Question

Consider the following struct:

struct Vector4D
{
   union
   {
      double components[4];
      struct { double x, y, z, t; } Endpoint;
   };
};

It seems to me that I have seen something similar in WinApi’s IPAddress struct. The idea is to give me the possibility to use the array components both by index and by name, for example:

Vector4D v;
v.components[2] = 3.0;
ASSERT(v.Endpoint.z == 3.0) //let's ignore precision issues for now

In the C++ standard there is a guarantee that there will be no “empty” space at the beginning of a POD-struct, that is, the element x will be situated right in the beginnig of the Endpoint struct. Good so far. But I don’t seem to find any guarantees that there will be no empty space or padding, if you will, between x and y, or y and z, etc. I haven’t checked out the C99 standard though.

The problem is that if there is an empty space between Endpoint struct elements, then the idea will not work.

Questions:

1. Am I right that there indeed is no guarantee that this will work either in C or C++.

2. Will this practically work on any known implementation? In other words, do you know of any implementation where this doesn’t work?

3. Is there any standard(I mean not compiler-specific) way to express the same idea? Maybe the C++0x alignment features might help?

By the way, this isn’t something I am doing in production code, don’t worry, just curious. Thanks in advance.

Answer 1

1. yes
2. depends on the alignment needs of the architecture and the compilers strategy
3. no, but you could make a object wrapper (but you will end up with .z() instead of just .z)

Most compilers should support squashing a structure using a pragma or an attribute. #pragma pack for example.