Can C++ pass empty pointer to a function? - c++

Question

For example, I want to declare pointer to array and to don't wanna initialize it. Then, I want pass it to a function and initialize it exactly in function. How can I do it in C++?
void F(int *B, const int& N) {
B = new int[N];
for (int i = 0; i < N; ++i) B[i] = i;
}
int main() {
int N = 4;
int *B = nullptr; // doesn't work
F(*B, N);
for (int i = 0; i < N; ++i)
cout << B[i] << endl;
return 0;
}
It evokes/produces error corresponds to the problem that B has no reference to point and, therefore he can't initialize array.
error: Process finished with exit code 11

Answer 1

The pointer you pass is a copy of the original pointer. When you modify B in F, B i main() remains the same.
You need to pass the pointer by reference or pass a pointer to the pointer:
void F(int*& B, const int& N) {
B = new int[N];
for (int i = 0; i < N; ++i) B[i] = i;
}
int main() {
int N = 4;
int *B = nullptr; // doesn't work
F(B, N); //no operator here, just pass the pointer by reference
for (int i = 0; i < N; ++i)
cout << B[i] << endl;
return 0;
}
or
void F(int** B, const int& N) {
*B = new int[N];
for (int i = 0; i < N; ++i) (*B)[i] = i;
}
int main() {
int N = 4;
int *B = nullptr; // doesn't work
F(&B, N); //need address-of operator
for (int i = 0; i < N; ++i)
cout << B[i] << endl;
return 0;
}
Also, since you decided to use manual memory management, you are also responsible to release the memory acquired with new. This is not necessary if you use std::vector as your container instead of raw pointer (or std::unique_ptr or other containers).
for (int i = 0; i < N; ++i)
cout << B[i] << endl;
delete[] B; //release memory after use
return 0;

Answer 2

You have at least two problems.
The first in in the call where you use *B. *B is the same as B[0], which means you're attempting to pass a single int element to the function. But since B is a null pointer, the dereference will lead to undefined behavior.
Correct (to fit the shown function signature) would be plain B:
F(B, N);
The second problem is that passing arguments to functions by default is done by value. Which means that the value of B is copied into the local variable B inside the function F.
When you assign to B in the function you only change the local copy, and the original value in the main function will be unmodified.
To solve this you need to pass B by reference:
void F(int*& B, const int N) { ... }