How to change value in array with pointer?

Question

I have simply program to write. I try to make it with pointers. It is program which is changing uppercase to lowercase without index from function argument. Problem is with changing value, my program is crashing there...

#include <iostream>
#include <cstring>

using namespace std;

char* male(char* nap, int n) {
    for (int i = 0; i < 9; ++i) {
        if (i != n && ((*nap >= 'A') && (*nap <= 'Z'))) {
            *nap = (char)(*nap+32);
        }
        nap++;
    }
    return nap;
}

int main() {

    char * nap = "aBCDEFGHI";
    male(nap, 2);

    return 0;
}

Could you tell me why *nap = (char)(*nap+32); is not a good way?

@lightness Poor choice of words. Its not ill-formed nor undefined behaviour because I have yet too see it result in anything but a crash. That's kind of the expected behaviour — , Apr 25 '16 at 19:27
@Dr.Jones: It's undefined behaviour. That you personally have _just so happened_ to see it result in a crash every time you personally have been observing, is irrelevant. — Lightness Races in Orbit, Apr 25 '16 at 19:31

score 2 · Accepted Answer · answered Apr 25 '16 at 19:15

2

This is C++ code so following:

char * nap = "aBCDEFGHI";

should be

const char * nap = "aBCDEFGHI";

because string literals are const in C++. So your code :

*nap = (char)(*nap+32);

is also Undefined Behaviour, which can result in crash.

minimal change to remove UB is to define nap as array:

char nap[] = "aBCDEFGHI";

answered Apr 25 '16 at 19:15

marcinj

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It can? It will! I have yet to see that code work anywhere – Apr 25 '16 at 19:20
Thanks, I change my string. – K.Dot Apr 25 '16 at 19:21
@Dr.Jones: "Can" is correct. Never rely on any effects of any UB. – Lightness Races in Orbit Apr 25 '16 at 19:22
@Dr.Jones see here: http://coliru.stacked-crooked.com/a/0442ccec9da7a7d1, no segmentation fault, it will crash when string is sent to cout. – marcinj Apr 25 '16 at 19:28

score 2 · Answer 2 · edited May 23 '17 at 12:16

Your compiler is supposed to complain of this line...

char * nap = "aBCDEFGHI";

Your major problem is that you are trying to modify a string literal at run-time. Its Undefined Behavior to do so. See Why is this string reversal C code causing a segmentation fault?

This answer explains your workaround.

Could you tell me why *nap = (char)(*nap+32); is not a good way?

Its not all that bad per se. But you can get into trouble with it... That line is effectively...

*nap = *reinterpret_cast<char*>(const_cast<char*>(nap+32));

C-style cast is a ruthless cast. as you can see, you are also casting away all const and/or volatile qualifications... There are some code bases that will hurt, especially if the object was stored in a read only marked memory... See When should static_cast, dynamic_cast, const_cast and reinterpret_cast be used?

How to change value in array with pointer?

2 Answers2