<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <html> <head> <meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> <title>Design Overview</title> <link rel="stylesheet" href="../../../doc/src/boostbook.css" type="text/css"> <meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> <link rel="home" href="../index.html" title="The Boost C++ Libraries BoostBook Documentation Subset"> <link rel="up" href="../variant.html" title="Chapter 46. Boost.Variant"> <link rel="prev" href="../boost/visitor_ptr.html" title="Function template visitor_ptr"> <link rel="next" href="misc.html" title="Miscellaneous Notes"> </head> <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> <table cellpadding="2" width="100%"><tr> <td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../boost.png"></td> <td align="center"><a href="../../../index.html">Home</a></td> <td align="center"><a href="../../../libs/libraries.htm">Libraries</a></td> <td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> <td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> <td align="center"><a href="../../../more/index.htm">More</a></td> </tr></table> <hr> <div class="spirit-nav"> <a accesskey="p" href="../boost/visitor_ptr.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../variant.html"><img src="../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="misc.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> </div> <div class="section"> <div class="titlepage"><div><div><h2 class="title" style="clear: both"> <a name="variant.design"></a>Design Overview</h2></div></div></div> <div class="toc"><dl class="toc"><dt><span class="section"><a href="design.html#variant.design.never-empty">"Never-Empty" Guarantee</a></span></dt></dl></div> <div class="section"> <div class="titlepage"><div><div><h3 class="title"> <a name="variant.design.never-empty"></a>"Never-Empty" Guarantee</h3></div></div></div> <div class="toc"><dl class="toc"> <dt><span class="section"><a href="design.html#variant.design.never-empty.guarantee">The Guarantee</a></span></dt> <dt><span class="section"><a href="design.html#variant.design.never-empty.problem">The Implementation Problem</a></span></dt> <dt><span class="section"><a href="design.html#variant.design.never-empty.memcpy-solution">The "Ideal" Solution: False Hopes</a></span></dt> <dt><span class="section"><a href="design.html#variant.design.never-empty.double-storage-solution">An Initial Solution: Double Storage</a></span></dt> <dt><span class="section"><a href="design.html#variant.design.never-empty.heap-backup-solution">Current Approach: Temporary Heap Backup</a></span></dt> <dt><span class="section"><a href="design.html#variant.design.never-empty.optimizations">Enabling Optimizations</a></span></dt> <dt><span class="section"><a href="design.html#variant.design.never-empty.roadmap">Future Direction: Policy-based Implementation</a></span></dt> </dl></div> <div class="section"> <div class="titlepage"><div><div><h4 class="title"> <a name="variant.design.never-empty.guarantee"></a>The Guarantee</h4></div></div></div> <p>All instances <code class="computeroutput">v</code> of type <code class="computeroutput"><a class="link" href="../boost/variant.html" title="Class template variant">variant</a><T1,T2,...,TN></code> guarantee that <code class="computeroutput">v</code> has constructed content of one of the types <code class="computeroutput">T<span class="emphasis"><em>i</em></span></code>, even if an operation on <code class="computeroutput">v</code> has previously failed.</p> <p>This implies that <code class="computeroutput">variant</code> may be viewed precisely as a union of <span class="emphasis"><em>exactly</em></span> its bounded types. This "never-empty" property insulates the user from the possibility of undefined <code class="computeroutput">variant</code> content and the significant additional complexity-of-use attendant with such a possibility.</p> </div> <div class="section"> <div class="titlepage"><div><div><h4 class="title"> <a name="variant.design.never-empty.problem"></a>The Implementation Problem</h4></div></div></div> <p>While the <a class="link" href="design.html#variant.design.never-empty.guarantee" title="The Guarantee">never-empty guarantee</a> might at first seem "obvious," it is in fact not even straightforward how to implement it in general (i.e., without unreasonably restrictive additional requirements on <a class="link" href="reference.html#variant.concepts.bounded-type" title="BoundedType">bounded types</a>).</p> <p>The central difficulty emerges in the details of <code class="computeroutput">variant</code> assignment. Given two instances <code class="computeroutput">v1</code> and <code class="computeroutput">v2</code> of some concrete <code class="computeroutput">variant</code> type, there are two distinct, fundamental cases we must consider for the assignment <code class="computeroutput">v1 = v2</code>.</p> <p>First consider the case that <code class="computeroutput">v1</code> and <code class="computeroutput">v2</code> each contains a value of the same type. Call this type <code class="computeroutput">T</code>. In this situation, assignment is perfectly straightforward: use <code class="computeroutput">T::operator=</code>.</p> <p>However, we must also consider the case that <code class="computeroutput">v1</code> and <code class="computeroutput">v2</code> contain values <span class="emphasis"><em>of distinct types</em></span>. Call these types <code class="computeroutput">T</code> and <code class="computeroutput">U</code>. At this point, since <code class="computeroutput">variant</code> manages its content on the stack, the left-hand side of the assignment (i.e., <code class="computeroutput">v1</code>) must destroy its content so as to permit in-place copy-construction of the content of the right-hand side (i.e., <code class="computeroutput">v2</code>). In the end, whereas <code class="computeroutput">v1</code> began with content of type <code class="computeroutput">T</code>, it ends with content of type <code class="computeroutput">U</code>, namely a copy of the content of <code class="computeroutput">v2</code>.</p> <p>The crux of the problem, then, is this: in the event that copy-construction of the content of <code class="computeroutput">v2</code> fails, how can <code class="computeroutput">v1</code> maintain its "never-empty" guarantee? By the time copy-construction from <code class="computeroutput">v2</code> is attempted, <code class="computeroutput">v1</code> has already destroyed its content!</p> </div> <div class="section"> <div class="titlepage"><div><div><h4 class="title"> <a name="variant.design.never-empty.memcpy-solution"></a>The "Ideal" Solution: False Hopes</h4></div></div></div> <p>Upon learning of this dilemma, clever individuals may propose the following scheme hoping to solve the problem: </p> <div class="orderedlist"><ol class="orderedlist" type="1"> <li class="listitem">Provide some "backup" storage, appropriately aligned, capable of holding values of the contained type of the left-hand side.</li> <li class="listitem">Copy the memory (e.g., using <code class="computeroutput">memcpy</code>) of the storage of the left-hand side to the backup storage.</li> <li class="listitem">Attempt a copy of the right-hand side content to the (now-replicated) left-hand side storage.</li> <li class="listitem">In the event of an exception from the copy, restore the backup (i.e., copy the memory from the backup storage back into the left-hand side storage).</li> <li class="listitem">Otherwise, in the event of success, now copy the memory of the left-hand side storage to another "temporary" aligned storage.</li> <li class="listitem">Now restore the backup (i.e., again copying the memory) to the left-hand side storage; with the "old" content now restored, invoke the destructor of the contained type on the storage of the left-hand side.</li> <li class="listitem">Finally, copy the memory of the temporary storage to the (now-empty) storage of the left-hand side.</li> </ol></div> <p> </p> <p>While complicated, it appears such a scheme could provide the desired safety in a relatively efficient manner. In fact, several early iterations of the library implemented this very approach.</p> <p>Unfortunately, as Dave Abraham's first noted, the scheme results in undefined behavior: </p> <div class="blockquote"><blockquote class="blockquote"> <p>"That's a lot of code to read through, but if it's doing what I think it's doing, it's undefined behavior.</p> <p>"Is the trick to move the bits for an existing object into a buffer so we can tentatively construct a new object in that memory, and later move the old bits back temporarily to destroy the old object?</p> <p>"The standard does not give license to do that: only one object may have a given address at a time. See 3.8, and particularly paragraph 4."</p> </blockquote></div> <p> </p> <p>Additionally, as close examination quickly reveals, the scheme has the potential to create irreconcilable race-conditions in concurrent environments.</p> <p>Ultimately, even if the above scheme could be made to work on certain platforms with particular compilers, it is still necessary to find a portable solution.</p> </div> <div class="section"> <div class="titlepage"><div><div><h4 class="title"> <a name="variant.design.never-empty.double-storage-solution"></a>An Initial Solution: Double Storage</h4></div></div></div> <p>Upon learning of the infeasibility of the above scheme, Anthony Williams proposed in <a class="link" href="refs.html#variant.refs.wil02">[Wil02]</a> a scheme that served as the basis for a portable solution in some pre-release implementations of <code class="computeroutput">variant</code>.</p> <p>The essential idea to this scheme, which shall be referred to as the "double storage" scheme, is to provide enough space within a <code class="computeroutput">variant</code> to hold two separate values of any of the bounded types.</p> <p>With the secondary storage, a copy the right-hand side can be attempted without first destroying the content of the left-hand side; accordingly, the content of the left-hand side remains available in the event of an exception.</p> <p>Thus, with this scheme, the <code class="computeroutput">variant</code> implementation needs only to keep track of which storage contains the content -- and dispatch any visitation requests, queries, etc. accordingly.</p> <p>The most obvious flaw to this approach is the space overhead incurred. Though some optimizations could be applied in special cases to eliminate the need for double storage -- for certain bounded types or in some cases entirely (see <a class="xref" href="design.html#variant.design.never-empty.optimizations" title="Enabling Optimizations">the section called “Enabling Optimizations”</a> for more details) -- many users on the Boost mailing list strongly objected to the use of double storage. In particular, it was noted that the overhead of double storage would be at play at all times -- even if assignment to <code class="computeroutput">variant</code> never occurred. For this reason and others, a new approach was developed.</p> </div> <div class="section"> <div class="titlepage"><div><div><h4 class="title"> <a name="variant.design.never-empty.heap-backup-solution"></a>Current Approach: Temporary Heap Backup</h4></div></div></div> <p>Despite the many objections to the double storage solution, it was realized that no replacement would be without drawbacks. Thus, a compromise was desired.</p> <p>To this end, Dave Abrahams suggested to include the following in the behavior specification for <code class="computeroutput">variant</code> assignment: "<code class="computeroutput">variant</code> assignment from one type to another may incur dynamic allocation." That is, while <code class="computeroutput">variant</code> would continue to store its content <span class="emphasis"><em>in situ</em></span> after construction and after assignment involving identical contained types, <code class="computeroutput">variant</code> would store its content on the heap after assignment involving distinct contained types.</p> <p>The algorithm for assignment would proceed as follows: </p> <div class="orderedlist"><ol class="orderedlist" type="1"> <li class="listitem">Copy-construct the content of the right-hand side to the heap; call the pointer to this data <code class="computeroutput">p</code>.</li> <li class="listitem">Destroy the content of the left-hand side.</li> <li class="listitem">Copy <code class="computeroutput">p</code> to the left-hand side storage.</li> </ol></div> <p> Since all operations on pointers are nothrow, this scheme would allow <code class="computeroutput">variant</code> to meet its never-empty guarantee. </p> <p>The most obvious concern with this approach is that while it certainly eliminates the space overhead of double storage, it introduces the overhead of dynamic-allocation to <code class="computeroutput">variant</code> assignment -- not just in terms of the initial allocation but also as a result of the continued storage of the content on the heap. While the former problem is unavoidable, the latter problem may be avoided with the following "temporary heap backup" technique: </p> <div class="orderedlist"><ol class="orderedlist" type="1"> <li class="listitem">Copy-construct the content of the <span class="emphasis"><em>left</em></span>-hand side to the heap; call the pointer to this data <code class="computeroutput">backup</code>.</li> <li class="listitem">Destroy the content of the left-hand side.</li> <li class="listitem">Copy-construct the content of the right-hand side in the (now-empty) storage of the left-hand side.</li> <li class="listitem">In the event of failure, copy <code class="computeroutput">backup</code> to the left-hand side storage.</li> <li class="listitem">In the event of success, deallocate the data pointed to by <code class="computeroutput">backup</code>.</li> </ol></div> <p> </p> <p>With this technique: 1) only a single storage is used; 2) allocation is on the heap in the long-term only if the assignment fails; and 3) after any <span class="emphasis"><em>successful</em></span> assignment, storage within the <code class="computeroutput">variant</code> is guaranteed. For the purposes of the initial release of the library, these characteristics were deemed a satisfactory compromise solution.</p> <p>There remain notable shortcomings, however. In particular, there may be some users for which heap allocation must be avoided at all costs; for other users, any allocation may need to occur via a user-supplied allocator. These issues will be addressed in the future (see <a class="xref" href="design.html#variant.design.never-empty.roadmap" title="Future Direction: Policy-based Implementation">the section called “Future Direction: Policy-based Implementation”</a>). For now, though, the library treats storage of its content as an implementation detail. Nonetheless, as described in the next section, there <span class="emphasis"><em>are</em></span> certain things the user can do to ensure the greatest efficiency for <code class="computeroutput">variant</code> instances (see <a class="xref" href="design.html#variant.design.never-empty.optimizations" title="Enabling Optimizations">the section called “Enabling Optimizations”</a> for details).</p> </div> <div class="section"> <div class="titlepage"><div><div><h4 class="title"> <a name="variant.design.never-empty.optimizations"></a>Enabling Optimizations</h4></div></div></div> <p>As described in <a class="xref" href="design.html#variant.design.never-empty.problem" title="The Implementation Problem">the section called “The Implementation Problem”</a>, the central difficulty in implementing the never-empty guarantee is the possibility of failed copy-construction during <code class="computeroutput">variant</code> assignment. Yet types with nothrow copy constructors clearly never face this possibility. Similarly, if one of the bounded types of the <code class="computeroutput">variant</code> is nothrow default-constructible, then such a type could be used as a safe "fallback" type in the event of failed copy construction.</p> <p>Accordingly, <code class="computeroutput">variant</code> is designed to enable the following optimizations once the following criteria on its bounded types are met: </p> <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> <li class="listitem">For each bounded type <code class="computeroutput">T</code> that is nothrow copy-constructible (as indicated by <code class="computeroutput">boost::has_nothrow_copy</code>), the library guarantees <code class="computeroutput">variant</code> will use only single storage and in-place construction for <code class="computeroutput">T</code>.</li> <li class="listitem">If <span class="emphasis"><em>any</em></span> bounded type is nothrow default-constructible (as indicated by <code class="computeroutput">boost::has_nothrow_constructor</code>), the library guarantees <code class="computeroutput">variant</code> will use only single storage and in-place construction for <span class="emphasis"><em>every</em></span> bounded type in the <code class="computeroutput">variant</code>. Note, however, that in the event of assignment failure, an unspecified nothrow default-constructible bounded type will be default-constructed in the left-hand side operand so as to preserve the never-empty guarantee.</li> </ul></div> <p> </p> <p><span class="bold"><strong>Implementation Note</strong></span>: So as to make the behavior of <code class="computeroutput">variant</code> more predictable in the aftermath of an exception, the current implementation prefers to default-construct <code class="computeroutput">boost::blank</code> if specified as a bounded type instead of other nothrow default-constructible bounded types. (If this is deemed to be a useful feature, it will become part of the specification for <code class="computeroutput">variant</code>; otherwise, it may be obsoleted. Please provide feedback to the Boost mailing list.)</p> </div> <div class="section"> <div class="titlepage"><div><div><h4 class="title"> <a name="variant.design.never-empty.roadmap"></a>Future Direction: Policy-based Implementation</h4></div></div></div> <p>As the previous sections have demonstrated, much effort has been expended in an attempt to provide a balance between performance, data size, and heap usage. Further, significant optimizations may be enabled in <code class="computeroutput">variant</code> on the basis of certain traits of its bounded types.</p> <p>However, there will be some users for whom the chosen compromise is unsatisfactory (e.g.: heap allocation must be avoided at all costs; if heap allocation is used, custom allocators must be used; etc.). For this reason, a future version of the library will support a policy-based implementation of <code class="computeroutput">variant</code>. While this will not eliminate the problems described in the previous sections, it will allow the decisions regarding tradeoffs to be decided by the user rather than the library designers.</p> </div> </div> </div> <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> <td align="left"></td> <td align="right"><div class="copyright-footer">Copyright © 2002, 2003 Eric Friedman, Itay Maman<br>Copyright © 2014-2021 Antony Polukhin<p>Distributed under the Boost Software License, Version 1.0. (See accompanying file <code class="filename">LICENSE_1_0.txt</code> or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>) </p> </div></td> </tr></table> <hr> <div class="spirit-nav"> <a accesskey="p" href="../boost/visitor_ptr.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../variant.html"><img src="../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="misc.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> </div> </body> </html>