diff --git a/www/content.css b/www/content.css index dca6a32914369f77102d153de8d98f22df8f59ac..574b98e12b6d752dca4a5a5adb2f0fe8366a35ee 100644 --- a/www/content.css +++ b/www/content.css @@ -23,5 +23,8 @@ IMG.img_slide { .itemTitle { color:#2d58b7 } +span.error { color:red } +span.caret { color:green; font-weight:bold } + /* Tables */ tr { vertical-align:top } diff --git a/www/cxx_compatibility.html b/www/cxx_compatibility.html index 02dabbe8e3da6f16f533e581fba9b108f318db12..27defc2d8ac13f6f6434282732074157e63a5a1a 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="#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> @@ -215,6 +216,129 @@ This works whether the methods are static or not, but be careful: if <tt>DoThis</tt> is virtual, calling it this way will bypass virtual 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> <!-- ======================================================================= -->