Re: Explicit destructor call - problem with basic types and types from other namespaces

From:
=?ISO-8859-1?Q?Daniel_Kr=FCgler?= <daniel.kruegler@googlemail.com>
Newsgroups:
comp.lang.c++.moderated
Date:
Wed, 9 May 2012 14:57:02 -0700 (PDT)
Message-ID:
<joeivv$1uq$1@dont-email.me>
Am 09.05.2012 21:26, schrieb PiotrN:

Two questions:
    1) Why I cannot call destructor for builtin types like int:
            int a;
            int* p = new (&a) int(7);
            p->~int();

I found in the NET that standard says: "The notation for explicit call
of a destructor can be used for any scalar type name. Allowing this
makes it possible to write code without having to know if a destructor
exists for a given type. "


Yes, this is roughly the rationale for it: To be able to write generic
code without much trickery to discriminate scalar types from class
types. The more precise rule is that the pseudo-destructor name is valid
for scalar types only, if these are provided as /type-name/ (like a
typedef) or a decltype-specifier (like: decltype(0)).

And typedef int is "scalar type name" but int is not - so it works the
following:
          typedef int INT;
          p->~INT();
Why standard does allow calling destructor on typedef (and template
typename) - but it has objections to plain type? Is there any
rationale for this?


Only a weak one: Once you are in non-template code, you have knowledge
of the specific type, so why bothering with writing a no-op? This *is*
really a weak argument, because you have the same problem once you would
try to write a generic macro. In other words: I wish, we would not have
this constraint to support the pseudo-destructor only for type-names and
decltype-specifiers when we have a non-class type.

   2) Why C++ (gcc-4.3.4) has objections to this syntax:

      p->std::~string();

of course this works well:

    using namespace std;
    p->~string();


You can write this as:

p->std::string::~string();

HTH & Greetings from Bremen,

Daniel Kr?gler

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