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Add a note to the C++ compatibility page about templates with no

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valid instantiations.

llvm-svn: 100836
This commit is contained in:
John McCall 2010-04-09 01:07:07 +00:00
parent c68e140657
commit 81a4a72e84
1 changed files with 48 additions and 0 deletions
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48
clang/www/cxx_compatibility.html
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@ -25,6 +25,7 @@
<li><a href="#init_static_const">Initialization of non-integral static const data members within a class definition</a></li>
<li><a href="#dep_lookup">Unqualified lookup in templates</a></li>
<li><a href="#dep_lookup_bases">Unqualified lookup into dependent bases of class templates</a></li>
<li><a href="#bad_templates">Templates with no valid instantiations</a></li>
<li><a href="#default_init_const">Default initialization of const variable of a class type requires user-defined default constructor</a></li>
</ul>
@ -202,6 +203,53 @@ This works whether the methods are static or not, but be careful:
if <tt>DoThis</tt> is virtual, calling it this way will bypass virtual
dispatch!
<!-- ======================================================================= -->
<h2 id="bad_templates">Templates with no valid instantiations</h2>
<!-- ======================================================================= -->
The following code contains a typo: the programmer
meant <tt>init()</tt> but wrote <tt>innit()</tt> instead.
<pre>
template &lt;class T&gt; class Processor {
...
void init();
...
};
...
template &lt;class T&gt; void process() {
Processor&lt;T&gt; processor;
processor.innit(); // <-- should be 'init()'
...
}
</pre>
Unfortunately, we can't flag this mistake as soon as we see it: inside
a template, we're not allowed to make assumptions about "dependent
types" like <tt>Processor&lt;T&gt;</tt>. Suppose that later on in
this file the programmer adds an explicit specialization
of <tt>Processor</tt>, like so:
<pre>
template &lt;&gt; class Processor&lt;char*&gt; {
void innit();
};
</pre>
Now the program will work &mdash; as long as the programmer only ever
instantiates <tt>process()</tt> with <tt>T = char*</tt>! This is why
it's hard, and sometimes impossible, to diagnose mistakes in a
template definition before it's instantiated.
<p>The standard says that a template with no valid instantiations is
ill-formed. Clang tries to do as much checking as possible at
definition-time instead of instantiation-time: not only does this
produce clearer diagnostics, but it also substantially improves
compile times when using pre-compiled headers. The downside to this
philosophy is that Clang sometimes fails to process files because they
contain broken templates that are no longer used. The solution is
simple: since the code is unused, just remove it.
<!-- ======================================================================= -->
<h2 id="default_init_const">Default initialization of const variable of a class type requires user-defined default constructor</h2>
<!-- ======================================================================= -->