Question

I know this might be a common question but I have tried to search but still cannot find a clear answer.

I have the following code:

int* f() {
    int a[] = {1,2,3};
    return a;
}

int main() {

    int a[] = f(); // Error here

    getch();
    return 0;
}

This code produces the error message: “Cannot convert from 'int *' to 'int []'“

I found this quite strange because I have read that pointer and array are similar. For example, we can use a[i] instead of *(a + i).
Can anyone give me a clear explanation, please?

Answer 1

There are actually two errors in this code.

Firstly, you are returning the address of a temporary (the int array within f), so its contents are undefined after the function returns. Any attempt to access the memory pointed to by the returned pointer will cause undefined behaviour.

Secondly, there is no implicit conversion from pointers to array types in C++. They are similar, but not identical. Arrays can decay to pointers, but it doesn’t work the other way round as information is lost on the way – a pointer just represents a memory address, while an array represents the address of a continuous region, typically with a particular size. Also you can’t assign to arrays.

For example, we can use a[i] instead of *(a + i)

This, however, has little to do with the differences between arrays and pointers, it’s just a syntactic rule for pointer types. As arrays decay to pointers, it works for arrays as well.