Casting vector to const vector

Question

How can I cast vector<int> to const vector<const int>?

I've tried static_cast<const vector<const int>>(array) and using the value without a cast. Neither compiled.

You cannot. `vector` and `vector` are completely different types. Pass a `span` or `array_view` instead. — nwp, Jul 08 '16 at 22:16
@nwp then how would I implement my cache? Would plain old `const vector` work? — noɥʇʎԀʎzɐɹƆ, Jul 08 '16 at 22:18
To provide `const ` access to a `std::vector` use a `std::vector::const_iterator` — πάντα ῥεῖ, Jul 08 '16 at 22:18
@Great.And.Powerful.Oz Doesn't work unfortunately `static_cast from 'vector *' to 'vector *', which are not related by inheritance, is not allowed` — noɥʇʎԀʎzɐɹƆ, Jul 08 '16 at 22:21
@Great.And.Powerful.Oz Nope. `std::vector` is completely unrelated to `std::vector` — Kevin, Jul 08 '16 at 22:22
Check this post: http://stackoverflow.com/questions/2868485/cast-vectort-to-vectorconst-t -- — chema989, Jul 08 '16 at 22:22
`std::vector` isn't valid anyway since elements must be assignable. — chris, Jul 08 '16 at 22:26
@chema989 In fact this looks like a dupe of the question you linked to. — Sebastian Redl, Jul 08 '16 at 22:27
@uoɥʇʎPʎzɐɹC There's no need to ever make a `vector`, and in fact it's usually not even valid. — Sebastian Redl, Jul 08 '16 at 22:28
I realized there is an inherent problem with my caching implementation. I'll edit that part out and leave this for others to see. — noɥʇʎԀʎzɐɹƆ, Jul 08 '16 at 22:30
@Great.And.Powerful.Oz Of course that won't work. The types are unrelated. And don't think of using `reinterpret_cast`! That's UB waiting to happen (or if you're _lucky_, implementation-defined behaviour, which is no more useful IMHO). Even if you're tempted to say 'well, they'll really have the same member layout, so it's fine' - it's really not. — underscore_d, Jul 08 '16 at 22:58
@underscore_d Sorry, from a python background it looks like "Why wouldn't it work?" — noɥʇʎԀʎzɐɹƆ, Jul 08 '16 at 23:10
@SebastianRedl Even if it seems to work, the Standard doesn't seem to guarantee it's valid: [Does C++11 allow vector?](http://stackoverflow.com/questions/6954906/does-c11-allow-vectorconst-t), to which the answer from a very reliable source appears to be a distinct _no_. The real fleshing-out is in the comments, btw. — underscore_d, Jul 08 '16 at 23:14

score 3 · Accepted Answer · edited May 23 '17 at 10:30

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You cannot cast a std::vector<int> to const std::vector<const int>.

Besides, it does not make sense to use a std::vector<const int> at all. It doesn't give you any more safety than a const std::vector<int>.

Not only that, C++ does not allow construction of std::vector<const T>. See Does C++11 allow vector<const T>? for more info.

edited May 23 '17 at 10:30

Community

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answered Jul 08 '16 at 22:39

R Sahu

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...can a `vector` even _have_ `const`-qualified element type? It's never worked for me when I tried in the dim and distant past, and it doesn't work for me now with `int const`. – underscore_d Jul 08 '16 at 23:01
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@underscore_d, g++ does not allow creating a variable with `std::vector`. I don't know whether there is anything in the standard that prohibits such a variable. – R Sahu Jul 08 '16 at 23:06
I thought maybe it was to do with non-availability of copy/move ctors in the element types for which I tried it before, meaning reallocations and sorts and etc would need to use `operator=`, which obviously isn't `const`... but `int` has those, which torpedoes my theory. Sigh - off to the Standard I go. – underscore_d Jul 08 '16 at 23:08
@underscore_d a `const int` doesn't have `operator=` available to it. – R Sahu Jul 08 '16 at 23:10
To clarify, I was referring to it having the ctors, not assignment. – underscore_d Jul 08 '16 at 23:11
[Does C++11 allow vector?](http://stackoverflow.com/questions/6954906/does-c11-allow-vectorconst-t), to which the answer from a very reliable source appears to be a distinct _no_. The real fleshing-out is in the comments, btw. – underscore_d Jul 08 '16 at 23:14

score 1 · Answer 2 · answered Jul 08 '16 at 22:22

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You cannot change the type of the elements of the vector, but you can change the type of the vector to const std::vector<int> which will prevent making changes to the vector.

#include <vector>

int main()
{
    std::vector<int> v1;
    v.push_back(1);
    v.push_back(2);
    v.push_back(3);

    const std::vector<int>& v2 = v1;
    v2.push_back(4); // Causes an error
    v2[0] = 4; // Causes an error
    return 0;
}

answered Jul 08 '16 at 22:22

Alden

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Although it doesn't solve my caching problem, I'll leave it here because it might help others – noɥʇʎԀʎzɐɹƆ Jul 08 '16 at 22:24
This makes the _container_ const, but I'm guessing that by trying to use the disallowed concept of 'a container with `const` _elements_', OP hoped to be able to add to/reallocate the container (non-`const`) while not being able to alter elements after construction (`const`). One might assume this were possible post-C++11 as the compiler could use copy and move constructors for elements instead of assignment - but in reality, containers were never intended to have `const` elements - and currently, an incompatible default allocator prevents that anyway: http://stackoverflow.com/questions/6954906 – underscore_d Jul 08 '16 at 23:32

Casting vector to const vector

2 Answers2