Typedef-ing nonexisting struct in c

Question

I am currently learning pointers and structs, and I cant understand the following

typedef struct at atom;
struct at {
     int element = 5;
     struct at *next;
}

how come this code doesn't throw an error?

and when I create an atom using

atom x;
printf("%d", x.element);

it will print 5.

I don't understand the typedefing of non-existing structure, how does that work?

Nick Zuber · Answer 1 · 2015-12-06T23:01:40.343

4

typedef struct at atom;

Here, you are declaring a struct called at and giving it a typedef of atom. So that is completely valid C.

struct at {
     int element = 5;
     struct at *next;
}

Here, you are defining the struct at that you declared earlier.

NOTE:

It is important to note that in C, you are not allowed to initialize variables within a struct like you do here:

struct at {
     int element = 5; // <---- Illegal in C
     struct at *next;
}

However, this is perfectly okay in C++ so if you're trying to use C, remove the variable initialization, but if you're working with C++ then it's fine the way it is.

edited Dec 06 '15 at 23:01

answered Dec 06 '15 at 22:47

Nick Zuber

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But initialization of struct members (`int element = 5;`) is not allowed in C. – P.P Dec 06 '15 at 22:49
@l3x In C you're correct, however you can do this is C++ [**See here**](http://stackoverflow.com/questions/16782103/initializing-default-values-in-a-struct) – Nick Zuber Dec 06 '15 at 22:59

score 2 · Answer 2 · answered Dec 06 '15 at 22:47

It's a forward declaration — in C you are permitted to declare that a named thing will exist without giving details about it until later; that's most common used to allow cycles of knowledge; e.g. struct A has a pointer to struct B, struct B has a pointer to struct C, and struct C has a pointer to struct A. In that specific case, as long as you've said that the things are structs then the compiler knows enough about the type to know what an appropriate pointer will look like.

In this case you've declared that there will be a struct called at and that you'll want to refer to it as atom. If you were to try to declare an atom before telling the compiler what struct at looks like, that'd be an error. But until then the compiler has enough information to proceed.

Vlad from Moscow · Answer 3 · 2015-12-06T22:56:28.440

You forgot to place a semicolon

typedef struct at atom;
struct at {
     int element = 5;
     struct at *next;
}
^^^^

Take into account that it is a C++ code. In C you may not initialize data members of a structure within its definition.

This declaration

typedef struct at atom;

declares two things. First of all it declares new type struct at in the current declarative region. And it declares an alias for this type atom

It is not necessary that each declaration would be at the same time a definition.

At first struct at is declared and then it is defined

struct at {
     int element = 5;
     struct at *next;
};

Compare thsi approach for example with functions. Functions also may be several times declared and only one time defined. For example

void f();
void f();
//...
void f();

void f() { std::cout << "It is a joke" << std::endl; }

The same way a structure may be several times declared but only one time defined.

Typedef-ing nonexisting struct in c

3 Answers3