Re: function prototype - c++

Default User wrote:

> Art Cummings wrote:
>
> > Hello all,
> >
> > I'm trying to write a function prototype that includes an array of
> > structures. I'm not sure how to write it.
>
> struct st
> {
> int member;
> }
>
> void f(st *data);

Oh, I forgot to mention, you'd be far better off with vectors:

#include <vector>

struct st
{
int member;
}; // forgot the ; above, sorry

void f(std::vector<st> &data);




Brian

On Mon, 03 Dec 2007 15:58:03 -0500, Art Cummings wrote:

> Here's the code I have.
>
> I get an undefined reference to getInfo, error.
>
> Thanks
> Art

[snip]
Your declaration is:
> void getInfo(Student[]);
[snip]

Your definition is:
> void getInfo(Student &s)
> {
> }

Can you see the difference in argument type?

--
Tadeusz B. Kopec (tkopec@NOSPAMPLEASElife.pl)
If I told you you had a beautiful body, would you hold it against me?

Art Cummings wrote:
Please don't top-post. Your replies belong following or interspersed
with properly trimmed quotes. See the majority of other posts in the
newsgroup, or the FAQ.

Rearranged.

> "Tadeusz B. Kopec" <tkopec@NOSPAMPLEASElife.pl> wrote in message
> news:47546ff8$1@news.home.net.pl...
> > On Mon, 03 Dec 2007 15:58:03 -0500, Art Cummings wrote:
> >
> > > Here's the code I have.
> > >
> > > I get an undefined reference to getInfo, error.
> > >
> > > Thanks
> > > Art
> >
> > [snip]
> > Your declaration is:
> > > void getInfo(Student[]);
> > [snip]
> >
> > Your definition is:
> >> void getInfo(Student &s)
> >> {
> >> }
> >
> > Can you see the difference in argument type?

> Tadeusz, that's exactly the problem, i'm not sure how to define the
> header or the call or the function that receives it. I'm not sure
> when dealing with an array of structures exactly what this looks like
> for the prototype, call and function header. I've not got any
> examples in my text and that is exactly what i'm trying to find out.
> What i'm saying is I don't know how to define it.

Your function definition should the same parameter types as the
relevant declaration (if you have one). Your declaration specifices a
function getInfo() taking a pointer to Student, which you then call
correctly, but then you DEFINE getInfo() as taking a reference to
Student.

Because C++ allows function overloading, it's not an error to have them
disagree, it just means that you are referring to two different
versions of getInfo(), one which you declared and used, another that
you defined.

At link time, the implementation couldn't find a definition for the
version of getInfo() that you called.

Make the two signatures agree. Then look into vectors, so you need
magic numbers for the size of things.



Brian

Art Cummings wrote:

> Thanks Brian, we haven't dealt with vectors yet.

You really ought to start with those, and use raw arrays when you are
more experienced.




Brian

Art Cummings wrote:

> Thans Tadeusz, how do I define my function header as a pointer to an
> array of Students? That's the gist of my confusion. I don't know
> how to define a function prototype that is an array of structures to
> a pointer.

I wrote the post you replied to, AND you ignored what I wrote about
top-posting. See below.


> > Art Cummings wrote:
> > Please don't top-post. Your replies belong following or interspersed
> > with properly trimmed quotes. See the majority of other posts in the
> > newsgroup, or the FAQ.



Brian

Art Cummings wrote:
> Here's the code I have.
>
> I get an undefined reference to getInfo, error.
>
> Thanks
> Art
>
> #include <iostream>
> #include <fstream>
> #include <string>
> #include <conio.h> //necessary for the _getch function
>


> using namespace std;

I know that I'll be disagreed with, but I dislike using namespace std;


>
>
>
> const int SIZE = 15; //size for all arrays in student structure
>
> struct Student
> {
> char fName[SIZE]; //array for first name
> char lName[SIZE]; //array for last name
> char attendance[SIZE]; //array for attendance
> int days[SIZE]; //array for days
> };


I am somewhat confused by what Student is, or more to the point, why
SIZE is the same for fName, lName, attendance and days.


Are you trying to make a struct that has information for SIZE students?
Or is this information for just one student?

If its for one student think about:
const int NameSize = 15;
const int NumberOfDays = 15;
or something. Because I think it's a little confusing for the size of
the first name and last name to be the same symbolically as the size of
attendance and days.


If its for more than one student, then I think you're going to have
trouble storing the names.

Also, your comments are less than helpful. Sorry. Don't mean to be
brusque, but I know those things are arrays. In the case of attendance
and days, I'm not really certain what they're for. And lName and fName
might be clearer as lastName and firstName or something.



> void getInfo(Student[]);

getInfo is a function taking an array of Student as an argument and
returning void.

>
>
> int main()
> {
> Student studentInfo[30]; //define an array of 30 students

Magic numbers are generally frowned upon.

> getInfo(studentInfo);
> system("PAUSE");
> return 0;
> }
> void getInfo(Student &s)
> {
> }

But here, getInfo is a function taking a reference to a Student as an
argument and returning void.


Forget about structs for a while. Too complicated. Just do an array.

Personally, I think it saves time in small test programs to just put the
functions I'm calling before main. But since you specifically want to
think about prototypes...

So here's a simpler example.

#include <iostream>

int sum(const int [], const int);

int main() {
static const int x[] = { 5,7,9,};
const int s = sum(x, sizeof(x)/sizeof(x[0]));
std::cout << s << std::endl;
}

int sum(const int a[], const int n) {
int result = 0;
for(int i=0; i<n; i++) {
result += a[i];
}
return result;
}

Now, a struct is easy.

#include <iostream>

struct T {
int t_;

T &operator+=(const T &t) {
t_ += t.t_;
return *this;
}
};

T sum(const T [], const int);

int main() {
static const T x[] = { {5},{7},{9},};
const T s = sum(x, sizeof(x)/sizeof(x[0]));
std::cout << s.t_ << std::endl;
}

T sum(const T a[], const int n) {
T result = {0};
for(int i=0; i<n; i++) {
result += a[i];
}
return result;
}


Now, I don't want to say that the above code is any good. I don't think
it is. It's just for illustration. Just something to get you started.

HTH

Else thread Art Cummings wrote:
> I've not got any examples in my text and that is
> exactly what i'm trying to find out.

What text are you using?

Did your text cover std::vector<> yet?



LR

Art Cummings wrote:
> Thanks Lr, thanks everyone. I'm still not getting how to do this. [..]
> void getInfo(Student[],int);
>
> void getInfo(Student &s,int x)
> {
> }

The two declarations have to MATCH. The former declares that 'getInfo'
expects a pointer of 'Student' type, and an int. The latter declares
that 'getInfo' expects a REFERENCE to 'Student', and an int. In order
to make them match, you need the second one say

void getInfo(Student s[], int x)

You may need to pay a bit more attention to declarations (and what they
mean), otherwise C++ is going to be very difficult for you.

V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask

Art Cummings wrote:

> Thanks Lr, thanks everyone.

I asked you twice not to top-post, yet you continue to do so. Why
should anyone help you when you have demonstrated extreme rudeness to
the group?



Brian

Art Cummings wrote:

> Brian, I thought top posting was where you include the previous
> message in your reply.

Here's the information I posted twice already.

Please don't top-post. Your replies belong following or interspersed
with properly trimmed quotes. See the majority of other posts in the
newsgroup, or the newsgroup FAQ.




Also, see this:

<http://www.caliburn.nl/topposting.html>




Brian