How to add data to an array of pointers? - c++

Question

I created an array of pointers.
I want to know how to prp=operly add data to my array of pointers.
I have attempted the code below, unfortunately it is giving me a memory error and I do not know why.
class dataClass {
int data;
public:
void setdata(int d) {data = d;}
int getdata() const {return data;}
};
int main() {
dataClass** ptr = new dataClass*s[5];
int num = 9;
ptr[0] -> setdata(num);
return 0;
}

Answer 1

You just allocate the memory for the pointers (not for the objects), so ptr[0] points to an unspecified address and you get a segfault.
You need to add something like this:
for(int i = 0; i < 5; i++) {
ptr[i] = new dataClass;
}

Answer 2

When using a pointer-to-pointer-to-type, you must allocate storage for the required number of pointers and then allocate storage for each class which you then assign to each pointer.
For example, using 5 pointers as you attempt in your questions, you would first need to allocate for the pointers, e.g.
#define NPTRS 5
...
dataClass** ptrs = new dataClass *[NPTRS];
Now you have storage allocated for 5 pointers to type dataClass with the address for the initial pointer assigned to ptrs (I added the plural form for clarity)
The storage for the pointers is now allocated, but each of the allocated pointers does not yet point to valid storage for dataClass, they are just pointers. Before you can use the pointers to reference an instance of the class, you must allocate storage for each each class instance and then assign the address of the new block of memory to each of your pointers, e.g.
for (int i = 0; i < NPTRS; i++) {
ptrs[i] = new dataClass;
ptrs[i]->setdata (i+1);
}
(above you allocate storage for each class instance assigning the result to ptrs[i] and then set the data value to i+1)
Putting it altogether, you could do something similar to:
#include <iostream>
#include <cstdlib>
#define NPTRS 5
class dataClass {
int data;
public:
void setdata(int d) {data = d;}
int getdata() const {return data;}
};
int main() {
dataClass** ptrs = new dataClass *[NPTRS];
for (int i = 0; i < NPTRS; i++) {
ptrs[i] = new dataClass;
ptrs[i]->setdata (i+1);
}
for (int i = 0; i < NPTRS; i++)
std::cout << "ptrs[" << i <<"]->getdata() : "
<< ptrs[i]->getdata() << '\n';
return 0;
}
(note: don't forget you are responsible for keeping track of what you allocated and calling delete to free each allocation when it is not longer needed)
Example Use/Output
$ ./bin/dblptr2class
ptrs[0]->getdata() : 1
ptrs[1]->getdata() : 2
ptrs[2]->getdata() : 3
ptrs[3]->getdata() : 4
ptrs[4]->getdata() : 5
As others have mentioned, the C++ containers like vector are for less error prone and make your job a lot easier. Still, you should know how to handle both new/delete as well as the use of the containers.
Look things over and let me know if you have further questions.

Answer 3

You could first create an array of objects
(constructors will be called automatically).
After that create a pointer to your pointer:
#include <stdio.h>
class dataClass {
int data;
public:
dataClass(){
printf(" new instance \n");
}
void setdata(int d) {
data = d;
printf(" setdata \n");
}
int getdata() const {return data;}
};
int main() {
dataClass* ptr = new dataClass[5];
dataClass** pptr = &ptr;
int num = 9;
pptr[0]->setdata(num);
return 0;
}
result:
new instance
new instance
new instance
new instance
new instance
setdata