C++ Allocate a dynamic 2d array in function

Question

gals and everything in between, first off, I want to say sorry for noob/duplicate question.

TL;DR I want to allocate memory for a 2d array of chars, however I can only do so when the program calls alloc(). Any help is appreciated.

Since you guys love code, here is what I have:

#include <iostream>

using namespace std;
#define LOG(x) cout<<endl<<x<<endl

void alloc(char ***add){

    cout<<"Size: ";int s;cin>>s;
        *add = new char *[s];
        for (int i = 0; i < s; i++) {
            *(add)[i] = new char[s];
            cout<<add[i][i]<<" ; ";
        }
}

int main() {
    char **c;

    alloc(&c);
    for (int i =0;i<2;i++) {
        for (int j = 0; j < 2; j++) {
            cout<<c[i][j];
        }
        cout<<endl;
    }
    
cin.get();
}

Now whenever I run this code, I get the following error: Process finished with exit code 139 (interrupted by signal 11: SIGSEGV).

Also, in the debugger, when the function gets called and I input the size of the array, I get a use of undeclared identifier 'c' error.

Thank you and I apologize again for noob or duplicate question, but I did try the answer I got from Text, alas, to no avail.

***I get a use of undeclared identifier 'c' error*** That would prevent you from getting an executable so you could not debug anything. — drescherjm, Oct 12 '20 at 15:28
Ty for your input @drescherjm , but as I said, im a noob. Why it works and I am able to click the debug icon is beyond me at this point. — Stefan Campan, Oct 12 '20 at 15:35
What ever source you use for learning c++ you really should choose something else. The shown code is far from what you would do in c++. The way `alloc` is implemented prevents you from properly releasing the allocated memory and doing boundary tests. — t.niese, Oct 12 '20 at 17:35
@t.niese and I don't suppose you could recommend something else? I learned what I know mostly from the internet and stackoverflow. — Stefan Campan, Oct 13 '20 at 06:04
There is [The Definitive C++ Book Guide and List](https://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list) here on StackOverflow. StackOverflow is a Q&A platform for specific questions but not a good starting point to learn a language. You should first learn the basics and concepts of a language, and that is done by using a good book. — t.niese, Oct 13 '20 at 11:52

score 0 · Accepted Answer · answered Oct 12 '20 at 15:33

0

*(add)[i] does not look quite right, it indexes the add variable and dereferences the pointer afterwards. Those parentheses have no effect.

Try the opposite order, i.e. (*add)[i]

answered Oct 12 '20 at 15:33

Sven Nilsson

1,861
10
11

Just tried that and I get **Process finished with exit code 139 (interrupted by signal 11: SIGSEGV)** – Stefan Campan Oct 12 '20 at 15:37
add[i][i] is also wrong I think. You need to learn to use a debugger to see on which line it crashes... – Sven Nilsson Oct 12 '20 at 15:38
I use add[i][i] as a shorthand for *(*(add+i)+i) – Stefan Campan Oct 12 '20 at 15:49
Ok, made s=2 and just set a breakpoint at (*add)[i] = new char[s]; Now I get: *(c+1)=error: use of undeclared identifier 'c'. I get the sigsegv and the crash after the second time (*add)[i] = new char[s] is executed. – Stefan Campan Oct 12 '20 at 15:52
Good, I think you will figure it out. Just step the program until it crashes. – Sven Nilsson Oct 12 '20 at 15:56
Thats what Im doing, the program crashes after it allocates a new char[] for (*add)[i]. – Stefan Campan Oct 12 '20 at 15:59
Thing is, I dont know why it crashes. The same code worked in the link I provided... – Stefan Campan Oct 12 '20 at 16:05
It seems I am getting an invalid address when I try to cout add[i][i], but i can only take 2 values, 0 and 1, since I am trying to create a 2d array. How the frickfrack am I going out of bounds??? – Stefan Campan Oct 12 '20 at 16:09
Don't forget that "add" is of type char***, while "c" is of type char**. "add" is a pointer to your 2d array, not the 2d array itself. You need to keep that in mind when indexing it. – Sven Nilsson Oct 12 '20 at 16:16
**cout< – Stefan Campan Oct 12 '20 at 16:30

C++ Allocate a dynamic 2d array in function

1 Answers1