Declaring an array of pointers to objects dynamically in C++

Question

I have to declare an array of pointers to objects (of classes) in C++. I thought this was the only way, but apparently I was wrong, as it throws a syntax error when I try to compile it. Specifically, among the 7 errors I received, 2 of these errors are in the lines: where I create the array using "new", and in the line where I call the "setData()" function. Can you tell me where I went wrong? Thanks.

#include <iostream>

class Test
{
    public:
        int x;

        Test() { x=0; }
        void setData(int n) { x=n; }
};

void main()
{
    int n;
    Test **a;

    cin >> n;
    a=new *Test[n];

    for(int i=0; i<n; i++)
    {
        *(a+i)=new Test();
        *(a+i)->setData(i*3);
    }
}

deviantfan · Accepted Answer · 2014-07-18 04:06:48Z

3

Use a=new Test*[n];
Other than that, you have no delete´s in your program, trivial getter/setters
for public variables are strange, and *(a+i) could be a[i]

answered Jul 18, 2014 at 4:06

deviantfan

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4 Comments

user1637645 Over a year ago

Thanks a lot, it worked. But, can you tell me what's the difference in placing the asterisk before and after the class-name? and Why can't I use *(a+i) (although I know a[i] is better), still?

deviantfan Over a year ago

About the a[i]: You can use the other thing too, but why? About the asterisk: The language is just made this way; you can´t reorder everything. For a new int[10], [10]new int or new [10]int would be wrong too...

user1637645 Over a year ago

okay... but *(a+i)->setData() does not work, it throws up an error. I had to use a[i].

deviantfan Over a year ago

(*(a+i))->setData()

Remy Lebeau · Accepted Answer · 2014-07-18 04:13:14Z

Your syntax is close but slightly off. Use this instead:

Test **a;

...

a=new Test*[n];

for(int i=0; i<n; i++)
{
    a[i]=new Test();
    a[i]->setData(i*3);
}

...

// don't forget to free the memory when finished...

for(int i=0; i<n; i++)
{
    delete a[i];
}

delete[] a;

Since you are using C++, you should use std::vector instead. I would also suggest passing the desired value to the class constructor:

#include <iostream>
#include <vector>

class Test
{
    public:
        int x;

        Test(int n = 0) : x(n) { }
        Test(const Test &t) : x(t.x) { }
        void setData(int n) { x=n; }
};

int main()
{
    int n;
    std::vector<Test> a;

    cin >> n;
    a.reserve(n);

    for(int i=0; i<n; i++)
    {
        a.push_back(Test(i*3));
    }

    ...

    // memory is freed automatically when finished...

    return 0;
}

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