Dereferencing null pointer

Question

I am trying to write a very simple pointer assignment in C++ like this:

float  *src;
if(someCondition == true)
    src = somePointer;
else
    src = someOtherPointer;

However, when I try to access src, I get a warning saying "Dereference of null pointer( loaded from variable src)"

Also, if I try to give some initial value to src like this:

float  *src = aPointer;

if(someCondition == true)
    src = somePointer;
else
    src = someOtherPointer;

I get a warning saying that the initial value assigned to src is never read. Even though this doesnt seem to be affecting what I am trying to do, I am trying to get rid of all potential issues. What seems to be the problem here?

Any help is appreciated. Thanks!

Show us how somePointer and someOtherPointer are established please. — WhozCraig
– WhozCraig, Commented Mar 5, 2013 at 19:57
float * src = someCondition ? somePointer : someOtherPointer; — Nathan Monteleone
– Nathan Monteleone, Commented Mar 5, 2013 at 19:58
if(variableX > 350) src = &(anArray[0]) else src = &(anArray[50]). Thats the gist of the condition that I am trying to implement. Thanks! — halfwaythru
– halfwaythru, Commented Mar 5, 2013 at 19:58

andre · Accepted Answer · 2013-03-05 20:08:19Z

3

Since your using pointer and not reference expect NULL to be a possible value of your pointers.

It might be in your interest to check for NULL:

if(!src) {/*handle null case*/}

edited Mar 5, 2013 at 20:08

answered Mar 5, 2013 at 20:00

andre

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1 Comment

bames53 Over a year ago

'always' is a bit strong. A program can establish and maintain non-null invariants and therefore avoid having to always check for null.

mark · Accepted Answer · 2013-03-05 19:58:30Z

2

presumably one or both of somePointer and someOtherPointer are 0/NULL

answered Mar 5, 2013 at 19:58

mark

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2 Comments

halfwaythru Over a year ago

No, i know for a fact that they are valid pointers. I am using them elsewhere too. Also, even though this is potentially unsafe because of the warning that I get, this piece of code is working just fine. It wouldn't work if any of the two pointers were NULL/0. I am just trying to get rid of the warning and the possibly unsafe situation.

mark Over a year ago

it sounds to me like the compiler is seeing something you don't see. it might be some unexpected execution path, try to check them all. narrow in on the problem by commenting out blocks of code to see if/when the warning goes away.

ApproachingDarknessFish · Accepted Answer · 2013-03-05 20:15:28Z

0

It seems that the compiler isn't smart enough to realize that src will always be initialized. Although IDK why it would assume that the poiter would be NULL, more likely it would be some garbage value. This can probably be fixed by making the initialization a single statement using the ternary operator (?:) :

float* src = someCondition ? somePointer : someOtherPointer;

EDIT:

Try initializing src to NULL. That should get rid of the initialization warnings and I doubt that it will create an unused value warning.

edited Mar 5, 2013 at 20:15

answered Mar 5, 2013 at 20:08

ApproachingDarknessFish

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2 Comments

halfwaythru Over a year ago

I did try that, but it still gives the same warning. Inspite of the tertiary operator, the compiler doesnt realize that the pointer has been initialized. :(

halfwaythru Over a year ago

Sorry, that didn't help either.

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