Databases
StackOverflow2013
Postsdbo
POWhy is this use of union, class and lambda producing a segmentation fault?
 

Note that there are some explanatory texts on larger screens.

plurals
  1. POWhy is this use of union, class and lambda producing a segmentation fault?
    primarykey
    data
    text
    <p>I got a very strange bug in my code and managed to reduce it to the following single function.</p> <pre><code>#include "either.hpp" #include &lt;string&gt; #include &lt;iostream&gt; #include &lt;vector&gt; #include &lt;functional&gt; int main() { using std::string; using std::vector; auto callback = []() { auto either = data::either&lt;string,vector&lt;int&gt;&gt;(string("test")); return either; return data::either&lt;string,vector&lt;int&gt;&gt;(string("test")); }; callback(); } </code></pre> <p>When run, this program generate a segmentation fault on <code>std::string</code>'s destructor. But if we remove the second <code>return</code> statement, it work perfectly.</p> <p>Now, the <code>data::either</code> class is using a union with two members, and the lambda's return statement will cause the destruction of the stack-allcated variable.</p> <p>Is there a problem with my use of those features, is it undefined behaviour for a reason I don't know, or is it simply a compiler bug?</p> <p>Here is the <code>either</code> class:</p> <pre><code>#ifndef EITHER_HPP #define EITHER_HPP #include &lt;cassert&gt; #include &lt;functional&gt; #include &lt;type_traits&gt; #include &lt;utility&gt; // Model the haskel Data.Either data type. // http://hackage.haskell.org/package/base-4.6.0.1/docs/Data-Either.html namespace data { template&lt;class Left, class Right&gt; class either { static_assert( !std::is_same&lt; typename std::remove_cv&lt;Left&gt;::type, typename std::remove_cv&lt;Right&gt;::type &gt;::value, "data::either&lt;A,B&gt;: type A and B must be different."); bool m_is_right; union { Left m_left; Right m_right; }; public: explicit either(Left l) : m_is_right(false) , m_left(std::move(l)) { } explicit either(Right r) : m_is_right(true) , m_right(std::move(r)) { } either(either const&amp; other) : m_is_right(other.is_right()) { if (other.is_left()) { m_left = other.left(); } else { m_right = other.right(); } } either(either&amp;&amp; other) : m_is_right(other.is_right()) { if (other.is_left()) { m_left = std::move(other.left()); } else { m_right = std::move(other.right()); } } ~either() { if (is_right()) { right().~Right(); } else { left().~Left(); } } either&amp; operator=(either const&amp; other) { m_is_right = other.is_right(); if (other.is_left()) { m_left = other.left(); } else { m_right = other.right(); } return *this; } either&amp; operator=(either&amp;&amp; other) { m_is_right = other.is_right(); if (other.is_left()) { m_left = std::move(other.left()); } else { m_right = std::move(other.right()); } return *this; } bool is_left() const { return !is_right(); } bool is_right() const { return m_is_right; } Left&amp; left() { assert(is_left()); return m_left; } Left const&amp; left() const { assert(is_left()); return m_left; } Right&amp; right() { assert(is_right()); return m_right; } Right const&amp; right() const { assert(is_right()); return m_right; } }; } #endif </code></pre> <p>I tested it with the following compilers:</p> <pre><code>$ clang++ -std=c++11 test.cpp &amp;&amp; ./a.out Segmentation fault $ clang++ --version Debian clang version 3.2-11 (tags/RELEASE_32/final) (based on LLVM 3.2) Target: x86_64-pc-linux-gnu Thread model: posix $ g++ -std=c++11 test.cpp &amp;&amp; ./a.out Segmentation fault $ g++ --version g++ (Debian 4.7.3-4) 4.7.3 Copyright (C) 2012 Free Software Foundation, Inc. This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. </code></pre>
    singulars
    1. PO
      singulars
      1. PTAnswer
    1. This table or related slice is empty.
    1. PTQuestion
    1. USauthchir
    1. USauthchir
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. PO
      singulars
      1. PTAnswer
    1. This table or related slice is empty.
    1. CO@jxh: as of the 2011 revision of C++, `union`s can store types with constructors. However, it is necessary to take care of the proper initialization, destruction, and assignment manually. In C++03 you can only have PODs in a `union`.
      singulars
      1. POWhy is this use of union, class and lambda producing a segmentation fault?
      1. USDietmar Kühl
    2. CO@DietmarKühl I think [class.dtor]/5 even says the dtor of the union should be deleted, as the dtor of `string` (and `vector`) is not trivial.
      singulars
      1. POWhy is this use of union, class and lambda producing a segmentation fault?
      1. USdyp
    3. CO@DyP: agreed: the destructor is deleted by default if there is a non-trivial member stored in a `union`. Of course, you can provide a user-define destructor doing the right thing (e.g., nothing because the enclosing class does the appropriate destruction using a suitable descriptor and the `union` is `private`).
      singulars
      1. POWhy is this use of union, class and lambda producing a segmentation fault?
      1. USDietmar Kühl
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload