Is it legal to cast array of wrapper structs containing POD to the array of POD type it contains?

Question

Is this C++ code valid or undefined behaviour? The idea is to be able to wrap a POD you have no control over into another struct to provide helper functions but still be able to use it as if it was the original POD.

struct Data
{
  ...//POD
};
struct Wrapper
{
  Data data;//contains only this
  void HelperFuncA();
  void HelperFuncB();
  ...//only member functions
};

...
//is this legal?
std::vector<Wrapper> vec_of_wrappers;
Data* array_of_data = reinterpret_cast<Data*>(vec_of_wrappers.data());

Answer 1

Now, this code is not valid. There are several reasons for this. First, casting a pointer to the first member of the struct to the struct itself violates strict aliasing rule. This you can fix by making Wrapper a child class of the Data .

The second issue is more problematic, as you are trying to treat an array (vector in this case) polymorphically. sizeof Data is different from the sizeof Wrapper , so an attempt to index an array of Wrapper elements as if it was an array of Data elements will end up pointing into random areas of the array.