[cfe-commits] r98708 - in /cfe/trunk: docs/UsersManual.html www/cxx_compatibility.html
Rafael Espindola
rafael.espindola at gmail.com
Tue Mar 16 21:31:53 PDT 2010
Author: rafael
Date: Tue Mar 16 23:31:53 2010
New Revision: 98708
URL: http://llvm.org/viewvc/llvm-project?rev=98708&view=rev
Log:
Document common clang compatibility issues.
Patch by Zhanyong Wan.
Added:
cfe/trunk/www/cxx_compatibility.html
Modified:
cfe/trunk/docs/UsersManual.html
Modified: cfe/trunk/docs/UsersManual.html
URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/docs/UsersManual.html?rev=98708&r1=98707&r2=98708&view=diff
==============================================================================
--- cfe/trunk/docs/UsersManual.html (original)
+++ cfe/trunk/docs/UsersManual.html Tue Mar 16 23:31:53 2010
@@ -796,6 +796,13 @@
<p>Note that released Clang compilers will refuse to even try to use clang to compile C++ code unless you pass the <tt>-ccc-clang-cxx</tt> option to the driver. To turn on Clang's C++ support, please pass that flag. Clang compilers built from the Subversion trunk enable C++ support by default, and do not require the <tt>-ccc-clang-cxx</tt> flag.</p>
+<p>Clang strives to strictly conform to the C++ standard. That means
+it will reject invalid C++ code that another compiler may accept. If
+Clang reports errors in your code, please check
+the <a href="http://clang.llvm.org/cxx_compatibility.html">C++
+Compatibility</a> page to see whether they are C++-conformance bugs
+and how you can fix them.</p>
+
<!-- ======================================================================= -->
<h2 id="objcxx">Objective C++ Language Features</h2>
<!-- ======================================================================= -->
Added: cfe/trunk/www/cxx_compatibility.html
URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/www/cxx_compatibility.html?rev=98708&view=auto
==============================================================================
--- cfe/trunk/www/cxx_compatibility.html (added)
+++ cfe/trunk/www/cxx_compatibility.html Tue Mar 16 23:31:53 2010
@@ -0,0 +1,220 @@
+<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
+ "http://www.w3.org/TR/html4/strict.dtd">
+<html>
+<head>
+ <META http-equiv="Content-Type" content="text/html; charset=ISO-8859-1" />
+ <title>Clang - C++ Compatibility</title>
+ <link type="text/css" rel="stylesheet" href="menu.css" />
+ <link type="text/css" rel="stylesheet" href="content.css" />
+ <style type="text/css">
+</style>
+</head>
+<body>
+
+<!--#include virtual="menu.html.incl"-->
+
+<div id="content">
+
+<!-- ======================================================================= -->
+<h1>Clang's C++ Compatibility</h1>
+<!-- ======================================================================= -->
+
+<ul>
+<li><a href="#intro">Introduction</a></li>
+<li><a href="#vla">Variable-length arrays</a></li>
+<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">Dependent name lookup into dependent bases of class templates</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>
+
+<!-- ======================================================================= -->
+<h2 id="intro">Introduction</h2>
+<!-- ======================================================================= -->
+
+<p>Clang strives to strictly conform to the C++ standard. That means
+it will reject invalid C++ code that another compiler may accept.
+This page helps you decide whether a Clang error message means a
+C++-conformance bug in your code and how you can fix it.</p>
+
+<!-- ======================================================================= -->
+<h2 id="vla">Variable-length arrays</h2>
+<!-- ======================================================================= -->
+
+<p>GCC allows an array's size to be determined at run time. This,
+however, is not standard C++. Furthermore, it is a potential security
+hole as an incorrect array size may overflow the stack. If Clang tells
+you <tt>"variable length arrays are not permitted in C++"</tt>, here
+are some ways in which you can fix it:</p>
+
+<ol>
+<li>replace it with a fixed-size array if you can determine a
+ reasonable upper bound at compile time; sometimes this is as
+ simple as changing <tt>int size = ...;</tt> to <tt>const int size
+ = ...;</tt> (if the definition of <tt>size</tt> is a compile-time
+ integral constant);</li>
+<li>use an <tt>std::string</tt> instead of a <tt>char []</tt>;</li>
+<li>use <tt>std::vector</tt> or some other suitable container type;
+ or</li>
+<li>allocate the array on the heap instead using <tt>new Type[]</tt> -
+ just remember to <tt>delete[]</t> it.</li>
+</ol>
+
+<!-- ======================================================================= -->
+<h2 id="init_static_const">Initialization of non-integral static const data members within a class definition</h2>
+<!-- ======================================================================= -->
+
+The following code is ill-formed in C++'03:
+
+<pre>
+class SomeClass {
+ public:
+ static const double SomeConstant = 0.5;
+};
+
+const double SomeClass::SomeConstant;
+</pre>
+
+Clang errors with something similar to:
+
+<pre>
+.../your_file.h:42:42: error: 'SomeConstant' can only be initialized if it is a static const integral data member
+ static const double SomeConstant = 0.5;
+ ^ ~~~
+</pre>
+
+Only <i>integral</i> constant expressions are allowed as initializers
+within the class definition. See C++'03 [class.static.data] p4 for the
+details of this restriction. The fix here is straightforward: move
+the initializer to the definition of the static data member, which
+must exist outside of the class definition:
+
+<pre>
+class SomeClass {
+ public:
+ static const double SomeConstant;
+};
+
+const double SomeClass::SomeConstant<b> = 0.5</b>;
+</pre>
+
+<!-- ======================================================================= -->
+<h2 id="dep_lookup">Dependent name lookup into dependent bases of class templates</h2>
+<!-- ======================================================================= -->
+
+Some versions of GCC accept the following invalid code:
+
+<pre>
+template <typename T>
+class Base {
+ public:
+ void DoThis(T x) {}
+
+ static void DoThat(T x) {}
+};
+
+template <typename T>
+class Derived : public Base<T> {
+ public:
+ void Work(T x) {
+ DoThis(x); // Invalid!
+ DoThat(x); // Invalid!
+ }
+};
+
+void Test() {
+ Derived<int> d;
+ d.Work(42);
+}
+</pre>
+
+Clang correctly rejects it with the following errors:
+
+<pre>
+my_file.cpp:13:5: error: use of undeclared identifier 'DoThis'
+ DoThis(x);
+ ^
+ this->
+my_file.cpp:20:5: note: in instantiation of member function 'Derived<int>::Work' requested here
+ d.Work(42);
+ ^
+my_file.cpp:4:8: note: must qualify identifier to find this declaration in dependent base class
+ void DoThis(T x) {}
+ ^
+my_file.cpp:14:5: error: use of undeclared identifier 'DoThat'
+ DoThat(x);
+ ^
+ this->
+my_file.cpp:6:15: note: must qualify identifier to find this declaration in dependent base class
+ static void DoThat(T x) {}
+</pre>
+
+The reason the code is invalid is that in
+class <tt>Derived<T></tt>, the base class type <tt>Base<T></tt>
+depends on the template argument <tt>T</tt> (hence it's called a dependent base
+class in C++ jargon), and C++ doesn't look at the members of a
+dependent base class when resolving unqualified calls like <tt>DoThis(x)</tt>
+and <tt>DoThat(x)</tt> (see [temp.dep] p3 for details). The fix, as Clang tells
+you, is to prefix the calls with <tt>this-></tt>:
+
+<pre>
+...
+template <typename T>
+class Derived : public Base<T> {
+ public:
+ void Work(T x) {
+ <b>this-></b>DoThis(x);
+ <b>this-></b>DoThat(x);
+ }
+};
+...
+</pre>
+
+Alternatively, since DoThat() is a static method, you can also write
+
+<pre>
+ void Work(T x) {
+ <b>this-></b>DoThis(x);
+ <b>Base<T></b>::DoThat(x);
+ }
+</pre>
+
+<!-- ======================================================================= -->
+<h2 id="default_init_const">Default initialization of const variable of a class type requires user-defined default constructor</h2>
+<!-- ======================================================================= -->
+
+If a <tt>class</tt> or <tt>struct</tt> has no user-defined default
+constructor, C++ doesn't allow you to default construct a <tt>const</tt>
+instance of it like this ([dcl.init], p9):
+
+<pre>
+class Foo {
+ public:
+ // The compiler-supplied default constructor works fine, so we
+ // don't bother with defining one.
+ ...
+};
+
+void Bar() {
+ const Foo foo; // Error!
+ ...
+}
+</pre>
+
+To fix this, you can define a default constructor for the class:
+
+<pre>
+class Foo {
+ public:
+ Foo() {}
+ ...
+};
+
+void Bar() {
+ const Foo foo; // Now the compiler is happy.
+ ...
+}
+</pre>
+
+</div>
+</body>
+</html>
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