如何在Objective-C中创建可变参数方法 [英] How to create variable argument methods in Objective-C

问题描述

也许这对你们大多数人来说显然是简单的，但是请你举个例子来说明如何创建类似的方法（在Objective-C中）和C中的函数来创建像 NSString 的 stringWithFormat：或 NSLog（）

提醒：

pre $ [NSString stringWithFormat：@example tekst％i％@％.2f，122 ，@sth，3.1415];
NSLog（@account ID％i email％@，accountID，email）;


我想创建类似于 NSString 的方法 stringWithFormat： $ b

$ b

$ b

$ b




$ NSURL - urlWithFormat 一般来说，这些被称为可变参数函数（或方法，因为它）。

用，... 来创建这个方法，就像在
中一样 end logMessage：（NSString *）message，...;


在这一点上，你可能想把它包装在一个 printf 类似的函数中，从头开始尝试，最好。

$ p $ - （void）logMessage：（NSString *）格式，... {
va_list args;
va_start（args，format）;
NSLogv（format，args）;
va_end（args）;

$ / code>

使用 NSLogv 而不是 NSLog ;考虑 NSLog（NSString *，...）; vs NSLogv（NSString *，va_list）; 想要一个字符串; initWithFormat：参数：在 NSString * 。

---

另一方面，如果您不使用字符串，而是像

  +（NSArray *）arrayWithObjects：（id）object，... NS_REQUIRES_NIL_TERMINATION; 
  

事情变得容易多了。

在这种情况下，不要使用 vprintf 风格的函数，而是使用循环 args ，并在任何循环中解析它们。

   - （void）logMessage：（NSString *）format，... {
 va_list args; 
 va_start（args，format）; 
 
 id arg = nil; 
 while（（arg = va_arg（args，id）））{
 ///用arg在这里做你的事情
} 
 
 va_end（args）; 
 
 
 
 
 
 $ b 
当然，最后一个例子假定va_args列表没有终止。
 
 
 
 注意：为了使这项工作成功，您可以包含< ; STDARG.H> ;但是如果内存服务，这将被包含在NSLogv中，这意味着它通过Foundation.h，因此还有AppKit.h和Cocoa.h以及其他一些内容。所以这应该开箱即用。
 
maybe this will be obviously simple for most of you, but could you please give an example how to create similar methods (in objective-C) and functions in C to create functions like NSString's stringWithFormat:, or NSLog()

Just to remind:
[NSString stringWithFormat:@"example tekst %i %@ %.2f",122,@"sth",3.1415"];
NSLog(@"account ID %i email %@",accountID,email);
I'd like to create the similar to NSString's method stringWithFormat:, NSURL - urlWithFormat.

Thank you in advance.
解决方案
What these are called, generally, is "variadic functions" (or methods, as it were).

To create this, simply end your method declartion with , ..., as in 
- (void)logMessage:(NSString *)message, ...;
At this point you probably want to wrap it in a printf-like function, as implementing one of those from scratch is trying, at best.
- (void)logMessage:(NSString *)format, ... {
  va_list args;
  va_start(args, format);
  NSLogv(format, args);
  va_end(args);
}
Note the use of NSLogv and not NSLog; consider NSLog(NSString *, ...); vs NSLogv(NSString *, va_list);, or if you want a string; initWithFormat:arguments: on NSString *.



If, on the other hand, you are not working with strings, but rather something like
+ (NSArray *)arrayWithObjects:(id)object, ... NS_REQUIRES_NIL_TERMINATION;
things get a lot easier.

In that case, instead of a vprintf-style function, use a loop going through args, assuming id as you go, and parse them as you would in any loop.
- (void)logMessage:(NSString *)format, ... {
  va_list args;
  va_start(args, format);

  id arg = nil;
  while ((arg = va_arg(args,id))) {
  /// Do your thing with arg here
  }

  va_end(args);
}
This last sample, of course, assumes that the va_args list is nil-terminated.

Note: In order to make this work you might have to include <stdarg.h>; but if memory serves, this gets included in connection with NSLogv, meaning it comes down by way of "Foundation.h", therefore also "AppKit.h" and "Cocoa.h", as well as a number of others; so this should work out of the box.

                        
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