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POC++ how to handle tr1 and non-tr1 namespaces in portable code?
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copied!<p>Is there a canonical way to deal with the namespace issues that arise when trying to maintain portable code between a TR1 and non-TR1 toolchain?</p> <p>I have a VC++2010 project that <code>#include <type_traits></code>. I also have an LLVM 3.0 compiler that can handle this fine. This allows me to use templates such as:</p> <pre><code>std::enable_if<typename> std::is_enum<typename> </code></pre> <p>However I also need to build and maintain this code on an Xcode 4.5 clang compiler:</p> <pre><code>$ /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang --version Apple clang version 4.1 (tags/Apple/clang-421.11.66) (based on LLVM 3.1svn) Target: x86_64-apple-darwin11.4.2 Thread model: posix </code></pre> <p>This compiler doesn't seem to have a include file, instead it has a . However this is causing me problems because the namespace has changed from std:: to __gnu_cxx::, meaning I have to use:</p> <pre><code>__gnu_cxx::__enable_if<typename> </code></pre> <p>Somehow I was able to determine that the definition of the symbol <code>__GLIBCXX__</code> is sufficient to determine whether I should use one or the other (not even sure that's the right way to do it, but for now it works between the compilers I'm using).</p> <p>So I could resort to using preprocessor macros:</p> <pre><code>#ifdef __GLIBCXX__ # include <tr1/type_traits> # define ENABLE_IF __gnu_cxx::__enable_if #else # include <type_traits> # define ENABLE_IF std::enable_if #endif </code></pre> <p>But this seems like it might be more of a hack than a proper solution. (Actually I tried this and it doesn't work, because trying to use <code>__gnu_cxx::__enable_if</code> causes this error:</p> <pre><code>error: too few template arguments for class template '__enable_if' </code></pre> <ul> <li>further digging suggests that this version of enable_if actually takes two template arguments. I'm now very lost...)</li> </ul> <p>I thought about doing something like:</p> <pre><code>#ifdef __GLIBCXX__ # include <tr1/type_traits> namespace __gnu_cxx = foo; #else # include <type_traits> namespace std = foo; #endif ... foo::enable_if< ... > </code></pre> <p>However this doesn't work because the template is called <code>enable_if</code> in one namespace, but <code>__enable_if</code> in the other.</p> <p>I'm sure I'm not the first person to deal with this problem - can someone point me at the industry best practice for resolving this please? Or should I just use Boost instead?</p> <p>There is a similar question (I think) but only a partial answer <a href="https://stackoverflow.com/a/7095608/143397">here</a>. Are there better options?</p> <p>EDIT: I tried this, with <code><boost/type_traits.hpp></code>:</p> <pre><code>#include <boost/type_traits.hpp> template <typename ValueType> class Extractor <ValueType, typename boost::enable_if<boost::is_enum<ValueType>::value>::type> { public: ValueType extract(double value) { return static_cast<ValueType>(static_cast<int>(value)); // cast to int first, then enum, to satisfy VC++2010 } }; enum MyEnum { Enum0, Enum1 }; Extractor<MyEnum> e; MyEnum ev = e.extract(1.0); </code></pre> <p>However this gives me the following compiler error in Xcode 4.5:</p> <pre><code>error: expected a qualified name after 'typename' class Extractor <ValueType, typename boost::enable_if<boost::is_enum<ValueType>::value>::type> { ^ error: unknown type name 'type' </code></pre> <p>So it doesn't seem that std::enable_if and boost::enable_if are drop-in compatible. </p>

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