How to use nullptr properly?

Question

currently I am reading "A tour of C++" by Byarne Stroustrup. The thing that matters: on "pointers, arrays and references" he gave an example about using nullptr like this:

int count_x(char* p, char x)
// count the number of occurrences of x in p[]
// p is assumed to point to a zero-terminated array of char (or to nothing)
{

    if (p == nullptr) return 0;
        int count = 0;

    for (; p != nullptr; ++p)
        if (*p == x)
            ++count;

    return count;
}

In my my main:

int main(){

    char* str = "Good morning!";
    char c = 'o';
    std::cout << count_x(str, c) << std::endl;

    return 0;
}

When I run the program it crashes I get an exception thrown at line

if (*p == x)

If I change the loop to be like this:

for (; *p; p++)
    if (*p == x)
        ++count;

Now everything works fine! I am using MSVC++ 14.0.

The same code I ran on ideone I don't get an exception but the result is always 0 which should be 3:

https://ideone.com/X9BeVx

@DeiDei: Yes I just copied and pasted the code. What I add is main code. — WonFeiHong, Dec 16 '17 at 12:05
@DeiDei: How should this be correct: `*p != nullptr`? as I guess `nullptr` is only for addresses pointers point to not a the value they sore in address they point to. — WonFeiHong, Dec 16 '17 at 12:07
I didn't find this in any of the errata's for "A tour of C++". You should write to Bjarne and let him know. This is a mistake. — StoryTeller - Unslander Monica, Dec 16 '17 at 12:07
The code from this [online version](https://isocpp.org/images/uploads/2-Tour-Basics.pdf#page=11) uses a correct check - where did you copy your code from? — hnefatl, Dec 16 '17 at 12:12
@hnefatl: Yes that is correct. But I don't know why my book is incorrect? It is in extension "epub". Thank you. — WonFeiHong, Dec 16 '17 at 12:22
BTW, `char* str = "Good morning!";` is not a valid C++ code. `str` should be of type `const char*`. MSVC++ is too permissive. — Ruslan, Dec 16 '17 at 15:31

score 15 · Accepted Answer · edited Dec 16 '17 at 17:28

15

p != nullptr and *p perform very different checks.

The former checks that the pointer itself contains a non-null address. While the latter checks that the address being pointed at contains something that is not 0. One is clearly appropriate in the loop, where the buffer's content is checked, while the other is not.

You segfault because you never stop reading the buffer (a valid pointer is unlikely to produce null when it is incremented). So you end up accessing way beyond your buffer limit.

edited Dec 16 '17 at 17:28

Remy Lebeau

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answered Dec 16 '17 at 12:06

StoryTeller - Unslander Monica

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1

But this is considered incorrect: `*p = nullptr` isn't it? – WonFeiHong Dec 16 '17 at 12:09
3

@WonFeiHong - Yes, that would be incorrect. `*p` doesn't have a pointer type, so it won't be possible to compare it with `nullptr`. You'll get a compiler error when you try – StoryTeller - Unslander Monica Dec 16 '17 at 12:10
1

Yeah if you want to make the comparison more explicit you can do *p != '\0' though just *p works fine too... – Sean Burton Dec 16 '17 at 12:25

Pouyan · Answer 2 · 2017-12-16T14:02:27.023

Remember that you are using C language feature.

Your problem is in your for loop. After the pointer reaches the last element of your character array it points to the end of the array not the nullptr.

Imagine you have a character array such as const char *a ="world" and ptr points to the

     +-----+     +---+---+---+---+---+---+
ptr :| *======>  | w | o | r | l | d |\0 |
     +-----+     +---+---+---+---+---+---+

the last element which ptr points to is '\0' and in your for you should change your code as below:

for (; *p != 0; p++) {
        if (*p == x)
            ++count;
    }

output : 3

@hnefatl- What are you expecting me to do ? – Pouyan Dec 16 '17 at 13:42 — Pouyan, Dec 16 '17 at 13:42

How to use nullptr properly?

2 Answers2