Initializing a dynamic array with a raw array

Question

I have a dynamic array class, for which I would like to implement a functionality of initializing the dynamic array directly with a raw array, similar to std::vector.

struct Array {
   int *ptr = NULL;
   int size;
   int capacity;
   
   Array() : size(0), capacity(0) {}
   Array(int capacity) : size(0), capacity(capacity) {
       ptr = (int*)malloc(capacity*sizeof(int));
   }
};

We can initialize std::vector using a raw array like this:

 std::vector<int> v = {1, 2, 3, 4};

It seems that std::vector uses std::initializer_list to accomplish this. Is it possible to achieve the same result without using the STL library? Such that I can initialize it like this:

Array array = {1, 2, 3, 4};

or

Array array {1, 2, 3, 4};

If this is not possible without std::initializer_list, how can I copy the elements from it to the dynamic array?

Consider reading a good C++ programming book, a good C++ reference website then using standard C++ containers such as std::array or std::vector — Basile Starynkevitch
– Basile Starynkevitch, Commented Aug 15, 2020 at 3:48
With recent GCC or Clang C++ compilers, the implementation of std::vector is open source, so study its source code. — Basile Starynkevitch
– Basile Starynkevitch, Commented Aug 15, 2020 at 3:50
Also, your ptr should better be initialialized with new and destroyed with delete — Basile Starynkevitch
– Basile Starynkevitch, Commented Aug 15, 2020 at 3:52

asmmo · Accepted Answer · 2020-08-15 03:54:46Z

4

This

Array array = {1, 2, 3, 4};

is not initializing using a raw array. this uses an initializer list or a uniform initializer. If you want to use a raw array, you must define it firstly, like what follows.

I used templates and passing by reference to deduce the size.

#include <iostream>
#include <algorithm>

struct Array {
    size_t size;
    size_t capacity;
    int *ptr = nullptr;

    template<size_t n>
    Array(int (&arr) [n]):size{n}, capacity{2*size}, ptr{ new int[capacity]}{

        std::copy(std::cbegin(arr), std::cend(arr), ptr);
    }
    ~Array(){delete[] ptr;}
};

int main(){
    int arr [] = {1, 2, 3, 4};
    Array array = arr;
    std::for_each(array.ptr, array.ptr+array.size, [](auto  el){std::cout << el << " ";});
}

Demo

And this is another approach (using the uniform initializer), if you want to initialize your class as you showed

#include <iostream>
#include <algorithm>

struct Array {
    size_t size;
    size_t capacity;
    int *ptr = nullptr;


    template<class ... Args, class = std::enable_if_t<std::is_same_v<std::common_type_t<Args...>,int>>>
    Array(Args ... args):size{sizeof...(Args)}, capacity{2*size},ptr{ new int[capacity]}{

        int* temp = ptr;
        ((*(temp++) = args),...);
    }
    ~Array(){delete[] ptr;}
};

int main(){
    Array array = {1,2,3,4};
    std::for_each(array.ptr, array.ptr+array.size, [](auto  el){std::cout << el << " ";});
}

Demo

Using std::initializer_list<int>, it will be

#include <iostream>
#include <algorithm>

struct Array {
    size_t size;
    size_t capacity;
    int *ptr = nullptr;


    Array(std::initializer_list<int> && ls):size{ls.size()}, capacity{2*size},ptr{ new int[capacity]}{
        std::copy(ls.begin(), ls.end(), ptr);
    }
    ~Array(){delete[] ptr;}
};

int main(){
    Array array = {1,2,3,4};
    std::for_each(array.ptr, array.ptr+array.size, [](auto  el){std::cout << el << " ";});
}

Demo

edited Aug 15, 2020 at 3:54

answered Aug 15, 2020 at 3:11

asmmo

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5 Comments

Lenny White Over a year ago

This is very helpful thank you! If that's actually an initializer list how could we initialize the class using it instead?

David C. Rankin Over a year ago

warning: ‘Array::size’ will be initialized after [-Wreorder]

David C. Rankin Over a year ago

Thank @asmmo, he did the heavy lifting. Nice solutions both.

Lenny White Over a year ago

@asmmo Awesome thanks very much! So template parameter packs is what I needed, didn't know about these! I'm guessing this is how std::initializer_list implemented also.

Remy Lebeau Over a year ago

@LennyWhite "I'm guessing this is how std::initializer_list implemented also." - no. An initializer_list is a special kind of object that is constructed directly by the compiler itself while parsing user code.

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