C++ 可以将空指针传递给 function 吗？

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) { ... }