Code review: const char * modification

Question

I just wrote following code and it's working with g++. It shouldn't work. Why is it working?

#include <iostream>
using namespace std;

int main()
{
        //char const *cstr;
        const char *cstr;
        char *cptr;
        std::cin>>cptr;
        cstr = cptr;
        cstr = "CPP";
        cout<<cstr;
        return 0;
}

as it is apparent that cstr is const so it's modification in line cstr = "CPP"; should not work, but it is working. Why?

When you say "it's working with `g++`", do you mean it runs correctly, or just that it compiles. I would expect to crash with a core dump. — Martin Bonner supports Monica, Feb 06 '17 at 12:50
So it does! http://cpp.sh/7jcmn. I do get a nice warning though. You need to get rid of that error. — Martin Bonner supports Monica, Feb 06 '17 at 12:58
"Why is it working?" -- that's the wonderful thing about undefined behavior: it may do exactly what you expect it to do, purely by accident. It is **not** working. — Pete Becker, Feb 06 '17 at 15:04

score 4 · Accepted Answer · answered Feb 06 '17 at 12:42

4

cstr is a non-constant pointer to a constant. It can be assigned to and reassigned.

A constant pointer to a constant would be char const* const cstr.

answered Feb 06 '17 at 12:42

Maxim Egorushkin

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For completeness, a constant pointer to non-const would be `char * const cstr`; – Martin Bonner supports Monica Feb 06 '17 at 12:49
**I messed up**, I am always messing up definitions when it comes to `const` and `pointers`. – StewieGGriffin Feb 06 '17 at 12:53
@MartinBonner Maxim Egorushkin, Thank you for pointing it out. – StewieGGriffin Feb 06 '17 at 12:54
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If you have difficulty with `const` and pointers, remember "`const` goes to the *right* of what it is making constant". (There is a special hack that allows you to put it to the left if there is nothing to the right.) – Martin Bonner supports Monica Feb 06 '17 at 13:00
@MartinBonner I blame books that teach people to put `const` on the left. The right place is on the right. – Maxim Egorushkin Feb 06 '17 at 13:02
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@MaximEgorushkin it is not just books that use const on the left. The c++ standard itself also uses the syntax and it is common in many code bases. The choice is a matter of taste. Significant "blame" is on Dennis Ritchie who chose to not disallow const on left when he designed C. – eerorika Feb 06 '17 at 13:28

score 0 · Answer 2 · edited May 23 '17 at 11:45

0

This code causes undefined behaviour (if there is any input on cin):

std::cin>>cptr;

The meaning of that code is to read characters from cin and write them into the space pointed to by cptr. But you did not initialize cptr.

When a program has undefined behaviour, anything can happen, including "working as expected".

edited May 23 '17 at 11:45

Community

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answered Feb 06 '17 at 13:00

M.M

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I don't think this is an answer to the question the OP was asking (which was about a confusion between `char const * p` and `char * const p`). I agree that the code has UB btw. – Martin Bonner supports Monica Feb 06 '17 at 13:02

score 0 · Answer 3 · answered Feb 06 '17 at 13:09

0

const char *cstr;
as it is apparent that cstr is const

No, it isn't.

It is non-const, but the thing that it points to is const.

Furthermore, you're attempting to read into a char buffer that does not exist.

Stop using C-strings, and get yourself some shiny std::strings instead.

answered Feb 06 '17 at 13:09

Lightness Races in Orbit

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I am more comfortable with `std::string` but in interviews I mess up with C-Strings. – StewieGGriffin Feb 06 '17 at 13:17

score 0 · Answer 4 · answered Feb 06 '17 at 13:29

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To anyone who gets confused like I do when it comes to pointers with const declarations, use http://cdecl.org/.

answered Feb 06 '17 at 13:29

StewieGGriffin

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Code review: const char * modification

4 Answers4