions, e.g.

bNode<bItem> nodes[5] = { bItem(1),bItem(2),bItem(3),bItem(4),bItem(5) };

Or you can add another constructor taking int to bNode, then it could be copy-initialized from int too.

// forward it to bNode::bNode(itemType)
bNode(int val) : bNode(itemType(val)) {}

BTW: bNode<bItem> node2(6); performs direct initialization, which requiers only one implicit conversion (from int to bItem), and the converted bItme is passed to the constructor of bNode to construct node2. It doesn't have such issue as copy initialization. (Copy-initialization like bNode<bItem> node2 = 6; doesn't work either.)

In addition, the implicit conversion in copy-initialization must produce T directly from the initializer, while, e.g. direct-initialization expects an implicit conversion from the initializer to an argument of T's constructor.

I am really hoping to find a way to modify the node class to enable the kind of initialization I have in my main(). — bsobaid, Sep 01 '20 at 02:19
@bsobaid You can add a constructor in `bNode`, see revised answer. — songyuanyao, Sep 01 '20 at 02:24
thx songyuanyao. appreciate your detailed response and explaination. In bNode(int val) I am explicitly stating the type i-e int. May be this is the only way. but I think I am going to play with copy constructors to see if I find anything. — bsobaid, Sep 01 '20 at 02:26
@bsobaid You could define a template constructor for `bNode` that takes any type of argument and passes it to the `item` constructor. Like this: `template bNode(ArgType&& Arg) : item(std::forward(Arg)) {}` — Radoslav Voydanovich, Sep 01 '20 at 05:57

Implicit initialization by argument deduction for a templated class array

1 Answers1