[cfe-commits] r106076 - /cfe/trunk/www/cxx_compatibility.html
Jeffrey Yasskin
jyasskin at google.com
Tue Jun 15 18:12:12 PDT 2010
Author: jyasskin
Date: Tue Jun 15 20:12:12 2010
New Revision: 106076
URL: http://llvm.org/viewvc/llvm-project?rev=106076&view=rev
Log:
Fix template ordering compatibility docs. I missed another section that covered
the same thing.
Modified:
cfe/trunk/www/cxx_compatibility.html
Modified: cfe/trunk/www/cxx_compatibility.html
URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/www/cxx_compatibility.html?rev=106076&r1=106075&r2=106076&view=diff
==============================================================================
--- cfe/trunk/www/cxx_compatibility.html (original)
+++ cfe/trunk/www/cxx_compatibility.html Tue Jun 15 20:12:12 2010
@@ -25,7 +25,6 @@
<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="#declaration_ordering">Template uses of a function must either find the function by ADL or come after the declaration of the function</a></li>
<li><a href="#undep_incomplete">Incomplete types in 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>
@@ -118,33 +117,103 @@
<h2 id="dep_lookup">Unqualified lookup in templates</h2>
<!-- ======================================================================= -->
-Some versions of GCC accept the following invalid code:
+<p>Some versions of GCC accept the following invalid code:
<pre>
-template <typename T> struct Foo {
- void Work(T x) {
- func(x);
- }
-};
-...
-void func(int x);
-...
-template struct Foo<int>; // or anything else that instantiates Foo<int>::Work
+#include <iostream>
+#include <utility>
+
+template<typename T>
+void Dump(const T& value) {
+ std::cout << value << "\n";
+}
+
+namespace ns {
+ struct Data {};
+}
+
+std::ostream& operator<<(std::ostream& out, ns::Data) {
+ return out << "Some data";
+}
+
+void Use() {
+ Dump(std::make_pair(3, 4.5));
+ Dump(ns::Data());
+}
+
+template<typename T, typename U>
+std::ostream& operator<<(std::ostream& out, const std::pair<T, U>& p) {
+ return out << '(' << p.first << ", " << p.second << ")";
+}
</pre>
-The standard says that unqualified names like <tt>func</tt> are looked up
-when the template is defined, not when it's instantiated. Since
-<tt>void func(int)</tt> was not declared yet when <tt>Foo</tt> was
-defined, it's not considered. The fix is usually to
-declare <tt>func</tt> before <tt>Foo</tt>.
+<p>Clang complains:</p>
+
+<pre>
+<b>test.cc:6:13: <span class=error>error:</span> invalid operands to binary expression ('ostream' (aka 'basic_ostream<char>') and 'std::pair<int, double> const')</b>
+ std::cout << value << "\n";
+ <span class=caret>~~~~~~~~~ ^ ~~~~~</span>
+<b>test.cc:18:3: note:</b> in instantiation of function template specialization 'Dump<std::pair<int, double> >' requested here
+ Dump(std::make_pair(3, 4.5));
+ <span class=caret>^</span>
+<b>test.cc:6:13: <span class=error>error:</span> invalid operands to binary expression ('ostream' (aka 'basic_ostream<char>') and 'ns::Data const')</b>
+ std::cout << value << "\n";
+ <span class=caret>~~~~~~~~~ ^ ~~~~~</span>
+<b>test.cc:19:3: note:</b> in instantiation of function template specialization 'Dump<ns::Data>' requested here
+ Dump(ns::Data());
+ <span class=caret>^</span>
+2 errors generated.
+</pre>
+
+<p>The standard, in [temp.dep.candidate], says that unqualified names
+like <tt>operator<<</tt> are looked up when the template is
+defined, not when it's instantiated. Since
+<tt>operator<<(std::ostream&, const std::pair<>&)</tt>
+and <tt>operator<<(std::ostream&, ns::Data)</tt> were not
+declared yet when <tt>Dump</tt> was defined, they're not considered.
<p>This is complicated by <i>argument-dependent lookup</i> (ADL),
which is done when unqualified names are called as functions,
-like <tt>func(x)</tt> above. The standard says that ADL is performed
-in both places if any of the arguments are type-dependent, like
-<tt>x</tt> is in this example. However, ADL does nothing for builtin
-types like <tt>int</tt>, so the example is still invalid. See
-[basic.lookup.argdep] for more information.
+like <tt>operator<<</tt> above. The standard says that ADL is
+performed in both places if any of the arguments are type-dependent,
+like <tt>value</tt> and <tt>p</tt> are in this example.
+
+<p>The fix is usually to</p>
+<ol><li>Add a declaration before the use of the function,
+<li>Move the definition to before the use of the function, or
+<li>Move the function into the same namespace as one of its arguments
+so that ADL applies. (Note that it still needs to be declared before
+the template is <i>instantiated</i>, and that ADL doesn't apply to
+built-in types.)
+</ol>
+
+<pre>
+#include <iostream>
+#include <utility>
+
+template<typename T, typename U> // Fix 2
+std::ostream& operator<<(std::ostream& out, const std::pair<T, U>& p) {
+ return out << '(' << p.first << ", " << p.second << ")";
+}
+
+template<typename T>
+void Dump(const T& value) {
+ std::cout << value << "\n";
+}
+
+namespace ns {
+ struct Data {};
+
+ std::ostream& operator<<(std::ostream& out, Data) { // Fix 3
+ return out << "Some data";
+ }
+}
+
+void Use() {
+ Dump(std::make_pair(3, 4.5));
+ Dump(ns::Data());
+}
+</pre>
<!-- ======================================================================= -->
<h2 id="dep_lookup_bases">Unqualified lookup into dependent bases of class templates</h2>
@@ -217,129 +286,6 @@
dispatch!
<!-- ======================================================================= -->
-<h2 id="declaration_ordering">Template uses of a function must either find the function by ADL or come after the declaration of the function</h2>
-<!-- ======================================================================= -->
-
-<p>For example, gcc-4.4 accepts the following code:</p>
-
-<pre>
-#include <iostream>
-#include <utility>
-#include <vector>
-
-template<typename T>
-void Dump(const T& value) {
- std::cout << value << "\n";
-}
-
-template<typename T, typename U>
-std::ostream& operator<<(std::ostream& out, const std::pair<T, U>& i) {
- return out << '(' << i.first << ", " << i.second << ")";
-}
-
-namespace ns {
- struct Data {};
-}
-
-std::ostream& operator<<(std::ostream& out, ns::Data) {
- return out << "Some data";
-}
-
-void Use() {
- Dump(std::make_pair(3, 4.5));
- Dump(ns::Data());
- Dump(std::vector<const char*>(1, "Hello World"));
-}
-
-template<typename T>
-std::ostream& operator<<(std::ostream& out, const std::vector<T>& vec) {
- out << '[';
- for (size_t i = 0, size = vec.size(); i != size; ++i) {
- if (i != 0)
- out << ", ";
- out << vec[i];
- }
- return out << ']';
-}
-</pre>
-
-<p>while clang, following the rules in <tt>[temp.dep.candidate]</tt>
-complains:</p>
-
-<pre>
-<b>test.cc:7:13: <span class=error>error:</span> invalid operands to binary expression ('ostream' (aka 'basic_ostream<char>') and 'std::pair<int, double> const')</b>
- std::cout << value << "\n";
- <span class=caret>~~~~~~~~~ ^ ~~~~~</span>
-<b>test.cc:24:3: note:</b> in instantiation of function template specialization 'Dump<std::pair<int, double> >' requested here
- Dump(std::make_pair(3, 4.5));
- <span class=caret>^</span>
-<b>test.cc:7:13: <span class=error>error:</span> invalid operands to binary expression ('ostream' (aka 'basic_ostream<char>') and 'ns::Data const')</b>
- std::cout << value << "\n";
- <span class=caret>~~~~~~~~~ ^ ~~~~~</span>
-<b>test.cc:25:3: note:</b> in instantiation of function template specialization 'Dump<ns::Data>' requested here
- Dump(ns::Data());
- <span class=caret>^</span>
-<b>test.cc:7:13: <span class=error>error:</span> invalid operands to binary expression ('ostream' (aka 'basic_ostream<char>') and 'std::vector<char const *, std::allocator<char const *> > const')</b>
- std::cout << value << "\n";
- <span class=caret>~~~~~~~~~ ^ ~~~~~</span>
-<b>test.cc:26:3: note:</b> in instantiation of function template specialization 'Dump<std::vector<char const *, std::allocator<char const *> > >' requested here
- Dump(std::vector<const char*>(1, "Hello World"));
- <span class=caret>^</span>
-3 errors generated.
-</pre>
-
-<p>The fix is to</p>
-<ol><li>Add a declaration before the use of the function, or
-<li>Move the definition to before the use of the function, or
-<li>Move the function into the same namespace as one of its
-arguments. (Note that it still needs to be declared before the
-template is <i>instantiated</i>.)
-</ol>
-
-<pre>
-#include <iostream>
-#include <utility>
-#include <vector>
-
-template<typename T> // Fix 1.
-std::ostream& operator<<(std::ostream& out, const std::vector<T>& vec);
-
-template<typename T, typename U> // Fix 2.
-std::ostream& operator<<(std::ostream& out, const std::pair<T, U>& i) {
- return out << '(' << i.first << ", " << i.second << ")";
-}
-
-template<typename T>
-void Dump(const T& value) {
- std::cout << value << "\n";
-}
-
-namespace ns {
- struct Data {};
- std::ostream& operator<<(std::ostream& out, Data) { // Fix 3.
- return out << "Some data";
- }
-}
-
-void Use() {
- Dump(std::make_pair(3, 4.5));
- Dump(ns::Data());
- Dump(std::vector<const char*>(1, "Hello World"));
-}
-
-template<typename T>
-std::ostream& operator<<(std::ostream& out, const std::vector<T>& vec) {
- out << '[';
- for (size_t i = 0, size = vec.size(); i != size; ++i) {
- if (i != 0)
- out << ", ";
- out << vec[i];
- }
- return out << ']';
-}
-</pre>
-
-<!-- ======================================================================= -->
<h2 id="undep_incomplete">Incomplete types in templates</h2>
<!-- ======================================================================= -->
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