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Listin g 6-10. Variable Argument Methods in Cocoa
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@interface NSArray + (id)arrayWithObjects:(id)firstObj, ... @end @interface NSString + (id)stringWithFormat:(NSString*)format, ... @end int toastTimes[] = { 8, 22, 35, 45, 60, 90 }; NSArray *toastDescriptions = [NSArray arrayWithObjects:@"Warm", @"Light", @"Medium", @"Dark", @"Very dark", @"Burnt", nil]; int level = 3; int secs = toastTimes[level]; NSString *desc = [toastDescriptions objectAtIndex:level]; return [NSString stringWithFormat:@"%@ toast will take %d seconds.",desc,secs]; A method with variable arguments must assume the number and type of the parameters in the list. The parameter list doesn t include any type information or end-of-list indicator; it s just a raw sequence of bytes formed when the parameter values were pushed onto the stack. Consequently, the method must either assume the parameter types or infer them using information supplied in the named parameters. The two Cocoa methods shown in Listing 6-10 demonstrate the two most common solutions to this problem. The +arrayWithObjects: method assumes that all of the additional parameters will be object pointers and the list is terminated by a final nil parameter.
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CHAPTER 6 SENDING MESSAGES
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The +stringWithFormat: method uses the format parameter to infer the sequence of parameter values that follow. The format string is scanned for format specifications. The first specification (%@) is an object formatter. This tells +stringWithFormat: that the first additional parameter is a pointer to an object. The %@ is replaced with the -description (toString()) value of the object. The next specification encountered is %d, indicating that the next parameter is a signed int (d stands for decimal). It extracts one int from the parameter stream and continues, until all of the format specifications in the string have been replaced. In both cases, a programmer s agreement exists between the method and the caller. +arrayWithObjects: assumes that only object pointers are passed, terminated with a nil pointer. +stringWithFormat: assumes that the additional parameter agrees with the value types implied by the placeholders in the format string.
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Caution The compiler has no type information about variable parameters, so it assumes that all values are of
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the correct/expected type. The parameter 1 will pass an integer and 1.0 will pass a float. To remove any doubt, cast the parameter as in [NSString stringWithFormat:@"float %f, int %d, char %c",(float)34,(int)34,(char)34]. This will force the compiler to convert the parameter to the expected type before pushing onto the stack.
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Writing your own variable argument method is not difficult, but it does require a little planning. You must design the method so the type and number of the additional parameters is known or can be inferred. Listing 6-11 rewrites the containsAnyPoints:pointCount: method to use a variable argument list, instead of an array of pointers.
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Listin g 6-11. Variable Argument Implementation
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@interface BezierPathMapper : NSObject { NSBezierPath *path; } @property (assign) NSBezierPath *path; - (BOOL)containsAnyOfCountPoints:(unsigned int)count, ...; @end @implementation BezierPathMapper @synthesize path; - (BOOL)containsAnyOfCountPoints:(unsigned int)count, ... { va_list varList; BOOL hit = NO; NSPoint point; va_start(varList,count); while (count!=0) { point = va_arg(varList,NSPoint); // copy next point
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CHAPTER 6 SENDING MESSAGES
if ([path containsPoint:point]) { hit = YES; break; } count--; } va_end(varList); return hit; } @end if (![mapper containsAnyOfCountPoints:5, NSMakePoint(0.0,1.0), NSMakePoint(1.0,0.0), NSMakePoint(1.0,0.0), NSMakePoint(0.0,0.0), NSMakePoint(0.5,0.5)]) { NSLog(@"no points in path"); } The new method is passed a list of NSPoint structures directly as parameters. The initial count parameter tells the method how many NSPoint parameters to expect. To implement variable arguments in your method, follow these steps: 1. 2. 3. 4. 5. Follow the last parameter variable name in the method declaration with , ... . Define a va_list automatic variable. Call va_start() with the va_list variable and the last named parameter. The parameter is just used to tell the variable argument functions where the additional parameters begin. Call va_arg() with the va_list variable and the type of the next parameter. The value returned is a copy of the parameter. When finished, call va_end().
It is not necessary to retrieve all of the parameters before calling va_end(). Calling va_arg() for more parameter values than were passed is unpredictable and should be avoided.
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