Objective C 中函数隐式声明的问题

Question

我试图更多地了解 Objective-c，但目前我陷入了困境。我有 4 个错误，都是一样的。 “函数的隐式声明”，我用谷歌搜索但没有找到解决方案。

RadioStation .h

#import <Cocoa/Cocoa.h>
@interface RadioStation : NSObject {
  NSString* name;
  double frequency;
  char band;
}
+(double)minAMFrequency;
+(double)maxAMFrequency;
+(double)minFMFrequency;
+(double)maxFMFrequency;
-(id)initWithName:(NSString*)newName atFrequency:(double)newFrequency withBand:(char)newBand;
-(NSString*)name;
-(double)frequency;
-(char)band;
-(void)setName:(NSString*)newName;
-(void)setFrequency:(double)newFrequency;
-(void)setBand:(char)newBand;
@end

RadioStation .m

#import "RadioStation.h"

@implementation RadioStation
+(double)minAMFrequency{
 return 520.0;
};
+(double)maxAMFrequency{
     return 1610.0;
};
+(double)minFMFrequency{
 return 88.3;
};
+(double)maxFMFrequency{
 return 107.9;
};
-(id)initWithName:(NSString*)newName atFrequency:(double)newFrequency withBand:(char)newBand{
 self = [super init];
 if(self != nil){
  name = [newName retain];
  band = newBand;
  if (band == 'F') {
   if (newFrequency > maxFMFrequency()) {
    frequency = maxFMFrequency();
   }else if (newFrequency < minFMFrequency()) {
    frequency = minFMFrequency();
   }else {
    frequency = newFrequency;
   }

  }else if (band == 'A') {
   if (newFrequency > maxAMFrequency()) {
    frequency = maxAMFrequency();
   }else if (newFrequency < minAMFrequency()) {
    frequency = minAMFrequency();
   }else {
    frequency = newFrequency;
   }
  }
 }
 return self;
}
@end

这些行

if (newFrequency > maxFMFrequency()) {
if (newFrequency < minFMFrequency()) {
if (newFrequency > maxAMFrequency()) {
if (newFrequency < minAMFrequency()) {

都说“函数的隐式声明”

提前感谢， 迪格尔

Answer 1

这些是类方法，因此您需要按如下方式更改它们：

if (newFrequency > [RadioStation maxFMFrequency]) {
if (newFrequency < [RadioStation minFMFrequency]) {
if (newFrequency > [RadioStation maxAMFrequency]) {
if (newFrequency < [RadioStation minAMFrequency]) {

Answer 2

您混淆了方法和函数。

您调用该事物的代码

if (newFrequency > maxFMFrequency()) {

期望看到函数的声明，例如

double maxFMFrequency()

作为 C 函数实现的函数 - 这将无法从对象获取数据，因此您需要使用标头

确实声明了一个方法的方法，

+(double)maxFMFrequency;

但需要被称为

if (newFrequency > [RadioStation maxFMFrequency])

Answer 3

我认为这可能是因为你混合了 C 和 Objective C 语法。

尝试：

if (newFrequency > [self maxFMFrequency])

Answer 4

我遇到了同样的问题。通过修改函数调用进行排序，如下所示

//function declaration 
-(void) downloadPage:(NSString *)url;

//function definition 
-(void) downloadPage:(NSString *)url
{
userOutput.text = url;
}

//and now the fixed call to downloadPage

-(IBAction) onButtonOneClicked:(id) sender
{
userOutput.text = @"please wait...";
    //fixed call to downloadPage
[self downloadPage:[userInput text]];
}