OC message mechanism

What is the message mechanism

Compare with C++ The method of function call OC Message passing is adopted
Now let's make an analogy between the two methods
If you want to use a function in a function call , Generally, parameters are pushed onto the stack Then find the function address Take the parameter out of the stack Get the result and return to the original operation Call again . Or expand the code at the call when compiling
Messaging is whether you are polymorphic or not , Always look for the method to be executed at runtime .
So use the language of message structure , The code that should be executed when it runs is determined by the running environment ; The language in which function calls are used is determined by the compiler

What is a message selector

When sending a message to an object, it simply represents a method name such as [a func] here func It's the method name
When the source code is compiled, the selector will be pointed to a unique identifier in place of the method name Mainly for faster compilation
SEL Message selector SEL s1 = @selector(func);

Why message selector

For the sake of operating efficiency , There will not be used in the compiled code ASCII Code composition method name ,
In its place , The compiler will write each method name to a table , Then name each method
A unique identifier is used to identify a method at run time
SEL sor; （sor Message selector It can also be regarded as a unique identifier ）
-(void)who:(int)n;
sor = @selector(who:);
Be careful ！ This sor Only the method name is identified ！ It is not specified that this method is implemented