Question

I have the following code (include-guards omitted for simplicity’s sake):

= foo.hpp =

struct FOO
{
  int not_used_in_this_sample;
  int not_used_in_this_sample2;
};

= main.cpp =

#include "foo_generator.hpp"
#include "foo.hpp"

int main()
{
  FOO foo = FooGenerator::createFoo(0xDEADBEEF, 0x12345678);

  return 0;
}

= foo_generator.hpp =

struct FOO; // FOO is only forward-declared

class FooGenerator
{
  public:

    // Note: we return a FOO, not a FOO&
    static FOO createFoo(size_t a, size_t b);
};

= foo_generator.cpp =

#include "foo_generator.hpp"
#include "foo.hpp"

FOO FooGenerator::createFoo(size_t a, size_t b)
{
  std::cout << std::hex << a << ", " << b << std::endl;

  return FOO();
}

This code, as it stands, compiles perfectly fine without any warning. If my understanding is correct, it should output:

deadbeef, 12345678

But instead, it randomly displays:

12345678, 32fb23a1

Or just crashes.

If I replace the forward-declaration of FOO in foo_generator.hpp with #include "foo.hpp", then it works.

So here is my question: Does returning a forward-declared structure lead to undefined behavior ? Or what can possibly go wrong ?

Compiler used: MSVC 9.0 and 10.0 (both show the issue)

Answer 1

That should be fine according to 8.3.5.6: “The type of a parameter or the return type for a function declaration that is not a definition may be an incomplete class type.”