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Declared Properties

When the compiler encounters property declarations (see Declared Properties in The Objective-C 2.0 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.

In this section:

Property Type and Functions
Property Type String
Property Attribute Description Examples

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.

• If the property is defined as readonly, the string contains R, then a comma.

• If the property is defined as copy or retain, the string contains C or & respectively, then a comma.

• If the property defines custom getter or setter method names, the names follow a G or a S respectively, followed by a comma (for example, GcustomGetter,ScustomSetter:,).

  If the property is read-only and specifies a custom get accessor, the description finishes here.

• The string finishes with a V followed by the name of the backing instance variable.

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:

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