default value for struct member in C

Question

Is it possible to set default values for some struct member? I tried the following but, it\'d cause syntax error:

typedef struct
{
  int flag = 3;
} MyStruct         


        

Answer 1

If you only use this structure for once, i.e. create a global/static variable, you can remove typedef, and initialized this variable instantly:

struct {
    int id;
    char *name;
} employee = {
    .id = 0,
    .name = "none"
};

Then, you can use employee in your code after that.

Answer 2

You can create a function for it:

typedef struct {
    int id;
    char name;
} employee;

void set_iv(employee *em);

int main(){
    employee em0; set_iv(&em0);
}

void set_iv(employee *em){
    (*em).id = 0;
    (*em).name = "none";
}

Answer 3

you can not do it in this way

Use the following instead

typedef struct
{
   int id;
   char* name;
}employee;

employee emp = {
.id = 0, 
.name = "none"
};

You can use macro to define and initialize your instances. this will make easiier to you each time you want to define new instance and initialize it.

typedef struct
{
   int id;
   char* name;
}employee;

#define INIT_EMPLOYEE(X) employee X = {.id = 0, .name ="none"}

and in your code when you need to define new instance with employee type, you just call this macro like:

INIT_EMPLOYEE(emp);

Answer 4

Create a default struct as the other answers have mentioned:

struct MyStruct
{
    int flag;
}

MyStruct_default = {3};

However, the above code will not work in a header file - you will get error: multiple definition of 'MyStruct_default'. To solve this problem, use extern instead in the header file:

struct MyStruct
{
    int flag;
};

extern const struct MyStruct MyStruct_default;

And in the c file:

const struct MyStruct MyStruct_default = {3};

Hope this helps anyone having trouble with the header file.

Answer 5

You can implement an initialisation function:

employee init_employee() {
  empolyee const e = {0,"none"};
  return e;
}

Answer 6

Another approach to default values. Make an initialization function with the same type as the struct. This approach is very useful when splitting large code into separate files.

struct structType{
  int flag;
};

struct structType InitializeMyStruct(){
    struct structType structInitialized;
    structInitialized.flag = 3;
    return(structInitialized); 
};


int main(){
    struct structType MyStruct = InitializeMyStruct();
};