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Possible Duplicates:
Optional arguments in Objective-C 2.0?
Objective-C Default Argument Value

I'm writing a C function in Objective-C. I want a default value for my last parameter.

I've tried:

foo(int a, int b, int c = 0);

but that's C++.

I've also tried:

foo(int a, int b, int c)
{
    ...
}

foo(int a, int b)
{
   foo(a, b, 0);
}

But that's also C++.

Is there a way to do this in Objective-C instead?

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4 Answers 4

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133

There's no default parameters in ObjC.

You can create 2 methods though:

-(void)fooWithA:(int)a b:(int)b c:(int)c {
  ...
}
-(void)fooWithA:(int)a b:(int)b {
  [self fooWithA:a b:b c:0];
}

For C : there's nothing special added to the C subset by using ObjC. Anything that cannot be done in pure C can't be done by compiling in ObjC either. That means, you can't have default parameters, nor overload a function. Create 2 functions instead.

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8 Comments

@Kenny, yeah but this doesn't really belong to any class. it really is a stand alone C function...
Then as Seva sugest, try Objective-C++. With that, you will be able to mix C++ and Obj-C code, calling one from the other, etc... : )
I have changed header file extension to hh, source file extension to mm and In project settings it is set compile source as according to file type extension to compile it as objective-C++. but still it says C does not support default arguments. what is the correct extension for objective-C++ header files?
@neckTwi mm. However, the error may happen if the hh is included by non-ObjC++ files.
God forbid!!! making it Objective-C++ you'll bare the curse of C++ forever. E.g. you cannot deploy your binary to others, because they'll need to compile their own code EXACTLY the same way you did. Same compiler, runtime-libs and settings. C++ has no standard/consistent ABI. The strongest reason OBJ-C is "pure C" - is to have a "compatible-forever" binary, that runs everywhere. You only use Obj-C++ when you need to interface C++ code from Obj-C. Certainly not to save a few simple-to-understand code-lines. If you're all for elegance - do Swift instead (and enjoy same perils as C++)
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7

No, objective-c does not support default parameters. See similar question

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3

You can write a C function with a variable length argument list. You can use '...' as the data type for one of your function's declared parameters to specify where in the parameter list the variable argument list begins. (That allows you to have one or more required arguments before the start of the list.)

printf() is an example of a function that is written using this facility (known as varargs).

printf(const char *restrict format, ...);

Here, the first argument is required, and then can be followed by zero or more additional arguments.

If you wrote your function this way, it could supply a default value for the missing parameter.

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2

For a C function - no. For an Objective C class method - yes, you just do two methods, one of them one parameter short, calling the other method.

Or you can rename your sources to .mm and C functions magically become C++.

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2 Comments

That's an interesting possibility... I'll have to figure out the cost of that in other parts of my integration...
Note that this changes global name decoration. You won't be able to call such a function externally from a .m/.c source; if there's a way to declare a function as "extern C++", so to say, I'm not familiar with it. In other words, if you're going ObjC++, you better do this throughout the project.

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