Name Mangling - Name Mangling in Objective-C

Name Mangling in Objective-C

Essentially two forms of method exist in Objective-C, the class ("static") method, and the instance method. A method declaration in Objective-C is of the following form

+ method name: argument name₁:parameter₁ ... – method name: argument name₁:parameter₁ ...

Class methods are signified by +, instance methods use -. A typical class method declaration may then look like:

+ (id) initWithX: (int) number andY: (int) number; + (id) new;

with instance methods looking like

– (id) value; – (id) setValue: (id) new_value;

Each of these method declarations have a specific internal representation. When compiled, each method is named according to the following scheme for class methods:

_c_Class_methodname_name₁_name₂_ ...

and this for instance methods:

_i_Class_methodname_name₁_name₂_ ...

The colons in the Objective-C syntax are translated to underscores. So, the Objective-C class method + (id) initWithX: (int) number andY: (int) number;, if belonging to the Point class would translate as _c_Point_initWithX_andY_, and the instance method (belonging to the same class) - (id) value; would translate to _i_Point_value.

Each of the methods of a class are labeled in this way. However, in order to look up a method that a class may respond to would be tedious if all methods are represented in this fashion. Each of the methods is assigned a unique symbol (such as an integer). Such a symbol is known as a selector. In Objective-C, one can manage selectors directly — they have a specific type in Objective-C — SEL.

During compilation, a table is built that maps the textual representation (such as _i_Point_value) to selectors (which are given a type SEL). Managing selectors is more efficient than manipulating the textual representation of a method. Note that a selector only matches a method's name, not the class it belongs to — different classes can have different implementations of a method with the same name. Because of this, implementations of a method are given a specific identifier too — these are known as implementation pointers, and are given a type also, IMP.

Message sends are encoded by the compiler as calls to the id objc_msgSend (id receiver, SEL selector, ...) function, or one of its cousins, where receiver is the receiver of the message, and SEL determines the method to call. Each class has its own table that maps selectors to their implementations — the implementation pointer specifies where in memory the actual implementation of the method resides. There are separate tables for class and instance methods. Apart from being stored in the SEL to IMP lookup tables, the functions are essentially anonymous.

The SEL value for a selector does not vary between classes. This enables polymorphism.

The Objective-C runtime maintains information about the argument and return types of methods. However, this information is not part of the name of the method, and can vary from class to class.

Since Objective-C does not support namespaces, there is no need for mangling of class names (that do appear as symbols in generated binaries).