Zystkmt

Well this is really weird

Consider the following two codes :-

#include <iostream>

using namespace std;



int main() {

    int a={1,2,3,4,5};

    int* ptr[5];



    for(int i = 0; i < 5; i++)

    {

        *ptr[i]=a[i];

    }



    for(int i = 0; i < 5; i++)

    {

        cout<<*(ptr[i]);

    }

}

AND

#include <iostream>

using namespace std;



int main() {

    int a={1,2,3,4,5};

    int* ptr[5];





        *ptr[0]=a[0];

        *ptr[1]=a[1];

        *ptr[2]=a[2];

        *ptr[3]=a[3];

        *ptr[4]=a[4];









       cout<<*(ptr[0])<<endl;

       cout<<*(ptr[1])<<endl;

       cout<<*(ptr[2])<<endl;

       cout<<*(ptr[3])<<endl;

       cout<<*(ptr[4])<<endl;





}

The first one gives runtime errorr , while the seconde one gives 1 ,2 ,3 ,4 ,5 output , I can't find the difference between the two codes , can anyone help me to find the difference .

#include <iostream>

using namespace std;



int main() {

    int a={1,2,3,4,5};

    int* ptr[5];





        *ptr[0]=a[0];

        *ptr[1]=a[1];











       cout<<*(ptr[0])<<endl;

       cout<<*(ptr[1])<<endl;









}

the above code runs fine and gives output 1, 2
but

#include <iostream>

using namespace std;



int main() {

    int a={1,2,3,4,5};

    int* ptr[5];





        *ptr[0]=a[0];

        *ptr[1]=a[1];

        *ptr[2]=a[2];









       cout<<*(ptr[0])<<endl;

       cout<<*(ptr[1])<<endl;

       cout<<*(ptr[2])<<endl;









}

this code gives runtime error on codeblocks(gcc) , I am just getting more confused

I have both runtime errors.
The result of recoding with the pointer variable below.

#include <iostream>

using namespace std;



int main() {

    int a={1,2,3,4,5};

    int* ptr[5];



    for(int i = 0; i < 5; i++)

    {

        ptr[i]=&a[i];

    }



    for(int i = 0; i < 5; i++)

    {

        cout<<*(ptr[i]);

    }

}

It is already established that

*ptr[0] = a[0];

with unallocated pointer is undefined behavior.

The solution

ptr[0] = &a[0];

is also already given. This works, but it is limited because now, the ptr array is tied to a, which you not always want. In that case you might want

ptr[0] = new int(a[0]);

This initializes the array with values from a but keeps it independent. However, this introduces manual memory management and this can become tricky in more complex code (memory leaks, double deallocations, dangling pointers), so a more C++ like solution would be (assuming the pointers point to something more interesting than an integer, because now there seems to be no need for pointers).

#include <memory>

#include <vector>

#include <iostream>



int main() {

    int a={1,2,3,4,5};

    std::vector<std::unique_ptr<int>> ptrs;



    for(auto i: a)

    {

        ptrs.emplace_back(std::make_unique<int>(i));

    }



    for(auto& i: ptrs)

    {

        std::cout << *i;

    }

}