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From: Christian Gollwitzer <auriocus@gmx.de>
Newsgroups: comp.lang.python,comp.lang.c++
Subject: Re: Question about math.pi is mutable
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Date: Sun, 8 Nov 2015 09:00:27 +0100
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Xref: csiph.com comp.lang.python:98426 comp.lang.c++:38661

Am 08.11.15 um 08:45 schrieb Marko Rauhamaa:
> Grant Edwards <invalid@invalid.invalid>:
>
>> On 2015-11-07, Marko Rauhamaa <marko@pacujo.net> wrote:
>>> "const" is a very ineffective tool that clutters the code and forces
>>> you to sprinkle type casts around your code.
>>
>> But it allows the compiler to warn you if you pass a pointer to a
>> read-only data to a function that expects a pointer to writable data.
>
> Unfortunately, it doesn't:
>
> ========================================================================
> #include <stdio.h>
> #include <string.h>
>
> int main()
> {
>      const char name[] = "Tom";
>      char *p = strstr(name, "Tom");
>      strcpy(p, "Bob");
>      printf("name = %s\n", name);
>      return 0;
> }
> ========================================================================
>
>      $ cc -o prog prog.c
>      $ ./prog
>      Bob
>
> No warning.
>

That is strange. In C, I can see thet problem here, because it is 
impossible to define a const correct strstr. But in C++ I have expected 
that according to the overload, the const version of strstr would be 
selected (http://www.cplusplus.com/reference/cstring/strstr/ ) . Still:

apfelkiste:Tests chris$ cat prog.cpp
#include <cstdio>
#include <cstring>

int main()
{
     const char name[] = "Tom";
     char *p = strstr(name, "Tom"); // this line should be an error
     strcpy(p, "Bob");
     printf("name = %s\n", name);
     return 0;
}

apfelkiste:Tests chris$ g++ -Wall prog.cpp
apfelkiste:Tests chris$ ./a.out
Bus error: 10

It segfaults because on OSX, const can be stored in write-only memory.

apfelkiste:Tests chris$ g++ --version
Configured with: --prefix=/Applications/Xcode.app/Contents/Developer/usr 
--with-gxx-include-dir=/usr/include/c++/4.2.1
Apple LLVM version 5.1 (clang-503.0.40) (based on LLVM 3.4svn)
Target: x86_64-apple-darwin12.6.0
Thread model: posix


	Christian