Variable length array inside a c++ sturcture

Question

I wondering what the best solution is for a structure with variable length array for one of the fields. I've done a bunch of research and I haven't seen a clear answer yet. I've been playing with the below code and trying to get the varField to be set to an array the size of 10 bytes.

typedef struct TestStruct{
    int size;
    unsigned char varField[1];
}

I have tried doing zero sized array and that gives me a compile error. I also tried something like this and it gave me a compile error.

int size= 10;
struct TestStruct*test = malloc(sizeof(struct TestStruct) + (size- 1));
test->size= size;

Thank you so much for help.

Is there some compelling reason to not use a standard container? — shuttle87, Jun 17 '14 at 15:35
Flexible arrays is [not part of C++ except by extension](http://stackoverflow.com/a/22608418/1708801) if this is C99 instead use [this method](http://stackoverflow.com/a/20221073/1708801). — Shafik Yaghmour, Jun 17 '14 at 15:36
You say "C++" but this code is entirely C, with conventions that C++ has abandoned. — Drew Dormann, Jun 17 '14 at 15:36
I am using C++. I just playing around with different ways of converting message fields to a byte message. But I don't know the size of the one field until runtime. Can I do a class that will map directly to bytes that won't include overhead of the class??????? — user3470688, Jun 17 '14 at 15:42
That last question makes no real sense to me. Adding lots of question marks doesn't make it any clearer. I think you need to try to explain the real problem and let people guide you to a good solution. This looks like an XY problem where you decide what the solution is without telling us the problem. What is your real underlying problem? — David Heffernan, Jun 17 '14 at 15:44
I have a structure that needs to be in a certain format for a certain message ICD. In the ICD byte 1 has to be lenght. The next field is a variable length field based upon the lenght. But I don't know this lenght until runtime. I know with a structure, if the size was NOT variable, how to convert this structure to a byte array. But since I don't know the size of one of the fields and if I just use a Vector, and convert it to bytes, you are going to have all the vector object as part of the message. how do I get ALL of the bytes for the stcructure at one time. — user3470688, Jun 17 '14 at 15:55

score 2 · Answer 1 · answered Jun 17 '14 at 15:35

The preferred way is to use the dynamically re-sizable std::vector. This class has the rule of five built in.

struct TestStruct {
    std::vector<unsigned char> varField;
}

If you're allergic to the standard library, you could use:

unsigned char *varfield;

And supply the necessary constructors/destructors.

score 1 · Answer 2 · answered Jun 17 '14 at 16:57

If you are implementing messages, a better solution is to set up a hierarchy:

struct Message_Base
{
  unsigned int message_length_in_bytes;
  unsigned int message_id;
  virtual Checksum_Type calculate_checksum(void) = 0;
  virtual bool send_message(Receiver& r) = 0;
  virtual bool receive_message(Sender& s) = 0;
  virtual void process_message(void) = 0;
};

Each child class would be a different message with possible different lengths. Some possible common methods to all message are listed.

This is how to implement using Object Oriented and C++.

The class C language implementation is to declare a zero length array at the end for the message's unique data.

score 0 · Accepted Answer · answered Jun 17 '14 at 15:35

0

You look like you want a std::vector<unsigned char>:

struct TestStruct{
    std::vector<unsigned char> varField;
}

and you get the size with:

ts.varField.size();

answered Jun 17 '14 at 15:35

Paul Evans

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I can't use a vector for this, since I need to convert it to a byte array for a message. – user3470688 Jun 17 '14 at 15:37
2

@user3470688 You are trying to solve the wrong problem. The right problem to solve here is to use something else than an unformatted byte stream for messaging. There are better solutions galore. Google released a new one just today. – Konrad Rudolph Jun 17 '14 at 15:37
2

not a problem, use the `data()` member of `vector` to get that byte array – Paul Evans Jun 17 '14 at 15:37
Why did you decide that you cannot convert a `std::vector` to a byte array? – David Heffernan Jun 17 '14 at 15:45
If I use a vector I can get the data bytes, but I also need the rest of the structure around it. – user3470688 Jun 17 '14 at 15:46

sigy · Answer 4 · 2014-06-17T15:45:21.800

You can't. In C++ dynamic size arrays are illegal. The size of an array must be a compile time constant expression.

The options you have basically are

Use an STL container like std::vector or the like. The benefits are that they also take care of memory allocation and deallocation for you.
Use a pointer in your struct and allocate the memory for it dynamically. Don't forget to use delete[] instead of just delete!

score 0 · Answer 5 · answered Jun 17 '14 at 15:46

In most compilers, the following will work:

template<unsigned N>
struct TestStruct {
  unsigned size = N;
  unsigned char varField[N];
};
struct ITestStruct {
  unsigned size;
  unsigned char varField[1]; // variable
};

template<unsigned N>
ITestStruct* make_test_struct() {
  return reinterpret_cast<ITestStruct*>(new TestStruct<N>());
};
ITestStruct* make_test_struct( unsigned n ) {
  char* buff = new char[ sizeof(ITestStruct)+n-1 ];
  ITestStruct* retval = reinterpret_cast<ITestStruct*>(buff);
  retval->size = n;
  return retval;
}

If you replace char with another non-POD type, things will get hairy.

Variable length array inside a c++ sturcture

5 Answers5