Objective-C 中方法的重載，覆蓋和隱藏

在C++中，類方法有重載（overload）、覆蓋（override）和隱藏（hide）的概念（當然，函數也可重載。）。

C++的覆蓋和隱藏是個非常容易混淆的概念，尤其隱藏。學到Objective-C，因爲它與C++有相似的地方，所以對此也格外留心。

從下面代碼測試結果可以看出，OC遠沒有C++那麼“靈活”。具體結論輸出信息一目瞭然。

/** * test_Hide.m */ #import <Foundation/Foundation.h> // ---- @interface section ---- // Base @interface Base: NSObject /* {} */ // It's optional. + (void) foo; + (void) foo: (int)param; + (void) foo: (int)param and: (int)param2; + (void) foo: (int)param and: (int)param2 and: (int)param3; - (void) bar; - (void) bar: (int)param; - (void) bar: (int)param and: (int)param2; - (void) bar: (int)param and: (int)param2 and: (int)param3; @end // Derived @interface Derived: Base + (void) foo: (int)param; - (void) bar: (int)param and: (int)param2; @end // ---- program section ---- int main(int argc, char* argv[]) { NSAutoreleasePool* pool = [[NSAutoreleasePool alloc] init]; [Base foo]; [Base foo: 1]; [Base foo: 1 and: 2]; [Base foo: 1 and: 2 and: 3]; Base* base = [[Base alloc] init]; [base bar]; [base bar: 1]; [base bar: 1 and: 2]; [base bar: 1 and: 2 and: 3]; [base release]; NSLog(@"--------< 1 >--------"); [Derived foo]; // Ok, either! NOT hided. [Derived foo: 11]; // OK [Derived foo: 11 and: 22]; // Ok, either! NOT hided. [Derived foo: 11 and: 22 and: 33]; // Ok, either! NOT hided. Derived* derived = [[Derived alloc] init]; [derived bar]; // Ok, either! NOT hided. [derived bar: 11]; // Ok, either! NOT hided. [derived bar: 11 and: 22]; // OK [derived bar: 11 and: 22 and: 33]; // Ok, either! NOT hided. [derived release]; NSLog(@"--------< 2 >--------"); [pool drain]; return 0; } // ---- @implementation section ---- // Base @implementation Base/* : NSObject */ // It's optional. + (void) foo { NSLog(@"Base::foo"); } + (void) foo: (int)param { NSLog(@"Base::foo: %i", param); } + (void) foo: (int)param and: (int)param2 { NSLog(@"Base::foo: %i and: %i", param, param2); } + (void) foo: (int)param and: (int)param2 and: (int)param3 { NSLog(@"Base::foo: %i and: %i and: %i", param, param2, param3); } - (void) bar { NSLog(@"Base::bar"); } - (void) bar: (int)param { NSLog(@"Base::bar: %i", param); } - (void) bar: (int)param and: (int)param2 { NSLog(@"Base::bar: %i and: %i", param, param2); } - (void) bar: (int)param and: (int)param2 and: (int)param3 { NSLog(@"Base::bar: %i and: %i and: %i", param, param2, param3); } @end // Derived @implementation Derived + (void) foo: (int)param { NSLog(@"Derived::foo: %i", param); } - (void) bar: (int)param and: (int)param2 { NSLog(@"Derived::bar: %i and: %i", param, param2); } @end

// Output 2010-12-24 03:31:20.642 test_Hide[3700] Base::foo 2010-12-24 03:31:20.658 test_Hide[3700] Base::foo: 1 2010-12-24 03:31:20.658 test_Hide[3700] Base::foo: 1 and: 2 2010-12-24 03:31:20.658 test_Hide[3700] Base::foo: 1 and: 2 and: 3 2010-12-24 03:31:20.673 test_Hide[3700] Base::bar 2010-12-24 03:31:20.673 test_Hide[3700] Base::bar: 1 2010-12-24 03:31:20.673 test_Hide[3700] Base::bar: 1 and: 2 2010-12-24 03:31:20.689 test_Hide[3700] Base::bar: 1 and: 2 and: 3 2010-12-24 03:31:20.689 test_Hide[3700] --------< 1 >-------- 2010-12-24 03:31:20.689 test_Hide[3700] Base::foo 2010-12-24 03:31:20.705 test_Hide[3700] Derived::foo: 11 2010-12-24 03:31:20.705 test_Hide[3700] Base::foo: 11 and: 22 2010-12-24 03:31:20.705 test_Hide[3700] Base::foo: 11 and: 22 and: 33 2010-12-24 03:31:20.720 test_Hide[3700] Base::bar 2010-12-24 03:31:20.720 test_Hide[3700] Base::bar: 11 2010-12-24 03:31:20.720 test_Hide[3700] Derived::bar: 11 and: 22 2010-12-24 03:31:20.736 test_Hide[3700] Base::bar: 11 and: 22 and: 33 2010-12-24 03:31:20.736 test_Hide[3700] --------< 2 >--------