Mutable/immutable objects in Python and C/C++

Question

You can't modify immutable Python objects, you may simply create new objects with new values:

n = 1
id(n)
output: 123
n = n + 1
id(n)
output: 140

You can modify mutable objects though:

m = [1]
id(m)
output: 210
m.append(2)
id(m)
output: 210

I'm wandering, what is the closest concept to Python's mutable/immutable objects in C/C++?

Ryan Haining Ryan Haining · Accepted Answer · 2014-12-11T20:21:36

Simple: const objects

More complex: since python follows reference semantics everything can be roughly thought of as a generic pointer. A pointer that can be reassigned, but what it points to can't be changed is a pointer to const, which is I think what you want.

int i;
int j;
const int *p =&i;
p = &j;
*p = 1; // error

However, since python dynamically allocates its objects, and C doesn't not unless told to do so, can't do something like the following and get the same behavior as python

*p = *p + 1;

So it's still lacking, but it's the closest I think you can get without making something really contrived.

Mutable/immutable objects in Python and C/C++

2 Answers