Validate locations of character pointer array after new char* allocation

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Hello all

I have the following array of character pointers

C++

char** test = new char*[256];

Now this array is passed to a function that fills "some" of its locations. My questions is how to check the status of other unfilled pointers? Or what is the status of these pointers?
for example I have this:

C++

test[0] = "hello";
test[1] = "world!";

How to check the status of any of the other 254 pointers? I tested them they are not NULL, not 0, not '\0'. My problem is I have a program that sometimes access these pointers and then crashes, no handling mechanism at all.

Posted 6-Jul-14 10:32am

Abdallah Al-Dalleh

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PIEBALDconsult 6-Jul-14 18:29pm

they are not NULL, not 0, not '\0'

Did you initialize the array?

Mohibur Rashid 6-Jul-14 20:57pm

test[] is a pointer of character; you need to allocate memory for these address;
example:
for(int i=0;i<256;i++){
test[i] = new char[somelength];
}
then you can assign value to test[]; but this is a character array;

3 solutions

Solution 2

Just initialize the pointer array with 0 like this:

~~char** test = new char*[256] = {};~~

C++

char* test[256] = {}

Note that since the initialization list is empty, it will be padded with 0 automatically. See here[^] for more info.

[edit]fixed intialization statement - sadly it won't work with heap variables[/edit]

[edit2]
After nv3 pointed it out I tried initializing the array using new, and it does indeed work with MS VisualSTudio 2010. I couldn't pinpoint a reference to this behaviour though, so I can only report what works with this compiler:

C++

char *test[256] = nullptr;
test = new char*[256](); // note the pair of rounded brackets!
assert(test[123] == 0); // works!
delete [] test;
test = new char*[256]; // no brackets this time
assert(test[123] == 0); // will most likely fail! (compiler dependend)

[/edit2]

Posted 6-Jul-14 22:24pm

Stefan_Lang

Updated 9-Jul-14 22:29pm

v3

Comments

nv3 7-Jul-14 8:36am

Stefan, is it "new char*[256] = {}" or "new char*{256] {}"?

Stefan_Lang 8-Jul-14 2:28am

*doh* shouldn't be a call to new at all - somehow I just copied the declaration ignoring the allocation :/

will fix it...

nv3 8-Jul-14 6:17am

Well, I was surprised to learn that it WILL work with the new operator as well. C++03 and C++11 brought some additions. "new char*[256]()" with a pair of round braces will do a default initialization, i.e. 0. In so far, you inspired me and perhaps others to look that up. My 5.

Stefan_Lang 9-Jul-14 2:41am

So it does work with new? I wasn't sure and didn't find a reference on that. And I don't trust MSVC to be conformant to the standard, so I didn't even bother to try.

Thanks for the tip though, I'll update my solution again.

Solution 1

Here is a sample to test. Access to **test will fail if not initialised (pointing to allocated memory).

C#

void CTest::Run()
{
char** test = new char*[256];
FILE *f;
fopen_s (&f,"b.txt","w");
fprintf(f,"**test: 0x%x\n",test);
for(int i=0;i<256;i++)
{
fprintf(f,"*test %d=> 0x%x\n",i, *test);
//fprintf(f,"test[i] %d=> 0x%x\n",i, **test);//access violation
test++;
}
fclose(f);
}

Output sample:

**test: 0x6d3f68
*test 0=> 0xcdcdcdcd
*test 1=> 0xcdcdcdcd

Posted 6-Jul-14 14:48pm

BacchusBeale

Comments

Abdallah Al-Dalleh 7-Jul-14 2:21am

That's the point how to detect the pointer access will fail before reading ?

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Abdallah Al-Dalleh · Accepted Answer · 2014-07-07T03:33:00

Solution 3

Hello all :D

I think it's already handled, it appears that all other unfilled locations in the char** test are given NULL after filling the array so all I had to do is to check if the value is null or not. Thanks all for the help :D

Posted 7-Jul-14 3:33am

Abdallah Al-Dalleh

Comments

nv3 7-Jul-14 17:14pm

Hello Abdallah! Good that your problem has been solved. Would you please honor one or more of the solutions by formally accepting them. This is the "thank you" for those who have spent their time to help you. -- And posting your own message as a solution is not the right way to do it and being frowned upon. A comment under your question would have been the way to do that.

Stefan_Lang 8-Jul-14 2:34am

FYI, you cannot rely on any value to be intialized to 0 automatically. This may or may not be true, depending on the compiler, or even compiler settings. Therefore you must initialize the entire array to 0 (or nullptr) yourself. If you define the array opn the stack rather than on the heap you can use the initialization syntax I posted in solution 2. Otherwise you should run a loop over all elements and assign them to 0.