Write Access Violation (ptr was nullptr) while returning a pointer in a function that was used to create a dynamic array

Asked Feb 06 '18 at 06:39

Active Feb 06 '18 at 06:46

Viewed 64 times

int foo(int *A);

int main()
{
  int* ptr = nullptr; 

  *ptr = foo(ptr); // Line X

  return 0;
}

int foo(int *A)
{
  A = new int[10];
  return *A;
}

When I perform this line of code it gives me a write access error the moment it returns the function call to on line x. It states ptr was nullptr.

I don't understand what this means. I'm trying to dynamically create an array inside this function, return a pointer back to the main, and then modify the values inside.

edited Feb 06 '18 at 06:46

stefanobaghino

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asked Feb 06 '18 at 06:39

Crazypete3

change `foo` to `int* foo(int *A){A = new int[10]; return A;}` and use `ptr = foo(ptr)`! – Barmak Shemirani Feb 06 '18 at 06:42
1

(1) You aren't modifying the pointer in `main`. (2) Even if you were to change it by passing a reference, you'd still have potential undefined behavior, depending on the version of C++ you are compiling as. – StoryTeller - Unslander Monica Feb 06 '18 at 06:42
@StoryTeller Why undefined behavior? – songyuanyao Feb 06 '18 at 06:44
@songyuanyao - In C++14, which side of the assignment expression is sequenced before the other? – StoryTeller - Unslander Monica Feb 06 '18 at 06:45
Thank you, it worked out nicely. I always get pointers confused and mixed up a lot! – Crazypete3 Feb 06 '18 at 06:50
@MartinBonner That's not really the right choice for duplicate. OP's error was due to assigning a value to a dereferenced `nullptr`. This would be a better duplicate: https://stackoverflow.com/q/2763617/224704 – Disillusioned Feb 06 '18 at 07:52
OP: What was your problem? Was it "what does 'null pointer assignment' mean?" or "why is my pointer still null?". The duplicate @CraigYoung suggests answers the first question; I closed the question with a duplicate for the second question. – Martin Bonner supports Monica Feb 06 '18 at 18:18

Write Access Violation (ptr was nullptr) while returning a pointer in a function that was used to create a dynamic array

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