Int append to char, does not behave as I expect

Question

I have this issue with assign an int to a char. I have a const char * data, that works like a storage of sizes of differents objects. I use this, to verify if the data have the same size that a object, if is thus, then I append a 2 integer to data. The problem is, that I expected that when I append 2 to data, move 2 positions in ASCII table, but how data behaves as a string, I cannot understand how works this addition.

Example: I have a const char* data that store a char '`', and that have a value of 96, if I make :

data += 2;

I would expect the result to be b, but is < with a value 60.

I use a function sys_log to show what happen.

function Boot:

     void Boot(const char* data){
     [...]
     /*
     * OBJECT PROTO
     */

    sys_log(0, "BOOT: OBJ PROTO data string: data = %s", data);
    sys_log(0, "BOOT: OBJ PROTO size of data: %d", decode_2bytes(data));

    if (decode_2bytes(data) != sizeof(TObjectProto))
    {
        sys_err("object proto table size error");
        thecore_shutdown();
        return;
    }
    sys_log(0, "BOOT: OBJ PROTO data after decode_2bytes(data): data = %s", data);
    data += 2;
    sys_log(0, "BOOT: OBJ PROTO data str + 2: data = %s", data);

    size = decode_2bytes(data);
    sys_log(0, "BOOT: OBJ PROTO data to size: size = %d", size);

    data += 2;
    sys_log(0, "BOOT: OBJ PROTO data str + 2: data = %s", data);
    sys_log(0, "BOOT: OBJ PROTO data to size: size = %d", decode_2bytes(data));

    CManager::instance().LoadObjectProto((TObjectProto *) data, size);

    sys_log(0, "To data  assign %d * %d", size, sizeof(TObject));
    data += size * sizeof(TObjectProto);
    sys_log(0, "BOOT: OBJ PROTO  data str: data = %s",  data);

    [...]

decode_2bytes():

inline WORD decode_2bytes(const BYTE *a)
{
    return (*((WORD *) a));
}

sys_log :

Apr 12 09:05:54.205886 :: BOOT: OBJ PROTO data string: data = `
Apr 12 09:05:54.205914 :: BOOT: OBJ PROTO size of data: 96
Apr 12 09:05:54.205924 :: BOOT: OBJ PROTO data after decode_2bytes(data): data = `
Apr 12 09:05:54.205933 :: BOOT: OBJ PROTO data str + 2: data = <
Apr 12 09:05:54.205948 :: BOOT: OBJ PROTO data to size: size = 60
Apr 12 09:05:54.205961 :: BOOT: OBJ PROTO data str + 2: data = ½6
Apr 12 09:05:54.205970 :: BOOT: OBJ PROTO data to size: size = 14013
Apr 12 09:05:54.209238 :: To data  assign 60 * 40
Apr 12 09:05:54.209247 :: BOOT: OBJ PROTO  data str: data = (

As can see in sys_log, I show what contain data, the value, and the result that I obtain when add 2.

It looks like your are adding 2 to the pointer, and not the dereferenced value — Nattrass, Apr 14 '17 at 21:57
You're not appending anything, you're moving a pointer. In ASCII, the backtick is 96, and `<` is 60. Your buffer apparently starts with `{96, 0, 60, 0, 171, 54, ...`. (And you're trying to print binary data as a string.) I recommend [The Definitive C++ Book Guide and List](http://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list). — molbdnilo, Apr 14 '17 at 21:59
You seem to be forgetting that `data` is a pointer. You're adding two to a pointer. — David Schwartz, Apr 14 '17 at 22:04
@molbdnilo ok thanks very much, now I understand little more, but I don't understand, why, if I print **data** as string no prints all these characters, just shows me one. — Alejoo, Apr 14 '17 at 23:00

zett42 · Answer 1 · 2017-04-14T22:06:04.117

2

data += 2;

I would expect the result to be b, but is < with a value 60.

You just increment the pointer by 2, so it will point to the 3rd character of the string, which is <. It may even be some random value if the strings happens to end before that.

To increment the value that the pointer points to, you have to dereference the pointer:

*data += 2;

But this will trigger a compiler error, because data is declared as const char*.

If you want to modify the string, change the parameter to char* data.

edited Apr 14 '17 at 22:06

answered Apr 14 '17 at 22:03

zett42

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I thought that `*data += 2` would first increment the pointer and then dereference, like in `while (*data++)`? – Stephan Lechner Apr 14 '17 at 22:04
@Stephan It first dereferences the pointer and then increments the value pointed to. – zett42 Apr 14 '17 at 22:08
Yes, you are right. I just found out that `*data+=1` is something different than `*data++`... – Stephan Lechner Apr 14 '17 at 22:11
2

@StephanLechner precedence. `*` is lower than `++` but higher than `+=`. http://en.cppreference.com/w/cpp/language/operator_precedence – user4581301 Apr 14 '17 at 22:12
@zett42, Thanks,I did not understanded why happen that. but still I can't understand, why, if I print data as string no prints all these characters, just shows me one. – Alejoo Apr 14 '17 at 23:02

Int append to char, does not behave as I expect

1 Answers1