Function that returns a Structure

Question

I want to create a structure called CarSpeed it has 4 int FR,FL,BR,BL (FrontRight,..) ; and i would like to create a function that returns this structure :

ActualSpeed Readspeed(void)
{
   code to read the speed of the car(already done)
}

Can someone help me define a function that returns this structure ? only the way how I should define it, the usual C way doesnt work :

typedefstruct CarSpeed{...}

and then

CarSpeed ActualSpeed;
ActualSpeed Readspeed(void){.. }

Answer 1

Since you're (probably) using a C++ compiler, it's usually easier to declare structures like this:

struct CarSpeed
{
    int fr, fl, br, bl;
};

(Note that there's no need for typedef.)

You can then return an instance of it directly from your function:

CarSpeed ReadSpeed()
{
    CarSpeed data;

    data.fr = ...;
    data.fl = ...;
    data.br = ...;
    data.bl = ...;

    return data;
}

You can call it from another function like this:

CarSpeed speed = ReadSpeed();

That will copy the contents of the structure into an object called speed. In this example, it's an 8 byte structure so it's a non-trivial copy. That shouldn't be a big problem unless you're calling it very frequently though.

As noted in another answer, using a pointer (or a reference) can be more efficient if necessary.

Answer 2

I would recommend to use pointers, it's faster and has the same result.

Something like this (disclaimer - I did not compile it):

typedef struct {
   int a;
   int b;
} pair;


int init_pair(pair *ptr) {
   if (!ptr)
      return -1;
   ptr->a = 6;
   ptr->b = 9;
   return 0;
}

int do_something() {
   pair mypair;
   return init_pair(&mypair);
}