How to initialize an array pointer object

Question

I have a little problem to initialize (constructor) an array pointer of object. See the class below. Class test has 2 variable member, a pointer (value) that will be an array, and his size (size); and a constructor with parameters, and a destructor. In main function, I will create an array pointer of objects, and I have problem with it. If I create a single object like:

test obj(4); it will create a object, and his instance, value array is big 4.

Then if i want to create an array of objects:

test *obj;
obj = new test[2]{4,7};

I will create 2 object: obj[0] that is big 4, and obj[1] that is big 7.

So if I want to create more object:

test *obj;
obj=new test[100]{/*here I must write 100 numbers*/}

and this is the problem. Because I cant write something like this:

test *obj;
obj=new int[100]{4}

I want that each value[] (instance of test class) is big 4, and I wont write 100 times "4". I thought the analogy of declaring array: If I write int array[5]={0,0,0,0,0}, I must write 4 times "0", or I can write also: int array[5]={0} and each value is set to 0. (it's also true that if write int array[5]={5}, first index will be 5 and others 0).

Should I use a default constructor? What should I do?

#include <iostream>

using namespace std;

class test
{
private:
int* value;
int size;

public:
test(int size)
{
    this->size = size;
    value = new int[size];
}

~test()
{
    delete[]value;
}
};

Yes it's another possible way. But i would know how to do (just for knowledge). — Riccardo
– Riccardo, Commented Jul 28, 2019 at 7:21
test doesn't follow rule of 5/3/0. leading to issue with copy/move. using std::vector for value would solve that issue. — Jarod42
– Jarod42, Commented Jul 28, 2019 at 7:46

frogatto · Accepted Answer · 2019-07-28 07:45:21Z

2

You can allocate the memory on the stack and get rid of dynamic allocation and memory management.

test array[100];
std::fill(std::begin(array), std::end(array), test(100));

Note that you would need a default constructor here.

answered Jul 28, 2019 at 7:45

frogatto

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Comments

score 1 · Accepted Answer · 2019-07-28 07:56:06Z

1

You can iterate over your pointer to initialize each element

test *obj = new test[100];
for(size_t i = 0; i != 100; ++i)
{
    obj[i] = test(/*parameters*/);
    /* Remember to provide a move assignment operator
which invalidates the pointer member, otherwise when the
temporary variable is destroyed the new object pointer
member will point to data no more available*/
}

// ...
delete [] obj;

However it would be better to use std::vector

std::vector<test> obj(100, test(/*parameters*/));

Using std::vector your test object is initialized 100 times passing its arguments, using a pointer the allocation (new test[100]) will default construct every element, then you are going to assign each element the new value, that's why std::vector is a better solution to your problem

edited Jul 28, 2019 at 7:56

answered Jul 28, 2019 at 7:26

anon

9 Comments

anon Over a year ago

Sorry, i accidentally wrote new int[100], I've edited the answer

melpomene Over a year ago

prog.cpp:25:25: error: no matching function for call to ‘test::test()’, and if you add a default constructor, it will crash at runtime.

anon Over a year ago

For the pointer alternative a default constructor is nedded. About the crash, can you post the error you are receinving?

melpomene Over a year ago

It's a double-free.

Riccardo Over a year ago

Yes, you need a default constructor. test::test(){ }

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