Help With Constant Pointers

[crystal twix]

I'm having trouble understanding constant pointers. If my function is
to compute the time difference between two structs of type

struct Time {
int hours, minutes, seconds;
}

and my function is:

Time *TimeDiff(const Time *t1, const Time *t2)

I thought I could create a long int and convert each of the times to
seconds, subtract the times, and convert back. However, I then
realized that doing:
t1->hours*3600 results in a Segmentation Fault because I am trying to
access the pointer in the wrong way. So how do you usually manipulate
constant pointers without declaring a new pointer? It seems odd and
inefficient if I were to create a new variable and set it to that
value and then do the manipulation. Like

int var = t1->minutes;
int newVar = var *3600;

Thanks!

 

[Michael Tsang]

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I'm having trouble understanding constant pointers. If my function is
to compute the time difference between two structs of type

struct Time {
int hours, minutes, seconds;
}

and my function is:

Time *TimeDiff(const Time *t1, const Time *t2)

Writing an operator- instead of function is much better.

I thought I could create a long int and convert each of the times to
seconds, subtract the times, and convert back. However, I then
realized that doing:
t1->hours*3600 results in a Segmentation Fault because I am trying to
access the pointer in the wrong way. So how do you usually manipulate
constant pointers without declaring a new pointer? It seems odd and
inefficient if I were to create a new variable and set it to that
value and then do the manipulation. Like

int var = t1->minutes;
int newVar = var *3600;

Thanks!

If running "t1->hours * 3600" gets a segmentation fault, your t1 does not
point to a valid object.
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[Ian Collins]

I'm having trouble understanding constant pointers. If my function is
to compute the time difference between two structs of type

struct Time {
int hours, minutes, seconds;
}

and my function is:

Time *TimeDiff(const Time *t1, const Time *t2)

I thought I could create a long int and convert each of the times to
seconds, subtract the times, and convert back. However, I then
realized that doing:
t1->hours*3600 results in a Segmentation Fault because I am trying to
access the pointer in the wrong way.

No, you are probably dereferencing a null pointer.

So how do you usually manipulate
constant pointers without declaring a new pointer? It seems odd and
inefficient if I were to create a new variable and set it to that
value and then do the manipulation. Like

You don't.

int var = t1->minutes;
int newVar = var *3600;

Don't concern yourself with such minor premature optimisations! The
compiler will take care of them for you.

 

[Jorgen Grahn]

I'm having trouble understanding constant pointers. If my function is
to compute the time difference between two structs of type

struct Time {
int hours, minutes, seconds;
}

and my function is:

Time *TimeDiff(const Time *t1, const Time *t2)

Wasn't this discussed in another thread created by you recently? It
helps the reader if you continue in the same thread (perhaps changing
the subject) rather than creating a new one, with no reference back to
the original thread.

/Jorgen