无法识别的选择器发送到类

Question

我正在学习Objective-C并尝试一些示例代码.我收到以下错误消息:

unrecognized selector sent to class

这是我的代码.

Basics.m

#import <Foundation/Foundation.h>
#import "Fraction.h"

int main (int argc, char *argv[])
{
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];

    Fraction *myFraction; 

    // Create an instance of a Fraction

    myFraction = [Fraction alloc];
    myFraction = [Fraction init];

    // set Fraction to 1/3
    [myFraction setNumerator: 1];
    [myFraction setDenominator: 3];

    // Display the value using the print method
    NSLog(@"The value of myFraction is:");
    [myFraction print];
    [myFraction release];

    [pool drain];
    return 0;
}

Fraction.h

#import <Foundation/Foundation.h>

// -- interface section --//
@interface Fraction : NSObject {
    int numerator;
    int denominator;
}

// public method signatures
-(void) print;
-(void) setNumerator: (int) n;
-(void) setDenominator: (int) d;
@end

Fraction.m

//--implementation section --//
@implementation Fraction 
-(void) print 
{
    NSLog(@"%i/%i",numerator,denominator);
}
-(void) setNumerator: (int) n
{
    numerator = n;
}
-(void) setDenominator: (int) d
{
    denominator = d;
}
@end

Answer 1

alloc是一个类方法,但它init是一个实例方法.在您的代码中,编译器抱怨它找不到任何命名的类方法init,这是准确的.要纠正这个问题,你应该调用init你收到的实例alloc,如下所示:

myFraction = [Fraction alloc];
myFraction = [myFraction init];

但最常见的模式是嵌套这样的调用:

// calls -init on the object returned by +alloc
myFraction = [[Fraction alloc] init];

这还可以帮助您避免在已分配但尚未初始化的对象上调用方法时可能发生的错误.