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iosobjective-coopdynamicbinding

In Objective-C, does the binding of method really happen at "run-time"?

I heard that Objective-C is influenced by the "message passing mechanism" of SmallTalk.

Objective-C, like Smalltalk, can use dynamic typing: an object can be sent a message that is not specified in its interface. This can allow for increased flexibility, as it allows an object to "capture" a message and send the message to a different object that can respond to the message appropriately, or likewise send the message on to another object.

And I felt for codes like [anObject someMethod], the binding of someMethod to the machine code may happen at run-time..

Therefore, I write a demo like this:

#import <Foundation/Foundation.h>

@interface Person : NSObject {
    @private char *name;
}
@property (readwrite, assign) char *name;
- (void)sayHello;
@end

@implementation Person
@synthesize name;

- (void)sayHello {
    printf("Hello, my name is %s!\n", [self name]);
}


@end

int main() {
    Person *brad = [Person new];
    brad.name = "Brad Cox";
    [brad sayHello];
    [brad sayHelloTest];


}

I tried [brad sayHelloTest] to send brad a message sayHelloTest which brad doesn't know how to handle with.. I expect the error will NOT happen at compile-time..

However, the compiler still throws an error:

main.m:24:11: error: instance method '-sayHelloTest' not found (return type defaults to 'id') [-Werror,-Wobjc-method-access]
    [brad sayHelloTest];
          ^~~~~~~~~~~~
main.m:3:12: note: receiver is instance of class declared here
@interface Person : NSObject {
           ^

Change [(id)brad sayHelloTest] to [(id)brad sayHelloTest]; doesn't work either.. (The compiling command is clang -Wall -Werror -g -v main.m -lobjc -framework Foundation -o main)

In Objective-C, does the binding of method really happen at "run-time"? If so, why will there be a compiler error like this?

If the binding doesn't happen at "run-time", why was "Objective-C" called "dynamic typing language"?

Does anyone have any ideas about this?

Solution

  • I figured out the reason finally..

    It throw errors during compiling because -Werror flag is included, which will turn warning into error..

    http://clang.llvm.org/docs/UsersManual.html#cmdoption-Werror

    After I delete -Werror flag, everything works as expected and the error only happens at run-time.