Cannot convert initializer_list to class<int>

Question

I am trying to initialize_list for a class template. I am fairly a newbie and have had some trouble with this. Error on main()

#include <iostream>
#include <initializer_list>
#include <stdexcept>

template <class T>
class Storage
{
private:
    int nrOfEl{ 0 };
    T *m_array{};
public:
//...
    Storage(std::initializer_list<T>& list)
        : Storage( static_cast<int>(list.size()) )
    {
        int count{ 0 };
        for (auto& element : list)
        {
            m_array[count] = element;
            ++count;
        }
    }

    ~Storage()
    {
        std::cout << "\nCalling ~Storage()\n";
        delete[] m_array;
    }

    T& operator[](const int &index)
    {
        if (index >= nrOfEl || index < 0)
            throw std::out_of_range("OUT OF BOUNDS");
        return *m_array[index];
    }

    Storage& operator=(std::initializer_list<T>& list)
    {
        int length{ static_cast<int>(list.size()) };
        if (length != nrOfEl)
        {
            delete[] m_array;
            nrOfEl = length;
            m_array = new int[length];
        }

        int count = 0;
        for (int element : list)
        {
            m_array[count] = element;
            ++count;
        }

        return *this;
    }
//...
};

int main()
{
    Storage<int> ints { 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0 };
        //error here
    return 0;
}

error C2440: 'initializing': cannot convert from 'initializer list' to 'Storage'

note: No constructor could take the source type, or constructor overload resolution was ambiguous

Evg · Accepted Answer · 2019-05-18 15:11:48Z

2

A note in addition to other answers: there is no reason to pass std::initializer_list by const&, it is just a pair of pointers (or a pointer and a size). You can pass it by value, just as it is done in the Standard library.

answered May 18, 2019 at 15:11

Evg

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1 Comment

Phil-ZXX Over a year ago

Best answer. If only I could up-vote multiple times!

TrebledJ · Accepted Answer · 2019-05-20 03:16:03Z

1

In main(), your initializer list is constructed as an r-value, so pass-by-reference will fail. Passing by const-reference, however, will work. You should allow the constructor to accept const-reference lists:

Storage(const std::initializer_list<T>& list)
//      ^^^^^
    : Storage( static_cast<int>(list.size()) )
{
    int count{ 0 }; 
    for (const auto& element : list)    // const here too
    {
        m_array[count] = element;
        ++count;
    }
}

You should also change the parameter likewise with operator=.

edited May 20, 2019 at 3:16

answered May 18, 2019 at 14:33

TrebledJ

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1 Comment

Eljay Over a year ago

C++ const correctness: yes, it's that important!

arayq2 · Accepted Answer · 2019-05-18 14:50:08Z

1

You have two issues:

You are passing the initiliazer list by non-const reference into the Storage() constructor.
You have not defined a Storage(int) constructor.

This should fix both:

Storage(std::initializer_list<T> const& list)
    : nrOfEl( static_cast<int>(list.size()) )
{ ... }

answered May 18, 2019 at 14:50

arayq2

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