CODEWITHSUNDEEP
cpointersstructure
Anonymous
Anonymous

Reputation: 347

Structures definition problem in C

I have a few structures with cross-pointers in my program, they are defined like this:

typedef struct
{
 ...
 struct myvar *next;
 struct myvar *prev;
 ...
} myvar;

typedef struct
{
 ...
 myvar *first;
 ...
} variables;

And I am getting a strange error on the next piece of code:

variables *Variables;
...
Variables->first->prev->next = Variables->first;

I am using MS Visual Studio, and it says

error C2037: left of 'next' specifies undefined struct/union 'myvar'

I have set it to Compile as C Code (/TC), in C++ mode all is ok. What is the problem and is there some workaround except something like this?

tmp = Variables->first->prev;
tmp->next = Variables->first;

Upvotes: 3

Views: 561

Answers (2)

emboss
emboss

Reputation: 39650

I constantly run into these problems, too :) Forward-declaring your typedefs should help you in your situation:

typedef struct _myvar myvar;
typedef struct _variables variables;

struct _myvar
{
    ...
    myvar *next;
    myvar *prev;
    ...
};

struct _variables
{
    ...
    myvar *first;
    ...
};

Upvotes: 4

Armen Tsirunyan
Armen Tsirunyan

Reputation: 133122

Here is your problem:

typedef struct
{
 ...
 struct myvar *next;
 struct myvar *prev;
 ...
} myvar;

You should change it to this 

typedef struct myvar_
{
 ...
 struct myvar_ *next;
 struct myvar_ *prev;
 ...
} myvar;

The problem is that in the first version, you're telling the compiler there is going to be a definition of struct myvar. But you never define a struct named myvar. You just have an anonymous struct and typedef is as myvar, which doesn't count. So next and prev remain of incomplete type.

Upvotes: 4

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