C++ Proper use of struct in function arguments

Question

Quick Question about passing a struct as a function arg. Maybe its a carry over from C that makes no difference in C++.

In many Linked List examples you see them re-declare the word struct in any function argument that takes a struct and I don't understand why. Or any allocation for that matter. A structure is its own object and declaring just the name of the struct is sufficient.

Example:

struct Node{

    int data;
    Node * next;
};

// Function to output data.
void printNode(struct Node* myNode){ 

    // Print data of node

}

why is the word struct re-declared in the function arg. Declaring type Node* works just fine.

Thanks.

This is a C thing. In C one often uses a `typedef` to avoid that, but in C++ you can always use the type without the `struct` or `class` keyword. — Greg Kikola, Feb 01 '17 at 19:21
Ahh , That would explain why we use typedef struct in Objective C — Miek, Feb 01 '17 at 19:23
Don't forget the semicolon after the `struct` definition though, I don't think your code will compile as is. — Greg Kikola, Feb 01 '17 at 19:24
The word is not redeclared, it's there because you name a struct by its tag. [And some would say that's preferable](http://yarchive.net/comp/linux/typedefs.html). — StoryTeller - Unslander Monica, Feb 01 '17 at 19:24

score 7 · Accepted Answer · answered Feb 01 '17 at 19:23

There is a difference between C and C++ use of declarations produced with struct keyword:

In C, struct defines a new struct tag, but not a new type
In C++, struct defines a new type.

The consequence is that C++ lets you use the name defined in struct, while C requires you to either prefix a struct to the tag, or use typedef to define a new type.

score 0 · Answer 2 · answered Feb 01 '17 at 19:24

0

C requires it. C++ does not require it, but allows it for backward compatibility. Do not do this for new code.

answered Feb 01 '17 at 19:24

Rob K

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You most certainly *should* do this in new code, if that code is a C binding. – StoryTeller - Unslander Monica Feb 01 '17 at 19:26
1

The question is tagged C++ so what C does bears no relevance for writing new C++ – Rob K Feb 01 '17 at 19:31
And new C binding are no longer written for C++ libraries? That's news to me. – StoryTeller - Unslander Monica Feb 01 '17 at 19:33
StroryTeller - That's good info moving forward. I'll just get in the habit of using typedef struct. – Miek Feb 01 '17 at 19:36
1

@Miek - Don't. It's not required for C++. And as discussed in the thread I linked to your question, it can actually make the C bindings better. – StoryTeller - Unslander Monica Feb 01 '17 at 19:37
If you're writing header files for use in C, then you're not writing C++ are you? – Rob K Feb 01 '17 at 19:39
`extern "C"` and the `__cplusplus` macro seem pretty C++ to me – StoryTeller - Unslander Monica Feb 01 '17 at 19:41
@Rob K Although you are correct, but for my situation, I write in C, C++ and Objective C and understanding now this issue effects all three is important for me to know. I appreciate the input. – Miek Feb 01 '17 at 19:49
You didn't say anything in the question about interfacing with code in other languages, so I inferred you wanted to know about pure C++. If you're writing compatibility code, code that's going to be both C and C++, then of course you don't have any choice. But for pure C++, code that's only going to be C++, you shouldn't use it. – Rob K Feb 01 '17 at 21:21

C++ Proper use of struct in function arguments

2 Answers2