Manually calling destructor is not evaluated as referencing variable

Question

Once my class doesn't have destructor defined, following code gives warning C4189: 'f' : local variable is initialized but not referenced

(f->~Fred() isn't recognised as referencing f)

Is this a bug, or is this standard behaviour?

struct Fred
{
    int a, b;
    //~Fred(){}
};

int main()
{
    char memory[sizeof(Fred)];

    void * place = memory;

    Fred* f = new(place)Fred();

    f->~Fred();
}

This code is of course meaningless (it is minimal working sample), but I get this error in real scenario when writing pool allocator and using it like

template <typename T>
void CallDestructor(T * t)
{
    t->~T();
}

I use visual studio 2013 warning level 4 I tested it on some on-line compilers without warning, but I'm not sure what is their warning level

Does this warning get triggered in both Debug and Release builds? What if the destructor actually does something? — Banex, Aug 19 '15 at 09:43
@Banex If destructor does something, everything is fine. Once destructor (even empty one) is defined, everything is fine. — relaxxx, Aug 19 '15 at 09:46
@relaxxx Well then calling an empty destructor is a no-op and it is like not calling it altogether, hence the warning. It should trigger also when you define an empty one but it is probably a compiler quirk. — Banex, Aug 19 '15 at 09:50

DevSolar · Accepted Answer · 2015-08-19T09:57:39.033

Is this a bug, or is this standard behaviour?

This particular warning ("local variable is initialized but not referenced") is something not required by the standard. As far as the language is concerned, initializing a local variable and then not referencing it is perfectly legal.

It is a sign that your code might not do what you intended it to do, though, so the compiler tries to be helpful and warn you about a questionable construct. ("Have you forgotten something here?") This is completely within the domain of the compiler, so it cannot be "standard behaviour" even if they tried. ;-)

Yes, the compiler should realize that f->... does constitute a referencing of f. So the warning is a false posititve. (Probably because the whole thing is optimized away, being a no-op.) Such things happen if you are using high warning levels.

But it's a warning, not an error. You may safely ignore it, or mask it out with #pragma warning (since you are working with MSVC).

Manually calling destructor is not evaluated as referencing variable

1 Answers1