Re: Using the STL for scientific programming

From:

=?ISO-8859-1?Q?Daniel_Kr=FCgler?= <daniel.kruegler@googlemail.com>

Newsgroups:

comp.lang.c++.moderated

Date:

Wed, 5 Oct 2011 14:38:34 -0700 (PDT)

Message-ID:

<j6iekv$m03$1@dont-email.me>

Am 05.10.2011 21:51, schrieb piotrN:

It is simple not true. You can instantiate std::vector with incomplete
type.

Watch out: Dave Harris' position is correct. It is UB, if you do that.

http://ideone.com/MEZKC

   #include<vector>
   class MyObject;

   struct MyData {
       std::vector<MyObject> myObjectVector;
   };

   class MyObject {};

   int main() { MyData myData; return 0; }

Of course if you instantiate an object - its type must be complete to
do the destruction for instance.

This code is *not* portable and still invokes undefined behaviour by the C++ Standard. There is no guarantee that will be well-formed, even though it often is for existing implementations. Just as a sketch, there is nothing that prevents a library implementation to define std::vector as follows (ignore the missing allocator parameter for the moment):

template<class T>
class vector {
static const int __size_of_value = sizeof(T);
typedef bool __check_incomplete_type[__size_of_value <= 0 ? -1 : 1];
//...
};

Try this

http://ideone.com/CT4X9

or with other compilers and you will typically notice a compiler error.

Btw.: Another myth is that a similar program would be well-defined with std::auto_ptr as a replacement for std::vector. Instantiating std::auto_ptr with an incomplete type is also not been granted by either C++03 nor C++11 (Use std::unique_ptr or std::shared_ptr, both provide this guarantee).

HTH & Greetings from Bremen,

Daniel Kr?gler

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