Declared Properties

When the compiler encounters property declarations (see “Declared Properties” in The Objective-C Programming Language), it generates descriptive metadata that is associated with the enclosing class, category or protocol. You can access this metadata using functions that support looking up a property by name on a class or protocol, obtaining the type of a property as an @encode string, and copying a list of a property's attributes as an array of C strings. A list of declared properties is available for each class and protocol.

Property Type and Functions

The Property structure defines an opaque handle to a property descriptor.

typedef struct objc_property *Property;

You can use the functions class_copyPropertyList and protocol_copyPropertyList to retrieve an array of the properties associated with a class (including loaded categories) and a protocol respectively:

objc_property_t *class_copyPropertyList(Class cls, unsigned int *outCount)
objc_property_t *protocol_copyPropertyList(Protocol *proto, unsigned int *outCount)

For example, given the following class declaration:

@interface Lender : NSObject {
    float alone;
}
@property float alone;
@end

you can get the list of properties using:

id LenderClass = objc_getClass("Lender");
unsigned int outCount;
objc_property_t *properties = class_copyPropertyList(LenderClass, &outCount);

You can use the property_getName function to discover the name of a property:

const char *property_getName(objc_property_t property)

You can use the functions class_getProperty and protocol_getProperty to get a reference to a property with a given name in a class and protocol respectively:

objc_property_t class_getProperty(Class cls, const char *name)
objc_property_t protocol_getProperty(Protocol *proto, const char *name, BOOL isRequiredProperty, BOOL isInstanceProperty)

You can use the property_getAttributes function to discover the name and the @encode type string of a property. For details of the encoding type strings, see “Type Encodings”; for details of this string, see “Property Type String” and “Property Attribute Description Examples.”

const char *property_getAttributes(objc_property_t property)

Putting these together, you can print a list of all the properties associated with a class using the following code:

id LenderClass = objc_getClass("Lender");
unsigned int outCount, i;
objc_property_t *properties = class_copyPropertyList(LenderClass, &outCount);
for (i = 0; i < outCount; i++) {
    objc_property_t property = properties[i];
    fprintf(stdout, "%s %s\n", property_getName(property), property_getAttributes(property));
}

Property Type String

You can use the property_getAttributes function to discover the name, the @encode type string of a property, and other attributes of the property.

The string starts with a T followed by the @encode type and a comma, and finishes with a V followed by the name of the backing instance variable. Between these, the attributes are specified by the following descriptors, separated by commas:

Table 7-1  Declared property type encodings

Code

Meaning

R

The property is read-only (readonly).

C

The property is a copy of the value last assigned (copy).

&

The property is a reference to the value last assigned (retain).

N

The property is non-atomic (nonatomic).

G<name>

The property defines a custom getter selector name. The name follows the G (for example, GcustomGetter,).

S<name>

The property defines a custom setter selector name. The name follows the S (for example, ScustomSetter:,).

D

The property is dynamic (@dynamic).

W

The property is a weak reference (__weak).

P

The property is eligible for garbage collection.

t<encoding>

Specifies the type using old-style encoding.

For examples, see “Property Attribute Description Examples.”

Property Attribute Description Examples

Given these definitions:

enum FooManChu { FOO, MAN, CHU };
struct YorkshireTeaStruct { int pot; char lady; };
typedef struct YorkshireTeaStruct YorkshireTeaStructType;
union MoneyUnion { float alone; double down; };

the following table shows sample property declarations and the corresponding string returned by property_getAttributes:

Property declaration

Property description

@property char charDefault;

Tc,VcharDefault

@property double doubleDefault;

Td,VdoubleDefault

@property enum FooManChu enumDefault;

Ti,VenumDefault

@property float floatDefault;

Tf,VfloatDefault

@property int intDefault;

Ti,VintDefault

@property long longDefault;

Tl,VlongDefault

@property short shortDefault;

Ts,VshortDefault

@property signed signedDefault;

Ti,VsignedDefault

@property struct YorkshireTeaStruct structDefault;

T{YorkshireTeaStruct="pot"i"lady"c},VstructDefault

@property YorkshireTeaStructType typedefDefault;

T{YorkshireTeaStruct="pot"i"lady"c},VtypedefDefault

@property union MoneyUnion unionDefault;

T(MoneyUnion="alone"f"down"d),VunionDefault

@property unsigned unsignedDefault;

TI,VunsignedDefault

@property int (*functionPointerDefault)(char *);

T^?,VfunctionPointerDefault

@property id idDefault;

Note: the compiler warns: no 'assign', 'retain', or 'copy' attribute is specified - 'assign' is assumed"

T@,VidDefault

@property int *intPointer;

T^i,VintPointer

@property void *voidPointerDefault;

T^v,VvoidPointerDefault

@property int intSynthEquals;

In the implementation block:

@synthesize intSynthEquals=_intSynthEquals;

Ti,V_intSynthEquals

@property(getter=intGetFoo, setter=intSetFoo:) int intSetterGetter;

Ti,GintGetFoo,SintSetFoo:,VintSetterGetter

@property(readonly) int intReadonly;

Ti,R,VintReadonly

@property(getter=isIntReadOnlyGetter, readonly) int intReadonlyGetter;

Ti,R,GisIntReadOnlyGetter

@property(readwrite) int intReadwrite;

Ti,VintReadwrite

@property(assign) int intAssign;

Ti,VintAssign

@property(retain) id idRetain;

T@,&,VidRetain

@property(copy) id idCopy;

T@,C,VidCopy

@property(nonatomic) int intNonatomic;

Ti,VintNonatomic

@property(nonatomic, readonly, copy) id idReadonlyCopyNonatomic;

T@,R,C,VidReadonlyCopyNonatomic

@property(nonatomic, readonly, retain) id idReadonlyRetainNonatomic;

T@,R,&,VidReadonlyRetainNonatomic