Variable Sized Struct C++

Question

Is this the best way to make a variable sized struct in C++? I don\'t want to use vector because the length doesn\'t change after initialization.

struct Pack         


        

Answer 1

If you never add a constructor/destructor, assignment operators or virtual functions to your structure using malloc/free for allocation is safe.

It's frowned upon in c++ circles, but I consider the usage of it okay if you document it in the code.

Some comments to your code:

struct Packet
{
    unsigned int bitlength;
    unsigned int data[];
};

If I remember right declaring an array without a length is non-standard. It works on most compilers but may give you a warning. If you want to be compliant declare your array of length 1.

Packet* CreatePacket(unsigned int length)
{
    Packet *output = (Packet*) malloc((length+1)*sizeof(unsigned int));
    output->bitlength = length;
    return output;
}

This works, but you don't take the size of the structure into account. The code will break once you add new members to your structure. Better do it this way:

Packet* CreatePacket(unsigned int length)
{
    size_t s = sizeof (Packed) - sizeof (Packed.data);
    Packet *output = (Packet*) malloc(s + length * sizeof(unsigned int));
    output->bitlength = length;
    return output;
}

And write a comment into your packet structure definition that data must be the last member.

Btw - allocating the structure and the data with a single allocation is a good thing. You halve the number of allocations that way, and you improve the locality of data as well. This can improve the performance quite a bit if you allocate lots of packages.

Unfortunately c++ does not provide a good mechanism to do this, so you often end up with such malloc/free hacks in real world applications.