Dangling and null

Questions 1 to 5 of 6

1. 1. What does this program print? ``cpp #include <iostream> int value_or_zero(const int* p) { return p != nullptr ? *p : 0; } int main() { int a = 4; int b = 9; int* p = &a; int* q = p; p = &b; std::cout << value_or_zero(p) << ' ' << value_or_zero(q) << ' ' << value_or_zero(nullptr) << '\n'; return 0; } ``

   4 9 09 9 09 4 09 4 -1
2. 2. Which statement describes the bug in this program? ``cpp #include <iostream> int main() { int* p = new int(7); int* q = p; delete p; p = nullptr; std::cout << *q << "\n"; return 0; } ``

   q still holds the address that delete p freed. Setting p to nullptr does not change q, so *q is a dangling read.The new int(7) leaks, because the program never calls delete on it.*q reads a null pointer, so the program crashes on that line every time.int* q = p; is a syntax error, because a pointer cannot be copied.
3. 3. What is the bug in this function? ``cpp int* make_number() { int local = 5; return &local; } ``

   local is never initialised, so it holds whatever was in memory before.&local returns the address of a variable that ends when the function returns, so the caller gets a dangling pointer.The return type should be int, because a pointer cannot hold an address.The function should return nullptr for every input, because a local cannot have an address.
4. 4. Why does a p != nullptr check not protect against a dangling pointer?

   The compiler removes the check when the code is optimised.A freed address is not zero, so the check passes and the read still happens.The check compares the value at the address, not the address itself.nullptr is only defined for int pointers.
5. 5. A Java program has String name = null; and then calls name.length(). What happens?

   It returns 0, because a null string has no characters.It reads a random number, the same as a dangling C++ read.It throws NullPointerException, and the program stops unless the exception is caught.It exits with code 139 and prints no message.