alloc a struct with zero length array using new

Question

In C (using gcc) I can declare a variable length struct as below:

typedef struct ProtocolFrame
{
     uint8_t      op;
     uint32_t     address;
     uint16_t     size;
     uint8_t      payload[0];
} ProtocolFrame;

then I can alloc different frame:

ProtocolFrame *frA;
ProtocolFrame *frB;

frA = malloc(sizeof(ProtocolFrame) + 50);
frB = malloc(sizeof(ProtocolFrame));

In this example frA has a payload field as big as 50 bytes, and frB has no payload

Can I do the same thing in C++ using the new operator?

Answer 1

template<size_t s>
struct ProtocolFrame
{
     uint8_t      op;
     uint32_t     address;
     uint16_t     size;
     uint8_t      payload[s];
} ProtocolFrame;

// specialize for no payload
template<>
struct ProtocolFrame<0>
{
     uint8_t      op;
     uint32_t     address;
     uint16_t     size;
} ProtocolFrame;

ProtocolFrame<50> *frA = new ProtocolFrame<50>;
ProtocolFrame<0> *frB = new ProtocolFrame<0>;

To decide what is the size at runtime you could use placement-new operator in cooperation with std::malloc:

void *buffer = std::malloc(sizeof(ProtocolFrame)+50);
ProtocolFrame *frA = new (buffer) ProtocolFrame;

You can read also this article on the codeproject.com which is contain the full sample.