CODEWITHSUNDEEP
c++
Hercules
Hercules

Reputation: 138

a trouble of copying array in C++

I am copying an array in C++, here is the code:

int arr1[10] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
int *source = arr1;
size_t sz = sizeof(arr1) / sizeof(*arr1); // number of elements
int *dest = new int[sz];                // uninitialized elements
while (source != arr1 + sz)
    *dest++ = *source++; //  copy element and increment pointers

int *p = dest;
while (p != dest + sz) {
    cout << *p++ << endl;
}

after running the code above-mentioned, I got:

714124054
51734
9647968
9639960
0
0
0
0
0
0

what's the trouble?

Upvotes: 1

Views: 104

Answers (2)

lulyon
lulyon

Reputation: 7225

The starting pointer is not saved with int *dest = new int[sz];, after *dest++ in the while loop, so the output is out of range of the array.

The revised code:

int arr1[10] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
int *source = arr1;
size_t sz = sizeof(arr1) / sizeof(*arr1); // number of elements
int *dest = new int[sz];                // uninitialized elements
int *temp_dest = dest;
while (source != arr1 + sz)
    *temp_dest++ = *source++; //  copy element and increment pointers

int *p = dest;
while (p != dest + sz) {
    cout << *p++ << endl;
}

Upvotes: 0

J.N.
J.N.

Reputation: 8421

The array is copied properly, though, by incrementing dest you are losing the actual beginning.

You need to keep a copy of dest to loop after it. Also don't forget to free the memory after you have allocated it.

Finally, in C++ you'll probably want to use std::vector instead of arrays that does all this in a transparent way while trading a minimum amount of performance.

int arr1[10] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
int *source = arr1;
size_t sz = sizeof(arr1) / sizeof(*arr1); // number of elements
int *dest = new int[sz];                // uninitialized elements
int *d = dest;
while (source != arr1 + sz)
    *d++ = *source++; //  copy element and increment pointers

int *p = dest;
while (p != dest + sz) {
    cout << *p++ << endl;
}

[...]
delete[] dest;

Upvotes: 5

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