How do you correctly initialize a struct with a member that is a const pointer to a const value?

Question

Currently I'm doing something like this:

struct foo
{
    const int
        *const a,
        *const b, 
        *const c;

    foo(int a, int b, int c)
            : a(_a), b(_b), c(_c)
    {
        *_a = a;
        *_b = b;
        *_c = c;
    }

private:
    int _a[1], _b[1], _c[1];

};

but is there a way to do this without putting in the second set of pointers (_a, _b, _c)?

This sounds like an XY problem to me. What are you trying to accomplish? — Jerry Coffin, May 12 '15 at 04:51
Let me rephrase. You say: "...is there a way to do this without ..." The question is what "this" do you want to do? Right now, I can't make enough sense of your code to figure out what the code should do. — Jerry Coffin, May 12 '15 at 05:06
@JerryCoffin probably a bit confusing because of scope... basically the question is how to create a new pointer to an int without declaring `int *ptr` — Joseph Nields, May 12 '15 at 14:39

score 4 · Accepted Answer · edited May 23 '17 at 11:58

4

but is there a way to do this without putting in the second set of pointers (_a, _b, _c)?

Sure. You can use:

foo(int a, int b, int c)
        : a(new int(a)), b(new int(b)), c(new int(c)) {}

Keep in mind The Rule of Three and implement copy constructor, copy assignment operator and destructor appropriately for foo.

edited May 23 '17 at 11:58

Community

1
1

answered May 12 '15 at 04:48

R Sahu

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The destructor for this will likely be considerably more entertaining. – WhozCraig May 12 '15 at 04:51
@Praetorian, my understanding is that he wants to obviate the need for `_a`, `_b`, and `_c`. – R Sahu May 12 '15 at 04:53
@WhozCraig, yes, it will :) – R Sahu May 12 '15 at 04:53
@RSahu you're right. Solved my problem and helped me understand how the `new` operator works in C++. Thanks! – Joseph Nields May 12 '15 at 04:54
@JosephNields consider it an exercise off-site to figure out how to properly `delete` those. – WhozCraig May 12 '15 at 04:55
@RSahu I'll be damned, you managed to figure out the intent! I was thinking the OP was looking for something like [this](http://coliru.stacked-crooked.com/a/93a2e6dc93ff6a11) But the obvious solution in this case is for `a,b,c` to be `(const) int` instead of pointers. – Praetorian May 12 '15 at 04:56
@Praetorian, that might be the real solution after the OP tries to figure out how to delete the objects. – R Sahu May 12 '15 at 05:01
@WhozCraig what's wrong with just `~foo() {delete a, b, c;}` ? – Joseph Nields May 12 '15 at 05:02
@JosephNields, the compiler won't let you call `delete` on a pointer that points to a `const` object. – R Sahu May 12 '15 at 05:03
@RSahu why does that line compile then? – Joseph Nields May 12 '15 at 05:04
@JosephNields, I am surprised too. I thought that violated the `const`-ness of the object but maybe not. After all, destructors have to called even for `const` objects when they get out out scope. BTW, that should be `{delete a; delete b; delete c;}`. – R Sahu May 12 '15 at 05:06
3

Yes, that's exactly the reason `delete` on a `const` pointer works, see [this](https://stackoverflow.com/a/2271055/241631). @JosephNields `~foo() {delete a, b, c;}` is very wrong. You've only deleted `a`, and leaked `b` and `c`. Read up on the comma operator. – Praetorian May 12 '15 at 05:10
Thanks to all of you. All this time I never bothered trying to fire `delete` on a const pointer. (probably because I never had a need to, i guess. gotta learn something every day.=P) – WhozCraig May 12 '15 at 05:15
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Exception safety goes out of the window, of course. If a later `new` throws, there's literally no way to release the memory previously allocated. – T.C. May 12 '15 at 05:16
That was an entertaining read. I could have bet that you cannot do `delete const T* const` – vsoftco May 12 '15 at 05:18

How do you correctly initialize a struct with a member that is a const pointer to a const value?

1 Answers1