How to defaultly set variable inside C struct-CodePudding

I have a struct in C, and need to set a value inside it to 1. This means that for all new structs created off that struct, the stage variable should be automatically 1. So far I have tried:

new_struct->stage = 1;

and

struct new_struct {
    stage = 1;
}

but none of these actually work. Is there a way to do this properly?

CodePudding user response：

Define an init_struct function, and call it religiously for every new_struct you allocate. It might look like this:

void init_struct(struct new_struct *nsp)
{
    nsp->stage = 1;
}

If you allocate an ordinary new_struct variable, initialize it by calling init_struct, passing a pointer to it:

struct new_struct ns;
init_struct(&ns);

If you call malloc to get a pointer to memory for a brand-new new_struct, pass that pointer to init_struct:

struct new_struct *p = malloc(sizeof(struct new_struct));
if(p != NULL) init_struct(p);

The init_struct function I've shown only initializes the stage member, since that's what you said you were worried about. Usually it's also a good idea to make sure that everything in the structure is cleanly initialized to zero, which you can do by adding a call to memset:

void init_struct(struct new_struct *nsp)
{
    memset(nsp, 0, sizeof(*nsp));
    nsp->stage = 1;
}

(Theoretically that memset call might not be adequate for allocating structure members of pointer or floating-point types, but this is a rather esoteric concern, and on any machine you're likely to use today, the memset call will be sufficient.)

CodePudding user response：

You could wrap the struct declaration and initialization into a function:

struct new_struct init_struct(void)
{
   struct new_struct s = { .stage = 1 };

   return s;
}

or a macro:

#define NEW_STRUCT(s) struct new_struct s = { .stage = 1 }

and use that everywhere you would otherwise use

struct s v;

struct new_struct v = init_struct();

// or

NEW_STRUCT(v1);

I would however question why you would need a bunch of structs that all have the same value in one element first place? Isn't there a better way to deal with whatever problem you try to solve with that?