Re: pointers as arguments
On 23 Oct, 16:43, crystal twix <jonwongfanc...@gmail.com> wrote:
I'm trying to understand how pointers and reference variables work in
functions as arguments and the return type. Like if I want to compute
the minimum with a function like this:
int *computeMin(const int *a, const int *b) {
//is this wrong?
oh, yes... In so many ways.
return *a < *b ? *a : *b;}
1. you are using pointers incorrectly
2. pointers are only useful in a limited number of places in C
3. these useful places become even more limited in C++
4. you should use templates for a simple generic function like min
5. there's a min() in the standard library
<snip>
I'll only bother with 1-3
1. You've tripped over a common newbie problem with C/C++
declarations. The declaration-mirrors-usage feature. This was either a
brilliant idea by the inventor of C or a complete disaster the
consequences of which we live with to this day.
Consider
int *i;
or int* i; as C++ people tend to write it.
i is an int* or ptr-to-int
and ALSO *i is an int. The declaration mirrors the usage.
Now look at your code. It returns either *a or *b. What type is *a?
It's an int isn't it? And what does your function return? An int* or
ptr-to-int. An int isn't an int* is it? So if you really want to use
pointers here (you don't, I assure you). You'd write it.
int* min (int* a, int* b)
{ return (*a < *b) ? a : b; }
int a, b, c;
c = *min (&a, &b);
But much more sane would be
int min (int a, int b)
{ return (a < b) ? a : b; }
int a, b, c;
c = min (a, b);
2. There are a few places in C where a pointer is useful. One is to
emulate call-by-reference (C has no references). min() is a daft
function to use call by reference (it doesn't modify its arguments).
But consider swop()
void swop (int* a, int* b)
{
int t;
t = *a;
*b = t;
}
int a,b;
swop (&a, &b);
the function modifies a and b so some sort of reference is needed.
3. but C++ has proper references so even swop() shouldn't be coded
like this. More like
void swop (int& a, int& b)
{
int t;
t = a;
b = t;
}
int a,b;
swop (a, b);
You need pointers as arguments in C++ much less often. If you want to
pass ownership around or the object may not be present (ie. the
pointer can be NULL) or it can be "reseatable" (the value can change-
references cannot be changed). Raw C pointers are often not a good
idea even then and you should seriously look at things called "smart
pointers". But your course may not have got that far yet!
Dig out the C++ FAQ and see what that has to say about pointers and
references.
nick keighley