Representing dynamic typing in C

问题

I'm writing a dynamically-typed language. Currently, my objects are represented in this way:

struct Class { struct Class* class; struct Object* (*get)(struct Object*,struct Object*); };
struct Integer { struct Class* class; int value; };
struct Object { struct Class* class; };
struct String { struct Class* class; size_t length; char* characters; };

The goal is that I should be able to pass everything around as a struct Object* and then discover the type of the object by comparing the class attribute. For example, to cast an integer for use I would simply do the following (assume that integer is of type struct Class*):

struct Object* foo = bar();

// increment foo
if(foo->class == integer)
    ((struct Integer*)foo)->value++;
else
    handleTypeError();

The problem is that, as far as I know, the C standard makes no promises about how structures are stored. On my platform this works. But on another platform struct String might store value before class and when I accessed foo->class in the above I would actually be accessing foo->value, which is obviously bad. Portability is a big goal here.

There are alternatives to this approach:

struct Object
{
    struct Class* class;
    union Value
    {
        struct Class c;
        int i;
        struct String s;
    } value;
};

The problem here is that the union uses up as much space as the size of the largest thing that can be stored in the union. Given that some of my types are many times as large as my other types, this would mean that my small types (int) would take up as much space as my large types (map) which is an unacceptable tradeoff.

struct Object
{
    struct Class* class;
    void* value;
};

This creates a level of redirection that will slow things down. Speed is a goal here.

The final alternative is to pass around void*s and manage the internals of the structure myself. For example, to implement the type test mentioned above:

void* foo = bar();

// increment foo
if(*((struct Class*) foo) == integer)
    (*((int*)(foo + sizeof(struct Class*))))++;
else
    handleTypeError();

This gives me everything I want (portability, different sizes for different types, etc.) but has at least two downsides:

Hideous, error-prone C. The code above only calculates a single-member offset; it will get much worse with types more complex than integers. I might be able to alleviate this a bit using macros, but this will be painful no matter what.
Since there is no struct that represents the object, I don't have the option of stack allocations (at least without implementing my own stack on the heap).

Basically, my question is, how can I get what I want without paying for it? Is there a way to be portable, have variance in size for different types, not use redirection, and keep my code pretty?

EDIT: This is the best response I've ever received for an SO question. Choosing an answer was hard. SO only allows me to choose one answer so I chose the one that lead me to my solution, but you all received upvotes.

回答1:

See Python PEP 3123 (http://www.python.org/dev/peps/pep-3123/) for how Python solves this problem using standard C. The Python solution can be directly applied to your problem. Essentially you want to do this:

struct Object { struct Class* class; };
struct Integer { struct Object object; int value; };
struct String { struct Object object; size_t length; char* characters; };

You can safely cast Integer* to Object*, and Object* to Integer* if you know that your object is an integer.

回答2:

C gives you sufficient guarantees that your first approach will work. The only modification you need to make is that in order to make the pointer aliasing OK, you must have a union in scope that contains all of the structs that you are casting between:

union allow_aliasing {
    struct Class class;
    struct Object object;
    struct Integer integer;
    struct String string;
};

(You don't need to ever use the union for anything - it just has to be in scope)

I believe the relevant part of the standard is this:

[#5] With one exception, if the value of a member of a union object is used when the most recent store to the object was to a different member, the behavior is implementation-defined. One special guarantee is made in order to simplify the use of unions: If a union contains several structures that share a common initial sequence (see below), and if the union object currently contains one of these structures, it is permitted to inspect the common initial part of any of them anywhere that a declaration of the completed type of the union is visible. Two structures share a common initial sequence if corresponding members have compatible types (and, for bit-fields, the same widths) for a sequence of one or more initial members.

(This doesn't directly say it's OK, but I believe that it does guarantee that if two structs have a common intial sequence and are put into a union together, they'll be laid out in memory the same way - it's certainly been idiomatic C for a long time to assume this, anyway).

回答3:

Section 6.2.5 of ISO 9899:1999 (the C99 standard) says:

A structure type describes a sequentially allocated nonempty set of member objects (and, in certain circumstances, an incomplete array), each of which has an optionally specified name and possibly distinct type.

Section 6.7.2.1 also says:

As discussed in 6.2.5, a structure is a type consisting of a sequence of members, whose storage is allocated in an ordered sequence, and a union is a type consisting of a sequence of members whose storage overlap.

[...]

Within a structure object, the non-bit-field members and the units in which bit-fields reside have addresses that increase in the order in which they are declared. A pointer to a structure object, suitably converted, points to its initial member (or if that member is a bit-field, then to the unit in which it resides), and vice versa. There may be unnamed padding within a structure object, but not at its beginning.

This guarantees what you need.

In the question you say:

The problem is that, as far as I know, the C standard makes no promises about how structures are stored. On my platform this works.

This will work on all platforms. It also means that your first alternative - what you are currently using - is safe enough.

But on another platform struct ~~String~~Integer might store value before class and when I accessed foo->class in the above I would actually be accessing foo->value, which is obviously bad. Portability is a big goal here.

No compliant compiler is allowed to do that. [I replaced String by Integer assuming you were referring to the first set of declarations. On closer examination, you might have been referring to the structure with an embedded union. The compiler still isn't allowed to reorder class and value.]

回答4:

There are 3 major approaches for implementing dynamic types and which one is best depends on the situation.

1) C-style inheritance: The first one is shown in Josh Haberman's answer. We create a type-hierarchy using classic C-style inheritance:

struct Object { struct Class* class; };
struct Integer { struct Object object; int value; };
struct String { struct Object object; size_t length; char* characters; };

Functions with dynamically typed arguments receive them as Object*, inspect the class member, and cast as appropriate. The cost to check the type is two pointer hops. The cost to get the underlying value is one pointer hop. In approaches like this one, objects are typically allocated on the heap since the size of objects is unknown at compile time. Since most `malloc implementations allocate a minimum of 32 bytes at a time, small objects can waste a significant amount of memory with this approach.

2) Tagged union: We can remove a level of indirection for accessing small objects using the "short string optimization"/"small object optimization":

struct Object {
    struct Class* class;
    union {
        // fundamental C types or other small types of interest
        bool as_bool;
        int as_int;
        // [...]
        // object pointer for large types (or actual pointer values)
        void* as_ptr;
    };
};

Functions with dynamically typed arguments receive them as Object, inspect the class member, and read the union as appropriate. The cost to check the type is one pointer hop. If the type is one of the special small types, it is stored directly in the union, and there is no indirection to retrieve the value. Otherwise, one pointer hop is required to retrieve the value. This approach can sometimes avoid allocating objects on the heap. Although the exact size of an object still isn't known at compile time, we now know the size and alignment (our union) needed to accommodate small objects.

In these first two solutions, if we know all the possible types at compile time, we can encode the type using an integer type instead of a pointer and reduce type check indirection by one pointer hop.

3) Nan-boxing: Finally, there's nan-boxing where every object handle is only 64 bits.

double object;

Any value corresponding to a non-NaN double is understood to simply be a double. All other object handles are a NaN. There are actually large swaths of bit values of double precision floats that correspond to NaN in the commonly used IEEE-754 floating point standard. In the space of NaNs, we use a few bits to tag types and the remaining bits for data. By taking advantage of the fact that most 64-bit machines actually only have a 48-bit address space, we can even stash pointers in NaNs. This method incurs no indirection or extra memory use but constrains our small object types, is awkward, and in theory is not portable C.

回答5:

The problem is that, as far as I know, the C standard makes no promises about how structures are stored. On my platform this works. But on another platform struct String might store value before class and when I accessed foo->class in the above I would actually be accessing foo->value, which is obviously bad. Portability is a big goal here.

I believe you're wrong here. First, because your struct String doesn't have a value member. Second, because I believe C does guarantee the layout in memory of your struct's members. That's why the following are different sizes:

struct {
    short a;
    char  b;
    char  c;
}

struct {
    char  a;
    short b;
    char  c;
}

If C made no guarantees, then compilers would probably optimize both of those to be the same size. But it guarantees the internal layout of your structs, so the natural alignment rules kick in and make the second one larger than the first.

回答6:

I appreciate the pedantic issues raised by this question and answers, but I just wanted to mention that CPython has used similar tricks "more or less forever" and it's been working for decades across a huge variety of C compilers. Specifically, see object.h, macros like PyObject_HEAD, structs like PyObject: all kinds of Python Objects (down at the C API level) are getting pointers to them forever cast back and forth to/from PyObject* with no harm done. It's been a while since I last played sea lawyer with an ISO C Standard, to the point that I don't have a copy handy (!), but I do believe that there are some constraints there that should make this keep working as it has for nearly 20 years...

来源：https://stackoverflow.com/questions/1485505/representing-dynamic-typing-in-c

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data-representation