Creating struct variable using the tag name

Question

I have read a article in the following link

http://www.embedded.com/electronics-blogs/programming-pointers/4024450/Tag-vs-Type-Names

Here author says that, use of follwing is wrong.

struct s
{
--
};

s var;

But in my sample code its works perfectly.

  1 #include<iostream>
  2 using namespace std;
  3
  4 struct s
  5 {
  6    int sd;
  7 };
  8 s v;
  9
 10
 11
 12 int main()
 13 {
 14
 15    v.sd=10;
 16    cout<<v.sd;
 17    return 0;
 18 }

EDIT:

What the actual difference? why it works in c++ and not works in c;

Also have a look at http://stackoverflow.com/questions/252780/why-should-we-typedef-a-struct-so-often-in-c — user1781290, Jan 06 '14 at 14:55
@Vinoth: Because they are different languages. The article you link to explains this difference quite clearly. — Mike Seymour, Jan 06 '14 at 14:59
@MikeSeymour You are correct but what i ask is in c it creates the different symbol table but what about c++? is normal identifier and Struct identifers are same in c++ — VINOTH ENERGETIC, Jan 06 '14 at 15:03
@juanchopanza I find it appropriate to use both tags here, because the question is about the difference. It's also interesting for writing a header compatible with both C/C++. — leemes, Jan 06 '14 at 15:10
@Vinoth: In C++, either the *class name* (`s`) or the *elaborated type specifier* (`struct s` or `class s`) can be used as a type specifier. In C, only the *struct specifier* (`struct s`) and not the *tag* (`s`) can be used as a type specifier. As I said, they are different languages, and have quite different rules concerning declarations of structure/class types, despite the superficial similarities. — Mike Seymour, Jan 06 '14 at 15:14

score 11 · Answer 1 · answered Jan 06 '14 at 14:54

11

It is the difference between C++ and C. The author you are citing speaks about C while you use C++ code instead of C code. In C you have to specify keyword struct, union or enum before declaring variables of correspondings types.

answered Jan 06 '14 at 14:54

Vlad from Moscow

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Also I would like to append that names of tags in C form their own namespace. So the same name can be used with a structure tag and with a variable. – Vlad from Moscow Jan 06 '14 at 14:57
is tags in c++ are considered as normal identifier... Typically in c its not am i correct @Vlad – VINOTH ENERGETIC Jan 06 '14 at 15:00
In C++ structures, unions and enums name considered together with names of variables. If for example a name of a structure coinside with a name of a variable then the name of the variable (or function) hides the name of the structure.In this case you should use an elaborated structure name, For example struct A {}; A A; /* it is a correct definition */ A = ( struct A )(); /* using of the elaborated name */ – Vlad from Moscow Jan 06 '14 at 15:03

leemes · Answer 2 · 2014-01-06T18:37:25.203

1

The article says that a user-defined type using

struct s {
   ...
};

defines a tag name s. To name the actual type, you can write struct s (C or C++) or class s (C++ only), but C++ makes both keywords optional (and almost never actually used). So whenever you write s in C++ it's actually interpreted as the correct type, while in C the keyword is obligatory to name the type.

So long story short: in C++ there is no difference in writing struct s, class s or s, they all mean the same.

In order to define the type name s in a C/C++ shared header, one typically writes

typedef struct {
   ...
} s;

Then the type s can be used in both C and C++ without the need to write struct.

edited Jan 06 '14 at 18:37

answered Jan 06 '14 at 15:08

leemes

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http://coliru.stacked-crooked.com/a/b071707eb46e1a31 In fact, 7.1.3/5 explicitly states that what you say is invalid C++, is valid. 7.1.3/3 explicitly states that the other one is valid too :) – Lightness Races in Orbit Jan 06 '14 at 18:05
Oh... Then I remember something wrong. I once tried to define a struct in a C++/CL shared header (not C), and in CL something didn't work which normally works in C/C++ shared headers. My workaround was to explicitly give the struct and the typedef different names. Thanks for pointing this out; I'll remove the part from my answer. – leemes Jan 06 '14 at 18:34

Creating struct variable using the tag name

2 Answers2