If you are even a little uncertain about how pointers work, this post aims to clear it up.
You use pointers in Objective-C a lot. As an iOS developer, every framework NSObject you create uses a pointer reference. You pass references to these objects.
weak. Sometimes you modify in place, sometimes you modify a copy. There’s a lot to understand here.
I think it’s important to understand pointers deeply. I figured out a few things, and here are some of the big “ah-ha” moments that helped me.
Address and Value all the things
Everything in your program is stored in memory. Maybe that’s obvious.
The key insight for me is that any variable, no matter the type, can access both the memory address and the stored value.
Both primitives (
long) and reference types (
NSObject *) have a memory location and a value. The difference is just which one is returned by “default”.
Here’s an example:
// Create a primitive int myNum = 42; myNum; // 42 (returned by "default") &myNum; // 0x7fff5fbff6bc (memory address) // Create a variable that points to an integer data type int *pointer = &myNum; pointer // 0x7fff5fbff6bc (returned by "default") *pointer // 42 (pointed-at value)
This shows that with a primitive value, you can get to the memory address with
&, and in a pointer, you can get to the value with
I think this has really sunk in now, and it explains a lot. Everything in programming has both an address and a value.
Even your Address has an Address
Even if your value is an address, that value has its own address. Huh? I’ll show you.
int myNum = 42; // integer has value 42 and lives at address A int *pointer = &myNum; // pointer has value A and lives at address B printf("The pointer stores a mem address: %p\n", pointer); // A printf("The pointer lives in memory here: %p\n", &pointer); // B
Everything has both a value and a memory address, even if the value itself is a memory address.
Whaaaaaaa?! …yes. It’s true. Here is a technical diagram:
Treehouse has an awesome tutorial on the topic of pointers. If you’re already a member check it out. If you’re not it’s worth considering a trial membership to watch.
The instructor is Amit Bijlani. He makes it clear how important an understanding of pointers is to Objective C (all you do is declare pointers over and over again and modify referenced objects). His explanations are succinct and thorough, with both concrete and abstract examples. Thanks Amit!
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