C++ way of creating a structure with variable number of items

Question

I need to create a memory region with variable number of items at the end. Something that I can write like this:

#pragma pack(push,0)
struct MyData
{
    int noOfItems;
    int buffer[0];
};
#pragma pack(pop)

MyData * getData(int size)
{
     bufferSize = size* sizeof(int) + sizeof(MyData ::noOfItems);
     MyData * myData= (MyData *)new char[m_bufferSize];
     return myData;
}

This code works on VS 2015 with a warning that array of size of zero is not standard

A bit of search showed me that this is a C hack and is not supported on C++.

Array with size 0

Array of zero length

What happens if I define a 0-size array in C/C++? .

How can I do this in C++

Answer 1

What about something like this? It works for trivially copyable types:

template <typename T, typename INT = size_t>
class packed_array {
  public:
    packed_array(INT size) {
      buf_ = new char[sizeof(INT) + size * sizeof(T)];
      memcpy(buf_, &size, sizeof(INT));
    }
    ~packed_array() { delete[] buf_; }

    void set(INT index, T val) {
      memcpy(buf_ + sizeof(INT) + index * sizeof(T), &val, sizeof(T));
    }
    T get(INT index) const {
        T temp;
        memcpy(&temp, buf_ + sizeof(INT) + index * sizeof(T), sizeof(T));
        return temp;
    }

    const char* data() const { return buf_; }

  private:
    char* buf_;

  static_assert(std::is_trivially_copyable<T>::value);
};

int main() {
  int n;
  std::cin >> n;
  packed_array<double, int> a(n);
  for (int i = 0; i < n; i++)
    a.set(i, pow(2.0, i)); 
  for (int i = 0; i < n; i++)
    std::cout << a.get(i) << std::endl;
}

Live demo: https://wandbox.org/permlink/Vc4ok756R1Sxieoj

Answer 2

template<class T, class D>
struct var_array_at_end {
  var_array_at_end( std::size_t N ) {
    ::new( (void*)data() ) std::aligned_storage_t<sizeof(T)*N, alignof(T)>;
    for (std::size_t i = 0; i < N; ++i) {
      ::new( (void*)( data()+sizeof(T)*i) ) ) T();
    }
  }
  char* data() { return reinterpret_cast<char*>(this)+sizeof(D); }
  char const* data() const { return reinterpret_cast<char*>(this)+sizeof(D); }
  T* ptr(std::size_t i = 0) { return reinterpret_cast<T*>( data()+sizeof(T)*i ); }
  T const* ptr(std::size_t i = 0) const { return reinterpret_cast<T*>( data()+sizeof(T)*i ); }
  T& operator[](std::size_t n) {
    return *ptr(n);
  }
  T const& operator[](std::size_t n) const {
    return *ptr(n);
  }
};

struct MyData:
  var_array_at_end<int, MyData>
{
private:
  explicit MyData( int count ):
    var_array_at_end<int, MyData>( count>0?(unsigned)count:0 ),
    noOfItems(count)
  {}
  struct cleanup {
    void operator()(MyData* ptr) {
      char* buff = reinterpret_cast<char*>(ptr);
      ptr->~MyData();
      delete[] buff;
    }
  };
public:
  using up = std::unique_ptr<MyData*, cleanup>;
  static up create( int count ) {
    if (count < 0) count = 0;
    std::unique_ptr<char> buff = std::make_unique<char[]>( sizeof(MyData)+sizeof(int)*count );
    auto* ptr = ::new( (void*)buff.get() ) MyData( count );
    (void)buff.release();
    return up( ptr, {} );
  }
  int noOfItems;
};

MyData * getData(int size)
{
   return MyData::create(size).release(); // dangerous, unmanaged memory
}

I believe this is standard compliant, assuming your implementation doesn't add padding on arrays of trivial types (like char). I am unware of any implementation thta does.

I didn't assume that MyData only contains plain old data; you could stuff a std::vector into it with the above. I might be able to simplify a few lines with that assumption.

It is more than a bit of a pain.

auto foo = MyData::create(100) creates a unique ptr to MyData that has a buffer of 100 int s after it. (*foo)[77] accesses the 77th element of the buffer.

Due to a defect in the standard, you there isn't an array after MyData but rather a buffer containing 100 distinct int objects in adjacent memory locations. There are vrey annoying differences between those two things; naive pointer arithimetic is guaranteed to work within arrays, but not between packed adjacent int s in a buffer. I am unaware of a compiler that enforces that difference.