Custom Array Class: Constructor for Initialization of List

Question

I'm working on a custom array class in C++ (as a self-led exercise), and I'm not sure how to create a constructor that allows me to do something along the lines of:

#include "array.h"
#include <iostream>
int main()
{
    array<int> test = {1, 2, 3, 4};
    std::cout << test(1) << std::endl;
    return 0;
}

The error that the compiler (VS Express 2013) gives me is "no instance of constructor array::array [with T = int]" matches the argument list. argument types are (int, int, int, int)."

I'm not sure what the constructor that takes the enumeration of a set of elements is called. I know I've properly overloaded operator()(const int&). I also know that this (for a reason that's not clear to me) works:

#include "array.h"
#include <iostream>
int main()
{
    array<char> test = "abcd";
    std::cout << test(1) << std:: endl; // prints 'a', as expected. 
    std::cout << test(4) << std::endl; // prints 'd', as expected.
    return 0;
}

This is achieved with a array(const T[]) constructor: will the solution for the array<int> test = {1, 2, 3, ..., n} case be similar?

Thanks in advance for any guidance.

EDIT: Including the code below, in case it's helpful.

template<typename T>
class array
{
public:
    typedef T* iterator;
    typedef const T* const_iterator;
private:
    iterator head;
    unsigned long elems;
public:
    array()
        : head(nullptr)
        , elems(0) {}
    array(const unsigned long &size)
        : head(size > 0 ? new T[size] : nullptr)
        , elems(size) {}
    array(const T[]);
    array(const array&);
    ~array() { delete[] head; }

    iterator begin() const { return head; }
    iterator end() const { return head != nullptr ? &head[elems] : nullptr; }
    unsigned long size() const { return elems; }

    array& operator=(const array&);
    T& operator()(const unsigned long&);
};

template<typename T>
array<T>::array(const T rhs[])
{
    unsigned long size = sizeof(rhs) / sizeof(T);
    head = new T[size];
    iterator pointer = begin();
    for (const_iterator i = &rhs[0]; i != &rhs[0] + size; i++)
        *pointer++ = *i;
}

template<typename T>
array<T>::array(const array<T> &rhs)
{
    head = new T[rhs.size()];
    iterator pointer = begin();
    for (const_iterator i = rhs.begin(); i != rhs.end(); i++)
        *pointer++ = *i;
}

template<typename T>
array<T>& array<T>::operator=(const array<T> &rhs)
{
    if (this != &rhs)
    {
        delete[] head;
        head = new T[rhs.size()];
        iterator pointer = begin();
        for (const_iterator i = rhs.begin(); i != rhs.end(); i++)
            *pointer++ = *i;
    }
    return *this;
}

template<typename T>
T& array<T>::operator()(const unsigned long &index)
{
    if (index < 1 || index > size())
    {
        // Add some error-handling here.
    }
    return head[index - 1];
}

That was a glaring omission on my part: apologies. I've included it above. — crcvd
– crcvd, Commented Oct 15, 2014 at 20:45
sizeof(rhs) in array<T>::array(const T rhs[]) certainly isn't doing what you think it is. — WhozCraig
– WhozCraig, Commented Oct 15, 2014 at 20:48
warning: sizeof on array function parameter will return size of 'const char *' instead of 'const char []' [-Wsizeof-array-argument]. — user3920237
– user3920237, Commented Oct 15, 2014 at 20:49

Piotr Skotnicki · Accepted Answer · 2015-06-27 21:04:19Z

4

#include <initializer_list>

// ...

template <typename T>
class array
{   
    // ... 
    array(std::initializer_list<T> il);    

// ...    
template <typename T>
array<T>::array(std::initializer_list<T> il)
{
    unsigned long size = il.size();
    head = new T[size];
    iterator pointer = begin();
    for (const T& i : il)
        *pointer++ = i;
}

// ...
array<int> test = {1, 2, 3, 4};

DEMO

Suggested improvements:

array(const T rhs[]); is an equivalent of array(const T* rhs);, that is, a pointer, which means that sizeof(rhs) / sizeof(T) expression will not give you the number of items. If you want a special constructor for const char*, then consider either an entire array<char> specialization or at least disabling this constructor from the overload resolution if T is not char
head = new T[size]; default-initializes all elements (calls default constructor for each instance of type T). Then you call an assignment operation: *pointer++ = *i;. This can be improved by utilizing a placement-new, like ::new ((void*)ptr) T(*i); where ptr is a pointer to a raw, uninitialized memory buffer, like new char[sizeof(T)*size] or returned from get_temporary_buffer.

And if you are wondering why the following works array<char> test = { "abcd" }; with your current implementation then here is an explanation:

array<T> class has a constructor taking const T[], which for T=char is instantiated as array<char>::array(const char*).
List-initialization can be used to call object's constructor.
Your const T[] constructor is not explicit, which means that you can use copy-initialization syntax like below:
```
array<char> test = { "test" };
```
The expression sizeof(rhs) / sizeof(T), although not valid as explained above, for T=char evaluates to sizeof(char*) / sizeof(char), which is (most-probably) 4 / 1 = 4.
Your "test" used for initialization has exactly 4 letters, your luck.

edited Jun 27, 2015 at 21:04

answered Oct 15, 2014 at 20:54

Piotr Skotnicki

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2 Comments

crcvd Over a year ago

Thanks for such a thorough answer, Piotr. One question: how does std::initializer_list<T> know the number of arguments passed? Isn't it just a pair of pointers (en.cppreference.com/w/cpp/header/initializer_list)? How are these pointers constructed given something like {1, 2, 3, 4}?

Piotr Skotnicki Over a year ago

Its a compile time object, Compiler counts them, use .size() member function to know how many are there.

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