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Diffstat (limited to 'www/cxx_compatibility.html')
| -rw-r--r-- | www/cxx_compatibility.html | 48 |
1 files changed, 48 insertions, 0 deletions
diff --git a/www/cxx_compatibility.html b/www/cxx_compatibility.html index e5fd63e..bd48c6c 100644 --- a/www/cxx_compatibility.html +++ b/www/cxx_compatibility.html @@ -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> @@ -203,6 +204,53 @@ 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 <class T> class Processor { + ... + void init(); + ... + }; + ... + template <class T> void process() { + Processor<T> 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<T></tt>. Suppose that later on in +this file the programmer adds an explicit specialization +of <tt>Processor</tt>, like so: + +<pre> + template <> class Processor<char*> { + void innit(); + }; +</pre> + +Now the program will work — 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> <!-- ======================================================================= --> |
