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Type Encoding

This is an advanced section. Type encodings are used extensively by the compiler and by the runtime, but you generally do not need to know about them to use Objective-C.

The Objective-C compiler generates type encodings for all the types. These type encodings are used at runtime to find out information about selectors and methods and about objects and classes.

The types are encoded in the following way:

_Bool B
char c
unsigned char C
short s
unsigned short S
int i
unsigned int I
long l
unsigned long L
long long q
unsigned long long Q
float f
double d
long double D
void v
id @
Class #
char* *
enum an enum is encoded exactly as the integer type that the compiler uses for it, which depends on the enumeration values. Often the compiler users unsigned int, which is then encoded as I.
unknown type ?
Complex types j followed by the inner type. For example _Complex double is encoded as “jd”.
bit-fields b followed by the starting position of the bit-field, the type of the bit-field and the size of the bit-field (the bit-fields encoding was changed from the NeXT’s compiler encoding, see below)

The encoding of bit-fields has changed to allow bit-fields to be properly handled by the runtime functions that compute sizes and alignments of types that contain bit-fields. The previous encoding contained only the size of the bit-field. Using only this information it is not possible to reliably compute the size occupied by the bit-field. This is very important in the presence of the Boehm’s garbage collector because the objects are allocated using the typed memory facility available in this collector. The typed memory allocation requires information about where the pointers are located inside the object.

The position in the bit-field is the position, counting in bits, of the bit closest to the beginning of the structure.

The non-atomic types are encoded as follows:

pointers ^ followed by the pointed type.
arrays [ followed by the number of elements in the array followed by the type of the elements followed by ]
structures { followed by the name of the structure (or ? if the structure is unnamed), the = sign, the type of the members and by }
unions ( followed by the name of the structure (or ? if the union is unnamed), the = sign, the type of the members followed by )
vectors ![ followed by the vector_size (the number of bytes composing the vector) followed by a comma, followed by the alignment (in bytes) of the vector, followed by the type of the elements followed by ]

Here are some types and their encodings, as they are generated by the compiler on an i386 machine:

Objective-C type Compiler encoding
int a[10];
struct {
  int i;
  float f[3];
  int a:3;
  int b:2;
  char c;
int a __attribute__ ((vector_size (16)));
![16,16i] (alignment would depend on the machine)

In addition to the types the compiler also encodes the type specifiers. The table below describes the encoding of the current Objective-C type specifiers:

Specifier Encoding
const r
in n
inout N
out o
bycopy O
byref R
oneway V

The type specifiers are encoded just before the type. Unlike types however, the type specifiers are only encoded when they appear in method argument types.

Note how const interacts with pointers:

Objective-C type Compiler encoding
const int
const int*
int *const

const int* is a pointer to a const int, and so is encoded as ^ri. int* const, instead, is a const pointer to an int, and so is encoded as r^i.

Finally, there is a complication when encoding const char * versus char * const. Because char * is encoded as * and not as ^c, there is no way to express the fact that r applies to the pointer or to the pointee.

Hence, it is assumed as a convention that r* means const char * (since it is what is most often meant), and there is no way to encode char *const. char *const would simply be encoded as *, and the const is lost.

Legacy Type Encoding

Unfortunately, historically GCC used to have a number of bugs in its encoding code. The NeXT runtime expects GCC to emit type encodings in this historical format (compatible with GCC-3.3), so when using the NeXT runtime, GCC will introduce on purpose a number of incorrect encodings:

  • the read-only qualifier of the pointee gets emitted before the ‘^’. The read-only qualifier of the pointer itself gets ignored, unless it is a typedef. Also, the ‘r’ is only emitted for the outermost type.
  • 32-bit longs are encoded as ‘l’ or ‘L’, but not always. For typedefs, the compiler uses ‘i’ or ‘I’ instead if encoding a struct field or a pointer.
  • ``enum``s are always encoded as ‘i’ (int) even if they are actually unsigned or long.

In addition to that, the NeXT runtime uses a different encoding for bitfields. It encodes them as b followed by the size, without a bit offset or the underlying field type.

@encode``GNU Objective-C supports the ``@encode syntax that allows you to create a type encoding from a C/Objective-C type. For example, @encode(int) is compiled by the compiler into "i".

@encode does not support type qualifiers other than const. For example, @encode(const char*) is valid and is compiled into "r*", while @encode(bycopy char *) is invalid and will cause a compilation error.

Method Signatures

This section documents the encoding of method types, which is rarely needed to use Objective-C. You should skip it at a first reading; the runtime provides functions that will work on methods and can walk through the list of parameters and interpret them for you. These functions are part of the public ‘API’ and are the preferred way to interact with method signatures from user code.

But if you need to debug a problem with method signatures and need to know how they are implemented (i.e., the ‘ABI’), read on.

Methods have their ‘signature’ encoded and made available to the runtime. The ‘signature’ encodes all the information required to dynamically build invocations of the method at runtime: return type and arguments.

The ‘signature’ is a null-terminated string, composed of the following:

  • The return type, including type qualifiers. For example, a method returning int would have i here.
  • The total size (in bytes) required to pass all the parameters. This includes the two hidden parameters (the object self and the method selector _cmd).
  • Each argument, with the type encoding, followed by the offset (in bytes) of the argument in the list of parameters.

For example, a method with no arguments and returning int would have the signature i8@0:4 if the size of a pointer is 4. The signature is interpreted as follows: the i is the return type (an int), the 8 is the total size of the parameters in bytes (two pointers each of size 4), the @0 is the first parameter (an object at byte offset 0) and :4 is the second parameter (a SEL at byte offset 4).

You can easily find more examples by running the ‘strings’ program on an Objective-C object file compiled by GCC. You’ll see a lot of strings that look very much like i8@0:4. They are signatures of Objective-C methods.