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

Answer 1

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.