How to convert std::array to a class type

Question

I have the below code where I need to create an array of object thru a templated class. The templated class will receive another class as its type:

#include <iostream>
#include <array>
class B
{
    public:
    B() {std::cout << "B Called" <<std::endl;}
    B(int y){std::cout << "B int Called" <<std::endl;}
    static const int mysize;
};
const int B::mysize = 256;

template <typename anotherclass>
class A : public anotherclass
{
    public:
    A(){std::cout << "Called" << std::endl;}
    static anotherclass* obj[anotherclass::mysize];
    //static anotherclass[] init();
    static std::array<anotherclass*,anotherclass::mysize> init();
};

template <typename anotherclass>
std::array<anotherclass*, anotherclass::mysize> A<anotherclass>::init ()
{
    std::array<anotherclass*,256> objtemp[anotherclass::mysize];
     for (int i = 0 ; i < anotherclass::mysize; i++)
    { objtemp[i] = new anotherclass(2); }
    return  objtemp;
}
//A<B> obj[256] = [] () {for (int i = 0 ; i < 256; i++) { obj[i] = new A<B>(2)}};
template <typename anotherclass>
anotherclass* A<anotherclass>::obj[anotherclass:: mysize] = A<anotherclass>::init();

int main()
{
    A<B> a;
}

When I am constantly getting below error -

could not convert ‘objtemp’ from ‘A<B> [256]’ to ‘std::array<B, 256ul>
conversion from ‘std::array<B, 256ul>’ to non-scalar type ‘A<B>’ requested

I changed the objtemp as an std::array and error went away but I get below error -

   32:1 error: need ‘typename’ before ‘A<anotherclass>::obj’ because ‘A<anotherclass>’ is a dependent scope
     A<anotherclass>::obj = A<anotherclass>::init();

I am not sure what this error means?

I got the actual fix:

template <typename anotherclass>
anotherclass A<anotherclass>::obj[anotherclass:: mysize] = A<anotherclass>::init();

But below is the output I receive :

B Called
Called

Whereby I was expecting B' constructor to be called 256 times cause of below statement

objtemp[i] = new anotherclass(2)

I understand that since I need to create 256 objects of class B type its constructor should be called 256 times which is not happening in my case. So what should I do to initialise a static array of objects in my case -

 template <typename anotherclass>
anotherclass A<anotherclass>::obj[anotherclass:: mysize] = A<anotherclass>::init();

Whats the wrong in above code or I am missing something?

Declare `objtemp` as an `std::array` instead of a C-style array ie as the same type as you are returning from the method. — Richard Critten, Aug 20 '17 at 08:18

LogicStuff · Accepted Answer · 2017-08-20T08:24:29.160

0

Just make your mind whether you're going to use a C-style array or std::array. The latter seems to be a better choice, as ~~C-style arrays can't be copied that simply~~ you're using C++ (11).

So declare objtemp and obj as std::arrays.

edited Aug 20 '17 at 08:24

answered Aug 20 '17 at 08:19

LogicStuff

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I have changed the code as per your advise - thanks - but not sure now why I am getting mentioned error – Programmer Aug 20 '17 at 08:31
Just do what the error message says. [Explanation in depth](https://stackoverflow.com/questions/610245/where-and-why-do-i-have-to-put-the-template-and-typename-keywords). – LogicStuff Aug 20 '17 at 08:37
This fixed it - template anotherclass A::obj[anotherclass:: mysize] = A::init() ; but why B constructor is not called 256 times? – Programmer Aug 20 '17 at 09:13

How to convert std::array to a class type

1 Answers1