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diff --git a/share/doc/gcc/Template-Instantiation.html b/share/doc/gcc/Template-Instantiation.html new file mode 100644 index 0000000..da09d25 --- /dev/null +++ b/share/doc/gcc/Template-Instantiation.html @@ -0,0 +1,216 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> +<html> +<!-- This file documents the use of the GNU compilers. + +Copyright (C) 1988-2023 Free Software Foundation, Inc. + +Permission is granted to copy, distribute and/or modify this document +under the terms of the GNU Free Documentation License, Version 1.3 or +any later version published by the Free Software Foundation; with the +Invariant Sections being "Funding Free Software", the Front-Cover +Texts being (a) (see below), and with the Back-Cover Texts being (b) +(see below). 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Somehow the compiler and linker have to make sure that each +template instance occurs exactly once in the executable if it is needed, +and not at all otherwise. There are two basic approaches to this +problem, which are referred to as the Borland model and the Cfront model. +</p> +<dl compact="compact"> +<dt>Borland model</dt> +<dd><p>Borland C++ solved the template instantiation problem by adding the code +equivalent of common blocks to their linker; the compiler emits template +instances in each translation unit that uses them, and the linker +collapses them together. The advantage of this model is that the linker +only has to consider the object files themselves; there is no external +complexity to worry about. The disadvantage is that compilation time +is increased because the template code is being compiled repeatedly. +Code written for this model tends to include definitions of all +templates in the header file, since they must be seen to be +instantiated. +</p> +</dd> +<dt>Cfront model</dt> +<dd><p>The AT&T C++ translator, Cfront, solved the template instantiation +problem by creating the notion of a template repository, an +automatically maintained place where template instances are stored. A +more modern version of the repository works as follows: As individual +object files are built, the compiler places any template definitions and +instantiations encountered in the repository. At link time, the link +wrapper adds in the objects in the repository and compiles any needed +instances that were not previously emitted. The advantages of this +model are more optimal compilation speed and the ability to use the +system linker; to implement the Borland model a compiler vendor also +needs to replace the linker. The disadvantages are vastly increased +complexity, and thus potential for error; for some code this can be +just as transparent, but in practice it can been very difficult to build +multiple programs in one directory and one program in multiple +directories. Code written for this model tends to separate definitions +of non-inline member templates into a separate file, which should be +compiled separately. +</p></dd> +</dl> + +<p>G++ implements the Borland model on targets where the linker supports it, +including ELF targets (such as GNU/Linux), Mac OS X and Microsoft Windows. +Otherwise G++ implements neither automatic model. +</p> +<p>You have the following options for dealing with template instantiations: +</p> +<ol> +<li> Do nothing. Code written for the Borland model works fine, but +each translation unit contains instances of each of the templates it +uses. The duplicate instances will be discarded by the linker, but in +a large program, this can lead to an unacceptable amount of code +duplication in object files or shared libraries. + +<p>Duplicate instances of a template can be avoided by defining an explicit +instantiation in one object file, and preventing the compiler from doing +implicit instantiations in any other object files by using an explicit +instantiation declaration, using the <code>extern template</code> syntax: +</p> +<div class="smallexample"> +<pre class="smallexample">extern template int max (int, int); +</pre></div> + +<p>This syntax is defined in the C++ 2011 standard, but has been supported by +G++ and other compilers since well before 2011. +</p> +<p>Explicit instantiations can be used for the largest or most frequently +duplicated instances, without having to know exactly which other instances +are used in the rest of the program. You can scatter the explicit +instantiations throughout your program, perhaps putting them in the +translation units where the instances are used or the translation units +that define the templates themselves; you can put all of the explicit +instantiations you need into one big file; or you can create small files +like +</p> +<div class="smallexample"> +<pre class="smallexample">#include "Foo.h" +#include "Foo.cc" + +template class Foo<int>; +template ostream& operator << + (ostream&, const Foo<int>&); +</pre></div> + +<p>for each of the instances you need, and create a template instantiation +library from those. +</p> +<p>This is the simplest option, but also offers flexibility and +fine-grained control when necessary. It is also the most portable +alternative and programs using this approach will work with most modern +compilers. +</p> +</li><li> <a name="index-fno_002dimplicit_002dtemplates-1"></a> +Compile your code with <samp>-fno-implicit-templates</samp> to disable the +implicit generation of template instances, and explicitly instantiate +all the ones you use. This approach requires more knowledge of exactly +which instances you need than do the others, but it’s less +mysterious and allows greater control if you want to ensure that only +the intended instances are used. + +<p>If you are using Cfront-model code, you can probably get away with not +using <samp>-fno-implicit-templates</samp> when compiling files that don’t +‘<samp>#include</samp>’ the member template definitions. +</p> +<p>If you use one big file to do the instantiations, you may want to +compile it without <samp>-fno-implicit-templates</samp> so you get all of the +instances required by your explicit instantiations (but not by any +other files) without having to specify them as well. +</p> +<p>In addition to forward declaration of explicit instantiations +(with <code>extern</code>), G++ has extended the template instantiation +syntax to support instantiation of the compiler support data for a +template class (i.e. the vtable) without instantiating any of its +members (with <code>inline</code>), and instantiation of only the static data +members of a template class, without the support data or member +functions (with <code>static</code>): +</p> +<div class="smallexample"> +<pre class="smallexample">inline template class Foo<int>; +static template class Foo<int>; +</pre></div> +</li></ol> + +<hr> +<div class="header"> +<p> +Next: <a href="Bound-member-functions.html#Bound-member-functions" accesskey="n" rel="next">Bound member functions</a>, Previous: <a href="C_002b_002b-Interface.html#C_002b_002b-Interface" accesskey="p" rel="previous">C++ Interface</a>, Up: <a href="C_002b_002b-Extensions.html#C_002b_002b-Extensions" accesskey="u" rel="up">C++ Extensions</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Indices.html#Indices" title="Index" rel="index">Index</a>]</p> +</div> + + + +</body> +</html> |