Opposite of const_cast

Question

I have an std::vector<int*> v and I'd like to prevent further writes to it. The C++ compiler does not accept this

const std::vector<const int*>& w = v;

but it accepts this

const std::vector<const int*>& w = reinterpret_cast<const std::vector<const int*>&>(v);

The latter does work with Microsoft's compiler, meaning that the int* inside v and const int* inside w have the same addresses in memory.

Does this work by chance, as an unspecified behavior, or is it valid C++ ? Does it work with all types inside the vector, like std::vector<std::shared_ptr<int>> ? If invalid, is there another way to do this cast ?

I know I could also copy the vector, but I'd like to avoid that, since my vectors are pretty big.

What about declaring the vector as const: `const std::vector v;`? — vahancho, Dec 22 '20 at 12:35
@vahancho The problem is the `const int*`, not the `const vector`. — V. Semeria, Dec 22 '20 at 12:36
So, you want to prevent rather the modification of **elements** than modification of vector itself? — vahancho, Dec 22 '20 at 12:38
https://stackoverflow.com/questions/19122858/why-is-a-vector-of-pointers-not-castable-to-a-const-vector-of-const-pointers — fas, Dec 22 '20 at 12:45
Also see https://stackoverflow.com/questions/63422076/why-is-type-punning-considered-ub — Den-Jason, Dec 22 '20 at 12:50
[`std::experimental::propage_const`](https://en.cppreference.com/w/cpp/experimental/propagate_const) might help. — Jarod42, Dec 22 '20 at 12:50
Sounds like you should encapsulate it and enforce the `const` semantics you want. — Sopel, Dec 22 '20 at 12:58

bolov · Accepted Answer · 2020-12-22T18:15:06.873

8

This is Undefined Behavior.

std::vector<const int*> and std::vector<int*> are two different classes (generated by the same template, but that is irrelevant). They cannot alias each other and you cannot reinterpret_cast between them.

My solution is to use std::span:

const std::span s{const_cast<const int* const*>(v.data()), v.size()};

This will create a const std::span<const int* const>.

edited Dec 22 '20 at 18:15

answered Dec 22 '20 at 12:49

bolov

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This is certainly an elegant solution. The `const_cast` makes it somewhat ugly though. It'd be better if we could get rid of that. – Aykhan Hagverdili Dec 22 '20 at 12:52
@AyxanHaqverdili yes, there is a way nicer solution: to modify the original variable type. Since that is not possible for OP I feel this is the next best thing. The `const_cast` is not a problem for me because it is used to add const, not remove it. – bolov Dec 22 '20 at 12:54
I cannot change the type of the variable, because the vector really is modified up to a certain point in the algorithm, and then it becomes read-only and used as input for the rest of the algorithm. I am in C++14 so I cannot use span, but exposing const getters to the beginning and end of the vector should be enough. Thanks. – V. Semeria Dec 22 '20 at 13:32
@bolov do note that adding const like that _is_ dangerous. See [Conversion from int** to const int**](https://stackoverflow.com/q/16390294/10147399). Or else the conversion would have been implicit. – Aykhan Hagverdili Dec 22 '20 at 14:25
@Artyer oh wow, I was so wrong. Thank you for pointing it out. Edited. – bolov Dec 22 '20 at 18:15

Opposite of const_cast

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