conditional statement

[oldnewbie]

Can I use the conditional operator in such a way that,if the conditions meets,it will assign the value,and if not it will do nothing.
What I mean is this possible?

apointer=(afunc())?afunc():donothing;

Here, afunc() returns a pointer with the same type as apointer.This function returns NULL value if an error occurs so apointer will be assigned a NULL in this case.How can I avoid that?

Also,this expression
apointer=(afunc())?afunc():NULL;
causes

afunc()

to be called twice,can we make it be called once without using an extra dummy pointer variable?

[Lynx876]

I've never used a conditional statement before, but are you actually missing the condition?

i.e.
x = y > 3 ? 10 : 20

  If y is greater than 3  
    x = 10
  else
    x = 20

You don't actually have a condition there.
The function is being called twice because it gets called for the condition and then the assignment.

x = y > afunc() ? afunc() : 20

 call afunc() for condition  
    x = call afunc() for assignment  
  else
    x = 20

[oldnewbie]

Thanks Lynx.

I thought that (afunc()) expression below can be condition.I mean,the function will return

something

or

NULL

,so I thought NULL can signal false,and the condition will be true otherwise.

apointer=(afunc())?afunc():donothing;

Doesn't it mean
apointer=(afunc()!=NULL)?afunc():donothing;
?

[Bazzy]

Here, afunc() returns a pointer with the same type as apointer.This function returns NULL value if an error occurs so apointer will be assigned a NULL in this case.How can I avoid that?

Use a simple if instead of the conditional operator

Also,this expression
apointer=(afunc())?afunc():NULL;
causes

afunc()

to be called twice,can we make it be called once without using an extra dummy pointer variable?

No

[Lynx876]

I see what you mean now, sorry.

I think you may be better off doing the following. But as I mentioned before, I've never used a conditional statement.

if( afunc() )
{
    aPointer = afunc();
}

wow, actually, that's wrong too, it still gets called twice... I'm a bit dumb this morning... ahaha

My last edit now! This should work.

( return type ) tempVariable = afunc();

if( tempVariable )
{
    aPointer = tempVariable;
}

[oldnewbie]

Thanks Bazzy and Lynx.
So, without using an extra dummy variable/object it is not possible.

[Lynx876]

Is the reason for trying to do this without a dummy/object variable, to save on memory?

If so, you could create a new pointer and then delete it after the assignment to the pointer you will be using afterwards.

http://en.wikipedia.org/wiki/New_(C%2B%2B)

[Bazzy]

That would consume more memory, a pointer on the stack and a pointer on the heap
( instead of only a pointer on the stack )

[Lynx876]

But wouldn't it be more efficient to create a pointer on the heap if it was being created in the main function? Assuming that it is being made in the main function.

That way, the pointer would only be on the heap until the assignment was made, then it would be removed from the heap. Rather than creating a pointer on the stack until the program had been exited.

new pointer = afunc();

if( new pointer )
    aPointer = new pointer

delete new pointer

[filipe]

When I say:

foo *ptr = new foo;

ptr is not being stored on the heap, only the object it points to. If it goes out of scope, ptr is destroyed because it's an automatic variable, stack allocated, but the object it points to isn't.

[sachav]

You could also do

apointer = (apointer=afunc()) ? apointer : donothing;

The power of the assignment operator :)

[Bazzy]

@Lynx876
What you are saying:

type** new_ptr = new type*;
*new_ptr = afunc();
if ( *new_ptr )
    apointer = *new_ptr;
delete new_ptr;

vs stack allocation of the pointer:

type* ptr = afunc();
if ( ptr )
    apointer = ptr;

[oldnewbie]

You could also do

  	

apointer = (apointer=afunc()) ? apointer : donothing;



The power of the assignment operator :) 

Thanks sachav,that was the power I was looking for :)
although that donothing is still symbolic.

Btw,I think I should use a new dummy pointer to keep the result of the function,in an if statement,as Lynx gave above,to prevent apointer from taking NULL value,because that NULL value creates problem when I display all the pointed objects in the array.

[Bazzy]

apointer = (apointer=afunc()) ? apointer : donothing; Is NOT valid C , you can't assign the same variable twice in an expression.

[sachav]

Wait, what's not valid C?
The said expression is just the diminutive form of

if(apointer = afunc())
    apointer = apointer;
else
    donothing;

Right?

Is it the second line that isn't valid? If that's so just use apointer = apointer * 1;

[Bazzy]

Nope, it's a single expression. As I said, in Cplusplus you can't modify the same variable twice.

( ATM there's some issue with the plus sign on the forum )

[oldnewbie]

Its nice to read your discussions,but I passed that example using an extra dummy variable.
The reason I asked such a question is that,the author of the book is so stingy about memory usage.Otherwise, since Im not coding a program with thousands of hundreds of lines and variables, an extra variable is not problem :)

[Bazzy]

In modern computers, memory is the last of your problems. How old is your book?

[hanst99]

What Bazzy said. Unless you need to do that with a bazillion recursions you don't really have to care about the memory you need for one extra variable.