@property与@synthesize的学习

今天开始学习《Objective-C 2.0程序设计》第七章，学习@property与@synthesize，编译时报错，代码如下：

文件Fraction.m

#import "Fraction.h"

@synthesize numerator, denominator;

@implementation Fraction

-(void) print
{
    NSLog(@"%i/%i", numerator, denominator);
}

-(double) convertToNum
{
    if (denominator != 0)
        return (double) numerator / denominator;
    else
        return 1.0;
}

@end

文件Fraction.h

#import <Foundation/Foundation.h>

//the Fraction class
@interface Fraction : NSObject
{
    int numerator;
    int denominator;
}

@property int numerator, denominator;

-(void) print;
-(double) convertToNum;

@end

编译不过，报错：Missing context for property implementation declaration

查了google，得到如下信息：

This can happen when you attempt to synthesize a property outside of the scope of your class' implementation.

Incorrect:

@synthesize yourProperty;
@implementation YourClass
@end

Correct:

@implementation YourClass
@synthesize yourProperty;
@end

原来是自己编写代码时将@synthesize方法放到了@implementation......@end之外！
于是修改代码如下：

#import "Fraction.h"

@implementation Fraction

@synthesize numerator, denominator;

-(void) print
{
    NSLog(@"%i/%i", numerator, denominator);
}

-(double) convertToNum
{
    if (denominator != 0)
        return (double) numerator / denominator;
    else
        return 1.0;
}

@end

编译顺利通过！看来习惯很重要，一定注意@synthesize方法放到@implementation......@end之内~

本文仅作警示！