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Groups > comp.lang.python > #98365
| From | Christian Gollwitzer <auriocus@gmx.de> |
|---|---|
| Newsgroups | comp.lang.python |
| Subject | Re: Question about math.pi is mutable |
| Date | 2015-11-06 20:59 +0100 |
| Organization | A noiseless patient Spider |
| Message-ID | <n1j0nt$odf$1@dont-email.me> (permalink) |
| References | <mailman.77.1446804678.16136.python-list@python.org> <n1i6d4$bfg$1@dont-email.me> <mailman.90.1446826322.16136.python-list@python.org> <n1iv1c$hst$1@dont-email.me> <mailman.93.1446838846.16136.python-list@python.org> |
Am 06.11.15 um 20:40 schrieb Chris Angelico:
> On Sat, Nov 7, 2015 at 6:30 AM, Bartc <bc@freeuk.com> wrote:
>> Is there no way then in Python to declare:
>>
>> pi = 3.141519 # etc
>>
>> and make it impossible to override?
>
> Nope. Even in C++, where classes can define certain things as const,
> private, and other such restrictions, you can always get around them
> by manipulating pointers appropriately.
No, that is not right. If you cast away the constness from a pointer to
a constant, you can technically write to that memory, but the behaviour
is undefined - another way of saying that the program is illegal.
In many cases, the compiler will inline the constant - because you
promised, that it can't change - and the update to the constant will not
change in all parts of the program:
apfelkiste:Tests chris$ cat constub.cpp
#include <iostream>
void call_by_ref(const double &v) {
std::cout<<"In function: v="<<v<<std::endl;
}
int main() {
const double pi=3.14;
double *errptr = const_cast<double*>(&pi);
*errptr= 0.0;
std::cout<<"Now pi is "<<pi<<std::endl;
call_by_ref(pi);
return 0;
}
apfelkiste:Tests chris$ g++ constub.cpp
apfelkiste:Tests chris$ ./a.out
Now pi is 3.14
In function: v=0
Christian
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Re: Question about math.pi is mutable Christian Gollwitzer <auriocus@gmx.de> - 2015-11-06 20:59 +0100
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