Cast constexpr int[] to void*

Question

I have a static array of integers that I never want to change. I have a C-style function that wants to have this array as a void-pointer parameter. I am trying out different combinations of const_cast and reinterpret_cast, but I'm coming to the point where I have no clue of what I'm exactly doing and it keeps giving me errors.

class Foo
{
    static constexpr int bar[3] = {1,2,3};

    void method()
    {
        cfunction(reinterpret_cast<void*>(const_cast<int*>(&bar)));
    }
};

invalid const_cast from type 'const int ()[3]' to type 'int'

I see that it fails because the types don't match. I also tried const_cast<int[]>(bar), but const_cast wants to have a pointer or reference type.

Where can I read up on this subject? It's hard for me to understand what is going on here.

Honestly, dont do that. What if your function tries to change the array? If you cant change the function, dont make the array const — Narase, Jun 18 '19 at 08:25
@Yastanub No, the first one is the pointer, the second one the adress of the pointer — Narase, Jun 18 '19 at 08:27
@MichaelChourdakis no it is not `&functionname` is equivalent to `functionname` the `&` is optional — Yastanub, Jun 18 '19 at 08:41
@MichaelChourdakis oh i somehow misread the question i dont knwo why i thought it was about function pointers sry :) — Yastanub, Jun 18 '19 at 08:44

Michael Chourdakis · Accepted Answer · 2019-06-18T08:41:47.553

4

cfunction((void*)bar);

P.S. I 've seen lots of programmers struggling to use all these casts when, in reality, they only need the simple C cast. If you insist on the C++ cast style, then

cfunction(reinterpret_cast<void*>(const_cast<int*>(bar)));

(Remove the & from bar).

edited Jun 18 '19 at 08:41

answered Jun 18 '19 at 08:29

Michael Chourdakis

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should be `void*` but i think it doesnt matter – 463035818_is_not_an_ai Jun 18 '19 at 08:30
This works, but there one caveat. I have enabled -Werrors, so I will get a non-solvable warning here. Are there any alternatives? – Marnix Jun 18 '19 at 08:33
What is the warning? – Michael Chourdakis Jun 18 '19 at 08:33
2

`warning: use of old-style cast [-Wold-style-cast]` and `warning: cast from type 'const int*' to type 'void*' casts away qualifiers [-Wcast-qual]` – Marnix Jun 18 '19 at 08:38
The warning is correct. Either you disable it at the specific point (VS has a #pragma warning to do that), or you fix the function to want a const int*, not a void. – Michael Chourdakis Jun 18 '19 at 08:40
1

I 've added a C++ style one. – Michael Chourdakis Jun 18 '19 at 08:42
Okay, so I was actually pretty close. Do you have some references for me so I can learn more about this kind of stuff? – Marnix Jun 18 '19 at 08:52
It depends on how do you want to immerse into C++. [book list](https://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list) – Michael Chourdakis Jun 18 '19 at 08:59
Do you mean by *"in reality, they only need the simple C cast"* that I should always just use the *"simple C cast"*? – nada Jun 18 '19 at 10:42

score 3 · Answer 2 · answered Jun 18 '19 at 08:41

As the compiler says, &bar is a const int (*)[3] - a pointer to an array - and you can't const_cast that to an int*.

You want a pointer to the array's first element, not to the array.
That is,

const_cast<int*>(&bar[0])

or, equivalently,

const_cast<int*>(bar)

Of course, this will only be valid if the C function doesn't ever modify the array.
If there is any risk of that, you should not make it const.

score 2 · Answer 3 · answered Jun 18 '19 at 08:33

If the C function was promising to not modify the data, then it would be taking a const void*. Since it doesn't, it might modify it. So don't make your array const:

class Foo
{
    static int bar[3];

    void method()
    {
        cfunction(bar);
    }
};

And define the array in the .cpp file of your class:

int Foo::bar[3] = {1, 2, 3};

Cast constexpr int[] to void*

3 Answers3