C similar struct's

greeklegend · 12-15-2007, 03:40 AM

Hi guys
I have two similar structures that differ in only a couple of elements (called http_request and http_response). I have written code that processes HTTP headers which works the same way for both requests and responses.
Is there a way i can write a function that can take ether a http_request or a http_response argument and do exactly the same thing to it? (the relevant members that this function needs to do are identical in both struct's).

jlliagre · 12-15-2007, 06:07 AM

If the structure members you use are at the same offset, you may have your function accepting a void * and cast it to use either structure pointer.

This is however poor coding. A better and less risky approach would be splitting your structure in a common trunk and an union of two other structures containing the differing members.

An even better approach would be using a object oriented language as class inheritance is really what you are after.

xlq · 12-15-2007, 11:47 AM

Code:

struct http_headers {
    /* fields common to both */
};

struct http_request {
    struct http_headers h;
    /* fields specific to http_request */
};

struct http_response {
    struct http_headers h;
    /* fields specific to http_response */
};

void dostuff(struct http_headers *h)
{
    /* do stuff to http request/response */
}

Instead of passing the http_request or http_response structs to dostuff(), pass a pointer to the h field of either.

greeklegend · 12-16-2007, 01:57 AM

So somethig like this would work?

Code:

//It's important for the common members to be at the top of the struct yes?
typedef struct{int common_member1;
int common_member2;
int not_common_member;} http_request;

typedef struct{int common_member1;
int common_member2;
unsigned long double not_common_member;
char* not_common_member_too;} http_response;

void dostuff(void* arg)
{
http_response* tmp;
tmp = (http_response*)arg

tmp->common_member1 = 8; //works
tmp->not_common_member = 9000000.3423473; //doesn't work, if http_request was passed to the function i'm assigning unsigned long double to int
return;
}

I can see why this is poor coding.
I might consider modifying the structs to store common information in a different struct
As for object oriented language...meh i don't really know why i'm using C for this since i can use C++ just fine. mabye I just like having a more intimate understanding of what exactly my program is doing.
Btw thanks guys

xlq · 12-16-2007, 03:33 AM

Why aren't you using C++? Because C++ has operator overloading, and other such features which obscures the meaning of the code.

greeklegend · 12-16-2007, 01:40 PM

Lol hmm....is there any reason for C to exist anymore, given C++ is everything C is and better? What exactly does C have over C++?

paulsiu · 12-16-2007, 02:08 PM

Quote:

Originally Posted by greeklegend

Lol hmm....is there any reason for C to exist anymore, given C++ is everything C is and better? What exactly does C have over C++?

Well, it's possible that for embedded systems, C++ may add an overhead over C. C exists because a lot of stuff is written in C.

Paul

bigearsbilly · 12-17-2007, 06:18 AM

Quote:

Lol hmm....is there any reason for C to exist anymore, given C++ is everything C is and better? What exactly does C have over C++?

it's readable

er, this for one:

Code:

    stdout = (FILE *) fp;

greeklegend · 12-17-2007, 11:43 PM

You can assin to stdout and stdin like that? Hehe you learn something new every day.
In other news i've decided to use the ugly "typecast void* to http_request*" hack i posted earlier, since it turns out my struct's are identical- I managed to make the only difference the name of the members, and since nobody except me will ever see this code, readability can go to hell lol.

bigearsbilly · 12-18-2007, 03:23 AM

Quote:

and since nobody except me will ever see this code, readability can go to hell lol.

have you ever gone back a year later and said "what the hell was I on about?"

paulsiu · 12-18-2007, 07:32 AM

Quote:

Originally Posted by bigearsbilly

have you ever gone back a year later and said "what the hell was I on about?"

You know, I had this conversation with my lead programmer from a decade ago. We wanted him to commented because we couldn't read his code. He never commented because he said that code should be readable on his own. I notice he couldn't read his own code 6 months later.

PAix · 12-18-2007, 10:47 AM

I remember testing an application program a few years back and was not able to break it (I cheated and had a squint at the source code looking for clues to weaknesses). Imagine my surprise when I realised that I had actually written it. I was more than impressed at some of the constructs that I had used. Software developer - elaborate and refine.
I don't suppose there are many of us without that particular Tshirt.

xlq · 12-18-2007, 11:45 AM

C++ has polluted the namespace, broken void pointers, mangled all the identifiers and induced a gross lack of compilation speed. C++ is by no means a superset of C.