C++ and binary compatibility: returning a POD struct by value

Question

Consider the following C++ code:

struct X
{
        int a;
        int b;
};

X foobar()
{
        X x = { 1, 2 };
        return x;
}

Now assume this code is put in a shared library, which is used by third-party applications.

My question is: if I add another member at the end of X (e.g. int c), and initialize it in foobar(), will existing applications which call foobar() break? Note that this is about binary compatibility, not source compatibility.

How is the "new" struct going to be used? The client code needs X to be declared in a header. Which X are you going to declare? Bottom line, you don't want to do this, it's why things like COM were invented. — user2100815
X is declared and defined in a header which is used to compile the application. foobar() is declared in a header but defined in a .cpp file inside the shared library. Of course, if X changes, the application won't be recompiled and so won't "see" that the header changed. Hence the question. — Etienne Dechamps
Short answer: Don't do it. Long answer: Why would you want to do that? What's the real situation here? — Kerrek SB
@Kerrek SB: in my particular situation that's just the simplest way to write my code. Neither the library nor the applications are written yet; I was just wondering if I could write this and still maintain binary compatibility in case X evolves. It seems that's not the case, so I'll just write it another way. No worries. — Etienne Dechamps

Oliver Charlesworth Oliver Charlesworth · Accepted Answer · 2011-06-08T16:28:56

It depends entirely on what your compiler chooses to do (more specifically, what the platform ABI dictates).

You can imagine that if the return value is placed on the stack, you would now be writing more onto the stack than the caller is expecting, which may result in stamping on something.

In general, you shouldn't rely on any particular behaviour. You simply must re-compile the client applications. The only realistic alternative is something like the PIMPL idiom.

C++ and binary compatibility: returning a POD struct by value

2 Answers