Accessing elements of a struct instance variable in Objective-C using the getter

Question

I have an instance variable which is a struct, for example:

struct Data {
    UInt32 i;
    UInt32 arr[1];
};

And a property is defined in my class:

@property struct Data data; // and the corresponding @synthesize in the imp file

Now, I am aware that changing the values of i and arr through the getter of data is conceptually wrong, since I will be accessing the copy of data returned by the getter (the correct way is accessing it using self->data).

However some general Objective-C questions arise regarding the following lines:

self.data.i = 1;      // produces compile error        
self.data.arr[0] = 1; // compiles ok

First, why does the first line produces a compile error, and the 2nd line does not?

Second, if the dot syntax in the above line (self.data) is just a syntactic sugar to [self data], why do I get 2 different (although similar) compile errors?

self.data.i = 1;   // error: Expression is not assignable
[self data].i = 1; // error: Semantic Issue: Assigning to 'readonly' return result of an objective-c message not allowed

This is the same reason self.frame.size = CGSizeMake(320,460) doesn't work. — james_womack
– james_womack, Commented Jan 28, 2012 at 10:47

Joachim Isaksson · Accepted Answer · 2012-01-28 11:21:52Z

5

Actually, structs are passed by value in C (and Objective C). That means that your property actually returns a read only copy (rvalue) of the internal "Data" type. The assignment is to the temporary returned copy, which the compiler (rightfully) flags as a bit suspect.

The second line that compiles correctly does so since self.data.arr returns a read only UInt32*, but when you index it with [0] and write to that, you're not writing to the pointer, you're writing to the memory that it points to which is allowed.

edited Jan 28, 2012 at 11:21

answered Jan 28, 2012 at 9:43

Joachim Isaksson

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4 Comments

Itamar Katz Over a year ago

Yes, this is clear from the error message, but how come the 2nd line compiles ok?

Joachim Isaksson Over a year ago

@ItamarKatz Missed that part, added some info clarifying that to the answer.

Costique Over a year ago

@ItamarKatz The second statement compiles and works because self.data.arr returns a pointer to a C array, an element of which you then modify. In other words, you're not modifying the internal struct's fields.

Itamar Katz Over a year ago

Thanks, it's clear now. Coming form c++, it's like the difference between a const int * and int * const

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