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Document common clang compatibility issues. 

Patch by Zhanyong Wan.

llvm-svn: 98708
This commit is contained in:
Rafael Espindola 2010-03-17 04:31:53 +00:00
parent cd7eef900f
commit fcc4739089
2 changed files with 227 additions and 0 deletions
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7
clang/docs/UsersManual.html
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@ -796,6 +796,13 @@ ask on the mailing list about how you can help.</p>
<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>
<!-- ======================================================================= -->

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<!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 &lt;typename T&gt;
class Base {
public:
void DoThis(T x) {}
static void DoThat(T x) {}
};
template &lt;typename T&gt;
class Derived : public Base&lt;T&gt; {
public:
void Work(T x) {
DoThis(x); // Invalid!
DoThat(x); // Invalid!
}
};
void Test() {
Derived&lt;int&gt; 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-&gt;
my_file.cpp:20:5: note: in instantiation of member function 'Derived&lt;int&gt;::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-&gt;
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&lt;T&gt;</tt>, the base class type <tt>Base&lt;T&gt;</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-&gt;</tt>:
<pre>
...
template &lt;typename T&gt;
class Derived : public Base&lt;T&gt; {
public:
void Work(T x) {
<b>this-&gt;</b>DoThis(x);
<b>this-&gt;</b>DoThat(x);
}
};
...
</pre>
Alternatively, since DoThat() is a static method, you can also write
<pre>
void Work(T x) {
<b>this-&gt;</b>DoThis(x);
<b>Base&lt;T&gt;</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>