Objective-C Fundamentals

How messaging works
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Messages are slower than function calls
Message sending is somewhat slower than calling a function directly. Typically this
overhead is insignificant compared to the amount of work performed in the method.
The overhead is also reduced by some internal tricks in the implementation of
objc_msgSend, which manages to reduce the amount of repetitive work performed in
scenarios such as a message being sent to all objects in an array.
In the rare case that method sending causes an undue performance hit, it’s possible
to bypass several aspects of Objective-C’s dynamism. Bypassing should be performed,
however, only after analysis using tools such as Instruments.
Behind the scenes, an Objective-C method ends up being transformed and compiled
as a straight C function, designed to be invoked via objc_msgSend. A method like this
- (void)setAddress:(NSString *)newAddress { ... }
can be imagined to be translated into a C function similar to the following:
void CTRentalProperty_setAddress(
CTRentalProperty *self,
SEL _cmd,
NSString *str
)
{
...
}
The C-style function isn’t given a name because it can’t be accessed directly, so it isn’t
made visible to the linker. The C function expects two arguments in addition to those
explicitly declared in the Objective-C method declaration. We previously discussed the
self parameter, which is implicitly sent in a message send to indicate the object that
received the message; the lesser-known implicit parameter is named _cmd. This parameter
holds the selector (and hence name) of the message that was sent in order to
invoke this method implementation. Later in this chapter we discuss a couple of techniques
whereby a single method implementation can in fact respond to more than
one unique method selector.
8.3.1 Methods, selectors, and implementations
In the previous section, a number of new concepts were introduced: method, selector,
implementation, message, and message send. What exactly are these terms, and how
do they relate to source code?
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Method—A piece of code associated with a class and given a particular name,
such as (void)setAddress:(NSString *)newValue { ... }.
Selector—An efficient way to present the name of a message at runtime. Represented
by the SEL data type.
Implementation—A pointer to the machine code that implements a method.
Represented by the IMP data type.