Passing an array of strings without copying

Hi,

I have two structs say:

Code:

struct a
{
int xyz;
char aStringArray[20][80];
};

struct b
{
bool b;
char ** aStringArray2;
};

Then I have a function that receives first struct as input and calls another function passing the second struct.

Code:

void exampleFunc1(struct a * mem_a)
{
struct b mem_b; 
if (mem_a->xyz) mem_b.b = true;
else mem_b.b = false;

/* update mem_b.aStringArray2 using the info from mem_a*/

/* should I allocate the memory or I can use the memory already allocated for mem_a ? */

/* I know that mem_a will survive longer than mem_b */

/* call second function passing mem_b */

exampleFunc2(&mem_b);

/* if I allocated memory explicitly then destroy it now as we no longer need it */

/* mem_a memory is not headache of this function so let's return */
return;
}

Thanks for any ideas

If you change this
char ** aStringArray2;
into this
char (*aStringArray2)[80];

Then you can do
mem_b.aStringArray2 = mem_a->aStringArray;

And you can just use your regular
mem_b->aStringArray2[x][y]
to access the array inside your second level function.

I agree with Salem's approach. But I want to ask, why are you using two different structs here? If the array in struct b points to the exact same memory in struct a, can you not just pass struct a around? If you want a boolean representation of whether mem_a->xyz is zero or non-zero, then just use it in an if statement by itself, like you do on line 4. Beside, if mem_b's array is referring to the exact same memory as mem_a's, then why bother with the other struct. Just have your second function take a struct a:

Code:

void exampleFunc2(struct a *mem_a)
{
    if (mem_a->xyz)  // instead of mem_b->b
    ...
    mem_a->aStringArray[i][j] = 'x'
}

It amounts to the same thing, and avoids the complication of an extra struct.

If you have really good justification for needing a struct a and struct b, then go ahead with Salem's suggestion, but based on what you posted, I don't see it.

Thanks Salem and anduril462. Actually the struct a and struct b were just examples. In real, the two structures are very different and they cannot be modified as they are provided by a 3rd party library interface. So seems that I must allocate the duplicate memory in this scenario.

Lesson for the day - present the actual facts if you want an actual answer.

That may be a lesson but I got the answer to my question (and some clarity in my mind how to re-use the existing memory in case of double pointers)

That will be useful for me in future so thanks anyway!