Why use array size 1 instead of pointer?

Question

In one C++ open source project, I see this.

struct SomeClass {
  ...
  size_t data_length;
  char data[1];
  ...
}

What are the advantages

Answer 1

Usually you see this as the final member of a structure. Then whoever mallocs the structure, will allocate all the data bytes consecutively in memory as one block to "follow" the structure.

So if you need 16 bytes of data, you'd allocate an instance like this:

SomeClass * pObj = malloc(sizeof(SomeClass) + (16 - 1));

Then you can access the data as if it were an array:

pObj->data[12] = 0xAB;

And you can free all the stuff with one call, of course, as well.

The data member is a single-item array by convention because older C compilers (and apparently the current C++ standard) doesn't allow a zero-sized array. Nice further discussion here: http://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html