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190 lines
7.6 KiB
HTML
190 lines
7.6 KiB
HTML
4 years ago
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!-- This manual describes how to install and use the GNU multiple precision
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arithmetic library, version 6.1.0.
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Copyright 1991, 1993-2015 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document under
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the terms of the GNU Free Documentation License, Version 1.3 or any later
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version published by the Free Software Foundation; with no Invariant Sections,
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with the Front-Cover Texts being "A GNU Manual", and with the Back-Cover
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Texts being "You have freedom to copy and modify this GNU Manual, like GNU
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software". A copy of the license is included in
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GNU Free Documentation License. -->
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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>C++ Interface Limitations (GNU MP 6.1.0)</title>
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<meta name="description" content="How to install and use the GNU multiple precision arithmetic library, version 6.1.0.">
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<meta name="keywords" content="C++ Interface Limitations (GNU MP 6.1.0)">
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<meta name="resource-type" content="document">
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<meta name="distribution" content="global">
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<meta name="Generator" content="makeinfo">
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<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index">
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<link href="C_002b_002b-Class-Interface.html#C_002b_002b-Class-Interface" rel="up" title="C++ Class Interface">
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<link href="Custom-Allocation.html#Custom-Allocation" rel="next" title="Custom Allocation">
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<link href="C_002b_002b-Interface-Random-Numbers.html#C_002b_002b-Interface-Random-Numbers" rel="prev" title="C++ Interface Random Numbers">
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</style>
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</head>
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<body lang="en">
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<a name="C_002b_002b-Interface-Limitations"></a>
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<div class="header">
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<p>
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Previous: <a href="C_002b_002b-Interface-Random-Numbers.html#C_002b_002b-Interface-Random-Numbers" accesskey="p" rel="prev">C++ Interface Random Numbers</a>, Up: <a href="C_002b_002b-Class-Interface.html#C_002b_002b-Class-Interface" accesskey="u" rel="up">C++ Class Interface</a> [<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="C_002b_002b-Interface-Limitations-1"></a>
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<h3 class="section">12.6 C++ Interface Limitations</h3>
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<dl compact="compact">
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<dt><code>mpq_class</code> and Templated Reading</dt>
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<dd><p>A generic piece of template code probably won’t know that <code>mpq_class</code>
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requires a <code>canonicalize</code> call if inputs read with <code>operator>></code>
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might be non-canonical. This can lead to incorrect results.
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</p>
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<p><code>operator>></code> behaves as it does for reasons of efficiency. A
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canonicalize can be quite time consuming on large operands, and is best
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avoided if it’s not necessary.
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</p>
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<p>But this potential difficulty reduces the usefulness of <code>mpq_class</code>.
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Perhaps a mechanism to tell <code>operator>></code> what to do will be adopted in
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the future, maybe a preprocessor define, a global flag, or an <code>ios</code> flag
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pressed into service. Or maybe, at the risk of inconsistency, the
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<code>mpq_class</code> <code>operator>></code> could canonicalize and leave <code>mpq_t</code>
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<code>operator>></code> not doing so, for use on those occasions when that’s
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acceptable. Send feedback or alternate ideas to <a href="mailto:gmp-bugs@gmplib.org">gmp-bugs@gmplib.org</a>.
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</p>
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</dd>
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<dt>Subclassing</dt>
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<dd><p>Subclassing the GMP C++ classes works, but is not currently recommended.
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</p>
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<p>Expressions involving subclasses resolve correctly (or seem to), but in normal
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C++ fashion the subclass doesn’t inherit constructors and assignments.
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There’s many of those in the GMP classes, and a good way to reestablish them
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in a subclass is not yet provided.
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</p>
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</dd>
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<dt>Templated Expressions</dt>
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<dd><p>A subtle difficulty exists when using expressions together with
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application-defined template functions. Consider the following, with <code>T</code>
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intended to be some numeric type,
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</p>
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<div class="example">
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<pre class="example">template <class T>
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T fun (const T &, const T &);
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</pre></div>
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<p>When used with, say, plain <code>mpz_class</code> variables, it works fine: <code>T</code>
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is resolved as <code>mpz_class</code>.
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</p>
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<div class="example">
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<pre class="example">mpz_class f(1), g(2);
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fun (f, g); // Good
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</pre></div>
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<p>But when one of the arguments is an expression, it doesn’t work.
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</p>
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<div class="example">
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<pre class="example">mpz_class f(1), g(2), h(3);
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fun (f, g+h); // Bad
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</pre></div>
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<p>This is because <code>g+h</code> ends up being a certain expression template type
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internal to <code>gmpxx.h</code>, which the C++ template resolution rules are unable
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to automatically convert to <code>mpz_class</code>. The workaround is simply to add
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an explicit cast.
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</p>
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<div class="example">
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<pre class="example">mpz_class f(1), g(2), h(3);
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fun (f, mpz_class(g+h)); // Good
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</pre></div>
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<p>Similarly, within <code>fun</code> it may be necessary to cast an expression to type
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<code>T</code> when calling a templated <code>fun2</code>.
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</p>
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<div class="example">
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<pre class="example">template <class T>
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void fun (T f, T g)
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{
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fun2 (f, f+g); // Bad
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}
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template <class T>
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void fun (T f, T g)
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{
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fun2 (f, T(f+g)); // Good
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}
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</pre></div>
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</dd>
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<dt>C++11</dt>
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<dd><p>C++11 provides several new ways in which types can be inferred: <code>auto</code>,
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<code>decltype</code>, etc. While they can be very convenient, they don’t mix well
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with expression templates. In this example, the addition is performed twice,
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as if we had defined <code>sum</code> as a macro.
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</p>
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<div class="example">
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<pre class="example">mpz_class z = 33;
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auto sum = z + z;
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mpz_class prod = sum * sum;
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</pre></div>
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<p>This other example may crash, though some compilers might make it look like
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it is working, because the expression <code>z+z</code> goes out of scope before it
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is evaluated.
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</p>
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<div class="example">
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<pre class="example">mpz_class z = 33;
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auto sum = z + z + z;
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mpz_class prod = sum * 2;
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</pre></div>
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<p>It is thus strongly recommended to avoid <code>auto</code> anywhere a GMP C++
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expression may appear.
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</p></dd>
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</dl>
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<hr>
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<div class="header">
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<p>
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Previous: <a href="C_002b_002b-Interface-Random-Numbers.html#C_002b_002b-Interface-Random-Numbers" accesskey="p" rel="prev">C++ Interface Random Numbers</a>, Up: <a href="C_002b_002b-Class-Interface.html#C_002b_002b-Class-Interface" accesskey="u" rel="up">C++ Class Interface</a> [<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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</body>
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</html>
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