dynamic array created in another method..

Can anyone explain to me why when I use the new prefix on a pointer in another method than I created it, it does not work as intended?

Code:

---

#include <iostream>
void magic(int *T, int sz)
{
        T = new int[sz];
        for(int i=0;i<sz;++i)
                T[i] = i;
}

int main()
{
        int *T;
        magic(T , 10);
        for(int i=0;i<10;++i)
                std::cout << T[i] << " ";
}

---

output is random numbers from memory.

does T no longer point to the start of array when the method magic reaches it end?

Thanks!

That is correct. T no longer points to the start of the new array when the magic function ends. If you want changes to a variable to not disappear, you need to pass by reference, i.e., with

Code:

---

void magic(int* &T, int sz)

---

ah didnt catch that, I thought that when I passed a pointer that it would behave like that, thanks!

Pointers aren't magic; they are subject to the same call-by-value semantics as everything else. (You can use that pointer to do magic with what it already points to, but you cannot try to make it point somewhere else, as that is just changing the local copy and not the pointer back in the original calling function.)

is there anything i should worry about when I pass a reference to a pointer like that ?

I am trying to use similar code and I am getting crazy error about malloc fail,

Any idea? i will post snippit of my actual code that does receive error on its own.

compiles fine but has run-time error :(

Code:

---

#include <string>

#include <iostream>
using namespace std;
void magic(int* &mpNext, string x)
{
        int m = x.length();
        int i, j;
        mpNext = new int[m];
        for(int i=0;i<m;++i )
                mpNext[i] = 0;

        i = 0;
        j = mpNext[0] = -1;

        while (i < m) {
                while (j > -1 && x[i] != x[j])
                        j = mpNext[j];
                mpNext[++i] = ++j;
        }
}


int main()
{
        int *T;
        string hay;
        while(cin >> hay)
        {
                magic(T,hay);
                cout << "method ran well" << endl;
        }

return 0;
}

---

input:

Code:

---

rod
blue
cpp

---

output:

Code:

---

method ran well
method ran well
a.out: malloc.c:3074: sYSMALLOc: Assertion `(old_top == (((mbinptr) (((char *) &((av)->bins[((1) - 1) * 2])) - __builtin_offsetof (struct malloc_chunk, fd)))) && old_size == 0) || ((unsigned long) (old_size) >= (unsigned long)((((__builtin_offsetof (struct malloc_chunk, fd_nextsize))+((2 * (sizeof(size_t))) - 1)) & ~((2 * (sizeof(size_t))) - 1))) && ((old_top)->size & 0x1) && ((unsigned long)old_end & pagemask) == 0)' failed.
Aborted

---

I am able to print out method ran well but then gives me big malloc error

Since you ask for m numbers, m-1 is the largest index you can use. You are guaranteeing yourself one overrun -- the last go-round of the loop ++i will equal m and you will go over. It's possible j is fine, but you may want to double-check that (there's certainly no check in your code that j won't all of a sudden be 235000).

thank you tabstop, you are great !

why not to use std::vector instead of dynamic array?