default value for struct member in C

Solution 1:

Structure is a data type. You don't give values to a data type. You give values to instances/objects of data types.
So no this is not possible in C.

Instead you can write a function which does the initialization for structure instance.

Alternatively, You could do:

struct MyStruct_s 
{
    int id;
} MyStruct_default = {3};

typedef struct MyStruct_s MyStruct;

And then always initialize your new instances as:

MyStruct mInstance = MyStruct_default;

Solution 2:

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);

Solution 3:

I agree with Als that you can not initialize at time of defining the structure in C. But you can initialize the structure at time of creating instance shown as below.

In C,

 struct s {
        int i;
        int j;
    };

    struct s s_instance = { 10 ,20 };

in C++ its possible to give direct value in definition of structure shown as below

struct s {
    int i;

    s(): i(10)
    {
    }
};

Solution 4:

You can do:

struct employee_s {
  int id;
  char* name;
} employee_default = {0, "none"};

typedef struct employee_s employee;

And then you just have to remember to do the default initialization when you declare a new employee variable:

employee foo = employee_default;

Alternatively, you can just always build your employee struct via a factory function.