Forward declaring static C struct instances in C++

Question

I'm writing a code generator, well, actually a data generator that will produce data structures of this form (obviously the actual data structures are much more elaborate):

typedef struct Foo {
  int a;
  struct Foo* foo;
} Foo;

extern Foo f1;
extern Foo f2;

Foo f1 = {1, &f2};
Foo f2 = {2, &f1};

This is portable for all C and C++ compilers I have tried.

I would like to forward declare these struct instances as static so as not to pollute the global variable space, as in:

typedef struct Foo {
  int a;
  struct Foo* foo;
} Foo;

static Foo f1;
static Foo f2;

static Foo f1 = {1, &f2};
static Foo f2 = {2, &f1};

Although this works with gcc and probably all C compilers, the above code does not work with C++ compilers and results in a compile error:

error: redefinition of ‘Foo f1’
error: ‘Foo f1’ previously declared

I understand why this is happening in C++. Is there a simple workaround that does not involve using code at runtime to achieve the same effect that is portable to all C++ compilers without resorting to using a C compiler to compile certain files?

Answer 1

This should compile with either C or C++ and give you the same name to access the same thing in both compilers.

#ifdef __cplusplus
 namespace // use anonymous namespace to avoid poluting namespace.
 {
    struct StaticFoos
    {
       static Foo f1;
       static Foo f2;
    };

    Foo StaticFoos::f1 = {1, &StaticFoos::f2};
    Foo StaticFoos::f2 = {2, &StaticFoos::f1};
 }

 static const &Foo f1 = StaticFoos::f1;
 static const &Foo f2 = StaticFoos::f2;
#else
 static Foo f1 = {1, &f2_};
 static Foo f2 = {1, &f1_};
#endif

Now in C and C++, you can access f1 and f2.

Answer 2

This seems to have a similar effect to Josh's answer, but with less complexity:

#ifdef __cplusplus
namespace {
    extern Foo f1;
    extern Foo f2;

    Foo f1 = {1, &f2};
    Foo f2 = {2, &f1};
}
#else
    static Foo f1;
    static Foo f2;

    Foo f1 = {1, &f2};
    Foo f2 = {2, &f1};
#endif

When compiled for C++, the extern definitions for f1 and f2 are exposed in the object file with an externally linkable symbol; however, because they are inside an anonymous namespace the symbols are mangled in such a way that they will not conflict with symbols from another translation unit.

With macro magic you could set things up so there's only one place where f1 and f2 are declared and defined, but if this is being generated mechanically there's probably not much reason to do that.

Something like:

#ifdef __cplusplus
#define START_PRIVATES namespace {
#define END_PRIVATES   }
#define PRIVATE extern
#else
#define START_PRIVATES 
#define END_PRIVATES   
#define PRIVATE static
#endif

START_PRIVATES
    PRIVATE Foo f1;
    PRIVATE Foo f2;

    Foo f1 = {1, &f2};
    Foo f2 = {2, &f1};
END_PRIVATES

Answer 3

What you are trying to avoid is termed as Static Initialization Order Fiasco. You will be well served to use functions instead and also to initialize the individual objects with default values and then reseat the member pointers.

Your code samples mean radically different things. You will have to relook. The first one succeeds because you have a definition of one object and a declaration of another. This will work with both C and C++.

extern Foo f1;

This is a declaration and a tentative definition.

static Foo f1;

This is the declaration and definition of an object f1 of type struct Foo.

static Foo f2;

Ditto.

static Foo f1 = {1, &f2};

This is a redefinition. You have violated the One Definition Rule which says, there must be one and exactly one definition of a symbol in a translation unit. Outside of that you are allowed to have multiple definitions but of course each such occurrence has to have the same syntax and semantics.

static Foo f2 = {2, &f1};

Ditto.

extern Foo fn;
/* some code */
extern Foo fn;
/* some more ... */
Foo fn; /* finally a definition */

This is fine since it's ok to have multiple tentative declarations.

Answer 4

You can't forward declare objects, only types. The extern solution is the correct solution. Or if you really need to avoid global namespace pollution, make them static and initialize them using a function which you call before all others.

EDIT: Michael Burr mentioned the reason in a comment, I figured I'd add it to a post:

@dirkgently: it is valid C because the C standard says: "Within one translation unit, each declaration of an identifier with internal linkage denotes the same object or function".

C++ has no such rule.

EDIT:

As noted in one other post. You can use an anonymous namespace as well to limit the scope of the variable. Just wrap the namespace stuff in an #ifdef __cplusplus and you should be good to go.

Answer 5

I have encountered this problem. The restriction is frustrating and I don't see any reason why C++ has this gratuitous incompatibility with C.

My solution is to use static functions - which you can forward declare - which just return f1 and f2:

typedef struct Foo {
  int a;
  struct Foo* foo;
} Foo;

static Foo* link_f1();
static Foo* link_f2();

static Foo f1 = {1, link_f2()};
static Foo f2 = {2, link_f1()};

static Foo* link_f1() { return &f1; }
static Foo* link_f2() { return &f2; }

Unfortunately this isn't valid C so you'll still need different code for C and C++.

Answer 6

Here is what I did in my project. Instead of trying to solve this with an anonymous namespace, I used a named namespace.

[ And then thanks to Matt McNabb's helpful comments, it turns out that an anonymous namespace will do for a super tidy solution with fewer macros that generates no external name pollution. ]

This allows me to have two separate regions of program text, with regular file scope in between, for a tidy solution:

Everything is hidden behind these macros:

#ifdef __cplusplus
#define static_forward(decl) namespace { extern decl; }
#define static_def(def) namespace { def; }
#else
#define static_forward(decl) static decl;
#define static_def(def) static def;
#endif

And so we can do:

static_forward(struct foo foo_instance)

void some_function(void)
{
  do_something_with(&foo_instance);
}

static_def(struct foo foo_instance = { 1, 2, 3 })

The C expansion is straightforward, looking like this:

static struct foo foo_instance;

void some_function(void)
{
  do_something_with(&foo_instance);
}

static struct foo foo_instance = { 1, 2, 3 };

The C++ expansion looks like this:

namespace { extern struct foo foo_instance; }

void some_function(void)
{
  do_something_with(&foo_instance);
}

namespace { struct foo foo_instance = { 1, 2, 3 }; }

So, in a nutshell, thanks to anonymous namespaces, C++ does not actually have a static forward reference problem, only the problem of implementing it in a C incompatible way, which is bridgeable with macros.

Multiple anonymous namespace regions in the same translation unit are the same namespace, and the surrounding file scope outside of the namespace has visibility into it.

Answer 7

I would create two files (.cpp and .h):

code.h:

typedef struct Foo {
  Foo() {}
  Foo(int aa, struct Foo* ff) : a(aa), foo(ff) {}
  int a;
  struct Foo* foo;
} Foo;

static Foo f1;
static Foo f2;

code.cpp:

void myfun()
    {
    f1 = Foo(1, &f2);
    f2 = Foo(2, &f1);
    }

I would also prefer to put all variables like f1, f2 ... into some kind of "storage" object (of my own class or e.g. some STL container). Then, I would define this object as a static one.