Is it optional to use struct keyword before declaring a structure object?

Question

To declare a class object we need the format

classname objectname;

Is it the sameway to declare a structure object?

like

structname objectname;

I found here a structure object declared as

struct Books Book1;

where Books is the structure name and Book1 is its object name. So is there any need of usinge the keyword struct before declaring a structure object?

If you don't use `typedef` or `struct` the compiler doesn't know that `structname` is a type. — Riley, Oct 03 '16 at 19:04
In C, you either need to use the `struct` keyword, or you need to `typedef` the struct type. In C++, the `struct` keyword is optional. — user3386109, Oct 03 '16 at 19:05
in `struct Books book1;` `Books` is not the "structure name", it is the structure tag. — pmg, Nov 14 '19 at 13:40

score 26 · Answer 1 · answered Oct 03 '16 at 19:27

This is one of the differences between C and C++.

In C++, when you define a class you can use the type name with or without the keyword class (or struct).

// Define a class.
class A { int x; };

// Define a class (yes, a class, in C++ a struct is a kind of class).
struct B { int x; };

// You can use class / struct.
class A a;
struct B b;

// You can leave that out, too.
A a2;
B b2;

// You can define a function with the same name.
void A() { puts("Hello, world."); }

// And still define an object.
class A a3;

In C, the situation is different. Classes do not exist, instead, there are structures. You can, however, use typedefs.

// Define a structure.
struct A { int x; };

// Okay.
struct A a;

// Error!
A a2;

// Make a typedef...
typedef struct A A;

// OK, a typedef exists.
A a3;

It is not uncommon to encounter structures with the same names as functions or variables. For example, the stat() function in POSIX takes a struct stat * as an argument.

Thanks for a great answer. One question: [this answer](https://stackoverflow.com/a/8423005/10027592) says that even C++ *requires* the `struct` keyword in your example (`stat()` function in POSIX systems). So, in these kinds of cases where function and structure have the same name, `struct` *must* be used even in C++? — starriet, Jul 23 '22 at 13:36
@starriet: Yes. The difference is that in C++, `struct` may be omitted in other scenarios. — Dietrich Epp, Jul 23 '22 at 15:01

Sajad · Accepted Answer · 2019-11-14T12:56:10.187

10

You have to typedef them to make objects without struct keyword

example:

typedef struct Books {
     char Title[40];
     char Auth[50];
     char Subj[100];
     int Book_Id;
} Book;

Then you can define an object without the struct keyword like:

Book thisBook;

edited Nov 14 '19 at 12:56

answered Oct 03 '16 at 19:22

Sajad

492
2
10

2

You don't need the `Books` tag if you aren't going to self-reference the struct within itself as a pointer type. – Qix - MONICA WAS MISTREATED Oct 03 '16 at 19:39
@Qix: Another reason to use the `Books` tag is to allow it to be used from other header files without creating a dependency. – Dietrich Epp Oct 03 '16 at 20:28
@DietrichEpp Oh true :) – Qix - MONICA WAS MISTREATED Oct 03 '16 at 20:49

Rohit Takhar · Answer 3 · 2016-10-03T19:34:19.993

5

Yes. In case of C language, you need to explicitly give the type of the variable otherwise the compiler will throw an error: 'Books' undeclared.(in the above case)

So you need to use keyword struct if you are using C language but you can skip this if you are writing this in C++.

Hope this helps.

edited Oct 03 '16 at 19:34

answered Oct 03 '16 at 19:15

Rohit Takhar

95
8

3

Using `g++` means you're actually writing C++, not C. – milleniumbug Oct 03 '16 at 19:17
yes but the compiler-driver decides to which libraries the code will be linked. So I differentiated on the basis of compiler-drivers. – Rohit Takhar Oct 03 '16 at 19:25
4

@RohitTakhar: But if you use `g++` to compile a `.c` file, it will be compiled and linked as C++, you're just writing C++ with a funny extension at that point. – Dietrich Epp Oct 03 '16 at 19:30
@DietrichEpp Right. I have edited my post to make it more clear. – Rohit Takhar Oct 03 '16 at 19:35

Is it optional to use struct keyword before declaring a structure object?

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