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| author | Rafael Espindola <rafael.espindola@gmail.com> | 2010-03-17 04:31:53 +0000 |
|---|---|---|
| committer | Rafael Espindola <rafael.espindola@gmail.com> | 2010-03-17 04:31:53 +0000 |
| commit | 9b2fc9563fba142ae8d570d2eea43b4c24079326 (patch) | |
| tree | 4855862e978262f9ee4c5ebf2f82c1281a719fe6 /www/cxx_compatibility.html | |
| parent | 9b35b25db3641dfa8876d24b41cfa5b4d51477cb (diff) | |
Document common clang compatibility issues.
Patch by Zhanyong Wan.
git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@98708 91177308-0d34-0410-b5e6-96231b3b80d8
Diffstat (limited to 'www/cxx_compatibility.html')
| -rw-r--r-- | www/cxx_compatibility.html | 220 |
1 files changed, 220 insertions, 0 deletions
diff --git a/www/cxx_compatibility.html b/www/cxx_compatibility.html new file mode 100644 index 0000000000..c094f244df --- /dev/null +++ b/www/cxx_compatibility.html @@ -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> |