Question

I would avoid the use of malloc to initialize a structure and I’m looking for the best practice for the design a C software using an oo-style (where possible).

Only C99, not C++

First question, what is preferable when use a struct like an object? typedef its pointer or not?

These are my test(all works using gcc compiler):

case 1

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

typedef struct sItem{
    int n;
    char* text;
} oItem, *Item;

int main(int argc, char** argv) {
    Item i1=(&(oItem){.n=1, .text="A"});
    Item i2=(&(oItem){.n=100, .text="ABC"});
    printf("%d, %s, %d\n", i1->n, i1->text, sizeof(*i1)); // 1, "A", 8
    printf("%d, %s, %d\n", i2->n, i2->text, sizeof(*i2)); // 1, "ABC", 8
    return (EXIT_SUCCESS);
}

This works, but i think it should not because text is not initialized to contains strings.
Is this an invalid piece of code?

case 2

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

typedef struct sItem{
    int n;
    char text[5];
} oItem, *Item;

int main(int argc, char** argv) {
    Item i1=(&(oItem){.n=1, .text="A"});
    Item i2=(&(oItem){.n=100, .text="ABC"});
    printf("%d, %s, %d\n", i1->n, i1->text, sizeof(*i1)); // 1, "A", 12
    printf("%d, %s, %d\n", i2->n, i2->text, sizeof(*i2)); // 1, "ABC", 12
    return (EXIT_SUCCESS);
}

This works and I think it is correct, is it?

case 3

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define Item_new(i, n, s) (&(oItem){0});Item_ctor(i, n, s);
#define Item_neww(i, x, s) (&(oItem){\
        .n=x,\
        .text=s\
})

typedef struct sItem{
    int n;
    char text[5];
} oItem, *Item;


void Item_ctor(Item i, int n, char* text){
    i->n=n;
    strcpy(i->text, text);
}

int main(int argc, char** argv) {
    Item i1=Item_new(i1, 10, "ABC");
    Item i2=Item_neww(i2, 10, "ABC");
    printf("%d, %s, %d\n", i1->n, i1->text, sizeof(*i1)); // 10, "ABC", 12
    printf("%d, %s, %d\n", i2->n, i2->text, sizeof(*i2)); // 10, "ABC", 12
    return (EXIT_SUCCESS);
}

I think this is very nice, but hides the code, and perhaps might be harmful, what do you think?
I case 3, what is the best choice: macro or constructor function?

Answer 1

Don’t do 3, macros that contain unprotected ; make me extremely nervous.

Instead I would replace your “new” and “ctor” by the following

#define Item_new(i, n, s) Item_ctor(&(oItem){0}, n, s)

Item Item_ctor(Item i, int n, char* text){
    if (i) {
      i->n=n;
      strncpy(i->text, text, 4);
    }
    return i;
}

This doesn’t break the expectation of the user for Item_new: a real
function like macro that returns a value.

And the ctor should do the necessary checks and never overwrite the memory, i->text[4] will always be 0. (Better would be to have a symbolic constant instead of 5 and use it also for the strncpy call.)