CODEWITHSUNDEEP
cstructinitialization
Leem.fin
Leem.fin

Reputation: 42592

Initialize struct with single field

I have a struct:

 struct MY_TYPE {
      boolean flag;
      short int value;
      double stuff;
    };

I know I can intialize it by:

MY_TYPE a = { .flag = true, .value = 123, .stuff = 0.456 };

But, now I need to create a pointer variable My_TYPE* and I only want to initialize one field there? I tried e.g.:

MY_TYPE *a = {.value = 123};

But I get compiler error "Designator in intializer for scalar type 'struct MY_TYPE *'".

Is it possible to initialize the struct with one field?

Upvotes: 10

Views: 2247

Answers (3)

LPs
LPs

Reputation: 16213

You can use compound literals.

#include <stdio.h>
#include <stdbool.h>   

struct MY_TYPE
{
    bool flag;
    short int value;
    double stuff;
};

int main(void)
{
    struct MY_TYPE *a = &(struct MY_TYPE){.value = 123};

    printf("%d\n", a->value);
}

Upvotes: 5

Lundin
Lundin

Reputation: 213563

First of all, you are mixing up struct MY_TYPE and typedef. The code posted won't work for that reason. You'll have to do like this:

typedef struct 
{
  bool flag;
  short int value;
  double stuff;
} MY_TYPE;

You can then use a pointer to a compound literal, to achieve what you are looking for:

MY_TYPE* ptr = &(MY_TYPE){ .flag = true, .value = 123, .stuff = 0.456 };

But please note that the compound literal will have local scope. If you wish to use these data past the end of the local scope, then you have to use a pointer to a statically or dynamically allocated variable.

Upvotes: 17

Some programmer dude
Some programmer dude

Reputation: 409166

It is possible, if you first create an actual instance of the structure initialized the way you want, and then make the pointer point to that.

E.g.

struct MY_TYPE a = {.value = 123};
struct MY_TYPE *p = &a;

If you want to allocate a structure dynamically, then no it is not possible. Then you need to do it in two steps: Allocation; Followed by initialization:

struct MY_TYPE *p = calloc(1, sizeof *p);
p->value = 123;

The error you get have nothing to do with the initialization, but that you try to initialize a pointer with something that is not a pointer.

Upvotes: 4

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