Comment by 0xdeafbeef

#include <vector> #include <iostream>

  int main() {
      std::vector<int> vec = {1, 2, 3};
      vec.reserve(3);

      std::cout << "Initial capacity: " << vec.capacity() << std::endl;
      std::cout << "Initial data address: " << (void*)vec.data() << std::endl;

      int* ptr = vec.data();

      std::cout << "Pointer before push_back: " << (void*)ptr << std::endl;
      std::cout << "Value via pointer before push_back: " << *ptr << std::endl;

      std::cout << "\nPushing back 4...\n" << std::endl;
      vec.push_back(4);

      std::cout << "New capacity: " << vec.capacity() << std::endl;
      std::cout << "New data address: " << (void*)vec.data() << std::endl;

      std::cout << "\nAttempting to access data via the old pointer..." << std::endl;
      std::cout << "Old pointer value: " << (void*)ptr << std::endl;
      int value = *ptr;
      std::cout << "Read from dangling pointer (UB): " << value << std::endl;

      return 0;
  }

./a.out Initial capacity: 3 Initial data address: 0x517d2b0 Pointer before push_back: 0x517d2b0 Value via pointer before push_back: 1

Pushing back 4...

New capacity: 6 New data address: 0x517d6e0

Attempting to access data via the old pointer... Old pointer value: 0x517d2b0 Read from dangling pointer (UB): 20861