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Why is it that sometimes dot syntax is used to access member variables in a struct but other times -> is used? I have been learning some of the C libraries in iOS and while passing structures into functions the arrow syntax seems to be used often. I gather it has to do with pointers/memory address/ C type stuff, but I'm not sure on what the significance between '.' and'->' is when accessing fields on a struct.

fuz
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Alexander Bollbach
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1 Answers1

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The . operator is used to access a member of a structure or union. The -> is defined such that a->b where b is a structure or union member of *a is equal to (*a).b. This syntactic sugar exists for convenience because writing (*a).b all the time is tedious. Just remember: If a is a pointer, use ->, if it's a structure or union, use ..

Some more recent languages (like Go) did away with the distinction, but not having it complicates the compiler as you need to do more complex type-checking to find out how to compile . so I guess that's why Ken Thompson didn't do that back then.

fuz
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  • so to make sure i have got it. are these syntactically correct? AStruct *structPointer = (AStruct*)someParam; // structPointer->aMember // AStruct structVar = *structPointer; // structVar.aMember // – Alexander Bollbach Dec 14 '15 at 00:00
  • hmm it the comments don't seem to let me use the asterisk * – Alexander Bollbach Dec 14 '15 at 00:03
  • @AlexanderBollbach Try to escape the asterisk like this: `\*`. Or even better, put your code in backticks: `*(AStruct).someParam`. – fuz Dec 14 '15 at 07:51
  • So with a pointer you access members like this: `AStruct *structPointer = (AStruct*)someParam; // structPointer->aMember` but with a direct variable you access members like this: `AStruct structVar = *structPointer; // structVar.aMember` is that right? – Alexander Bollbach Dec 14 '15 at 19:58
  • @AlexanderBollbach Still somewhat garbled. – fuz Dec 14 '15 at 20:00
  • how so? i'm defining a stuck pointer accessing a member with -> and than dereferences it to a local var and than using dot syntax. what was wrong there? – Alexander Bollbach Dec 14 '15 at 20:06
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    Now it's better, I didn't saw your edit. Yes, your notion is correct. – fuz Dec 14 '15 at 20:08