Function pointer inside struct in C

Question

I am trying to understand C syntax and I have done the following example. I have placed a function pointer *func as a property of my Person struct, which returns a struct Person.

typedef struct
{
    int age, salary;
    struct Person(*func) (int age, int salary);
} Person;

Person other_func(int age, int salary)
{
    Person* person = malloc(sizeof(Person));
    person->age = age;
    person->salary = salary;
    return *person;
};

int main()
{
    Person p;
    p.func= other_func;
    p = p.func(30, 3000);
}

This gives me "Can not convert Person to Person" on the last line. I suppose this is because the one is Person and the second is struct Person, but inside the Person struct, I have have my function as struct Person(*func_1) (int age, int salary); because it raises a compilation time error if I use Person instead of struct Person. So I used struct Person instead. Is this the problem ? How would I achieve what I am trying to do ?

Try changing the very first line to typedef struct Person. — StilesCrisis
– StilesCrisis, Commented Apr 7, 2015 at 18:34
@BLUEPIXY can you explain where exactly is the memory leak ? because i do not see it — dimitris93
– dimitris93, Commented Apr 7, 2015 at 18:36
memory leak: 1 malloc(), 0 free() ... BTW, you need to #include <stdlib.h> — pmg
– pmg, Commented Apr 7, 2015 at 18:37
Person* person = malloc(sizeof(Person)); person->age = age; change to Person person = {age, salary}; return person; no need malloc. because you return Person(copy) Without release parson. — BLUEPIXY
– BLUEPIXY, Commented Apr 7, 2015 at 18:39

Community · Accepted Answer · 2017-05-23 12:06:27Z

4

typedef struct
{
    int age, salary;
    struct Person(*func) (int age, int salary);
} Person;

Shouldn't this be:

typedef struct Person
{
    int age, salary;
    struct Person(*func) (int age, int salary);
} Person;

In the first case, you don't name your struct, so struct Person isn't a valid type name. My version of GCC gives more helpful output:

$ gcc test.c
test.c: In function ‘main’:
test.c:20:11: warning: assignment from incompatible pointer type
     p.func= other_func;
           ^
test.c:21:5: error: invalid use of undefined type ‘struct Person’
     p = p.func(30, 3000);

Note that the two names don't need to match. All you're doing is combining a typedef and a struct declaration. This is would be equally valid:

struct X {
    int age, salary;
    struct X(*func) (int age, int salary);
};

typedef struct X Person;

You may want to read this excellent answer about struct typedef'ing.

edited May 23, 2017 at 12:06

CommunityBot

11 silver badge

answered Apr 7, 2015 at 18:33

Brendan Long

54.5k21 gold badges154 silver badges194 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

dimitris93 Over a year ago

you are right. what exactly is the difference between the two ? I thought it was exactly the same ?

Brendan Long Over a year ago

For anyone else wondering why defining a type like this isn't an error, it's because this is how you do forward declaration in C.

fukanchik · Accepted Answer · 2015-04-07 18:36:04Z

2

typedef struct X {} Y;

X - is a TAG which you can convert to something useful with syntax 'struct X val'. Y is typename which you can use directly: 'Y val;'

Then, type name Y (in your case 'Person' is not visible inside of struct as it is defined later. That is why you have to use TAG inside of struct:

typedef struct tag_Person {
    struct tag_Person (*func)(int arge, int salary);
} Person;

answered Apr 7, 2015 at 18:36

fukanchik

2,93427 silver badges35 bronze badges

1 Comment

Brendan Long Over a year ago

I don't think the tag/ type name distinction is correct (they are both type names), but you're right that the second one isn't available yet.

Collectives™ on Stack Overflow

Function pointer inside struct in C

2 Answers 2

2 Comments

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related