How can I concisely assign to the members of a struct depending on a condition?

Question

I have some code that looks like this:

struct mystruct
{
    /* lots of members */
};

void mystruct_init( struct mystruct* dst, int const condition )
{
    if ( condition )
    {
        /* initialize members individually a certain way */
    }
    else
    {
        /* initialize members individually another way */
    }
}

Options I'm considering:

Simplest would be to have a function that assigns to every member and call that. Should I simply hope the compiler optimizes that call away?
Define a macro to explicitly avoid the function call overhead.
Write everything the long way.

What is the proper way to handle such a scenario in C11?

There's nothing wrong with what you have. Readability is important. You could hoist some of the lines out, e.g. `dst->foo = condition ? 5 : 7;` but consider which option makes it more obvious to the reader what is going on and why. — M.M, Aug 25 '16 at 05:07
Consider using 'compound literals' and 'designated initializers': `*dst = (struct mystruct) { .member1 = value1, .member2 = value2, … };` in each branch of the `if`. — Jonathan Leffler, Aug 25 '16 at 05:30
If the `struct` needs to be `const`, you could create a number of macros containing different initializer lists, then `#ifdef` between them. Not pretty, but there's no other option then. — Lundin, Aug 25 '16 at 11:37

score 7 · Accepted Answer · edited May 23 '17 at 12:09

7

Just write a function that initializes a member, or if you want (opinion based), use a MACRO.

By the way, I would personally do it like this:

void mystruct_init( struct mystruct* dst, int const condition )
{
    if ( condition )
        init_first_way(..);
    else
        init_second_way(..);
}

or just use the ternary operator. Remember, you care about readability and always have in mind:

Simplicity is a virtue!

I really think worrying about optimization at this stage will make a victim of immature optimization, since I doubt it will be the bottleneck.

In general, if you want to optimize your code, profile your code(while it runs with optimization flags, many people do not know this, I was one of them: Poor performance of stl list on vs2015 while deleting nodes which contain iterator to self's position in list), find the bottleneck and try to optimize that bottleneck.

edited May 23 '17 at 12:09

Community

1
1

answered Aug 25 '16 at 05:08

gsamaras

71,951
46
188
305

3

+1 for mentioning `immature optimization`. As said, premature optimization is the source of all evil :) [http://c2.com/cgi/wiki?PrematureOptimization] – Arun Aug 25 '16 at 05:20
@Arun It was less about optimization and more about conciseness. I'm not very familiar with compiler optimizations, so I didn't want to needlessly have something that was less efficient. Thank you both for pointing that out though, it reminds me to stop worrying about things like that when I haven't even measured. – user2296177 Aug 25 '16 at 05:22
@user2296177 been there, done that, see my edit. I feel the wind that made you ask that question though, that's why I upvoted. – gsamaras Aug 25 '16 at 05:35

score 7 · Answer 2 · edited Aug 25 '16 at 05:37

7

I do not think that there is any clear rule here. To me, it depends on the taste of the author.

Two obvious ways are:

// initialize members that are independent of 'condition'

if (condition) {
  // initialize members one way
}
else {
  // initialize members another way
}

The same may be written as:

// initialize members that are independent of 'condition'

// initialize members based on 'condition'
dst->memberx = condition ? something : something_else;
// ...

Please do not worry about one function call overhead.

edited Aug 25 '16 at 05:37

gsamaras

71,951
46
188
305

answered Aug 25 '16 at 05:16

Arun

19,750
10
51
60

1

That last sentence touched my heart, nice! – gsamaras Aug 25 '16 at 05:17

Support Ukraine · Answer 3 · 2016-08-25T06:14:24.877

I agree with the answers already posted (@gsamaras and @Arun). I just wanted to show another approach that I have found useful a couple of times.

The approach is to make some constants with the two (or more) relevant initialization values and then make a simple assignment based on one (or more) conditions.

Simple example:

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

struct mystruct
{
  int a;
  float b;
};

const struct mystruct initializer_a = { 1, 3.4 };
const struct mystruct initializer_b = { 5, 7.2 };

int main (void)
{
  int condition = 0;
  struct mystruct ms = condition ? initializer_a : initializer_b;
  printf("%d %f\n", ms.a, ms.b);
  return 1;
}

How can I concisely assign to the members of a struct depending on a condition?

3 Answers3