How to safely allocate memory at a pointer using a function & then use it in main

[Kanj]

I have a piece of code like this.

# include <iostream>

using namespace std;

int make_array_at_p (int **p) {
	cout<<"-----------------\n";
	*p = new int [10];
	cout<<"No segmentation fault\n";
	return 10;
}

int main (int argc, char* argv[]) {
	int **pointer;
	int size = make_array_at_p(pointer);
	// do something with array pointed by pointer
	return 0;
}

I want to declare a pointer in main(), fill it with data using a function and then use the data further in main().

My actual code needs data in 2D array so I used triple pointer argument for make_array_at_p. But this sample code is sufficient to explain my problem.

Now this code was running fine earlier today on Cygwin in windows 7.

Now it's getting a segmentation fault: 11 on my mac and on online compilers.

I am getting the dashes but the next cout is not being executed.

So I guess, I am making a mistake somewhere.

Can anyone explain to me what am I doing wrong and what is the correct way to deal with this?

Thanks.

[SECMoG1T]

you dereferenced the pointer before you allocated anything to it, this should work.

# include <iostream>

using namespace std;

int make_array_at_p (int **p) {
	cout<<"-----------------\n";
	p = new int* [10];
	cout<<"No segmentation fault\n";
	return 10;
}

int main (int argc, char* argv[]) {
	int **pointer;
	int size = make_array_at_p(pointer);
	// do something with array pointed by pointer
	return 0;
}

what you're code was trying to do is assign int* [] to a location that should instead hold int []//"/" int and in a better sense that location should have been p[0] but by that time the location din't exist yet.

[Kanj]

This will cause error when I'll try to access memory pointed by pointer variable in main().

Check this-

# include <iostream>

using namespace std;

int make_array_at_p (int **p) {
	cout<<"-----------------\n";
	p = new int* [10];
	cout<<"No segmentation fault\n";
	for (int i=0; i<10; i++) p[i] = new int[2];
	p[0][1] = 39; // check if we can get this data in main()
	return 10;
}

int main (int argc, char* argv[]) {
	int **pointer;
	int size = make_array_at_p(pointer);
	// do something with array pointed by pointer
	cout<<size<<" rows created\n";
	cout<<pointer[0][1]<<"\n";
	return 0;
}

Now I can't use the data that I stored in make_array_at_p because p is a local variable of the function.

I do get your point about dereferencing the pointer even before any memory is allocated to it.

But how can I keep accessing memory allocated by a helper function back in main()?

[SECMoG1T]

Now I can't use the data that I stored in make_array_at_p because p is a local variable of the function

no p isn't local to the function,the problem you are getting is beause you're passing the pointer by value use reference instead.

# include <iostream>

using namespace std;

int make_array_at_p (int **&p) {
	cout<<"-----------------\n";
	p = new int* [10];
	cout<<"No segmentation fault\n";
	for (int i=0; i<10; i++) p[i] = new int[2];
	p[0][1] = 39; // check if we can get this data in main()
	return 10;
}

int main (int argc, char* argv[]) {
	int **pointer;
	int size = make_array_at_p(pointer);
	// do something with array pointed by pointer
	cout<<size<<" rows created\n";
	cout<<pointer[0][1]<<"\n";
	return 0;
}

[Kanj]

Alright. I've got it now.

Thanks a lot.