为什么不能在 c++0x 模式下使用 libc++ 链接这个 boost::program_options 示例?
为 boost::program_options 编译这个示例代码:http://svn.boost.org/svn/boost/trunk/libs/program_options/example/first.cpp
Compiling this example code for boost::program_options: http://svn.boost.org/svn/boost/trunk/libs/program_options/example/first.cpp
...在 MacOS Lion (10.7.2) 上，使用安装了 MacPorts 的 boost-1.48.0:
...on MacOS Lion (10.7.2), using boost-1.48.0 installed with MacPorts:
$ clang++ -v
Apple clang version 3.0 (tags/Apple/clang-211.12) (based on LLVM 3.0svn)
Target: x86_64-apple-darwin11.2.0
Thread model: posix
$ clang++ -std=c++0x --stdlib=libc++ -lc++ -I/opt/local/include -L/opt/local/lib -lboost_program_options first.cpp -o first
Undefined symbols for architecture x86_64:
"boost::program_options::options_description::options_description(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, unsigned int, unsigned int)", referenced from:
_main in cc-6QQcwm.o
"boost::program_options::operator<<(std::__1::basic_ostream<char, std::__1::char_traits<char> >&, boost::program_options::options_description const&)", referenced from:
_main in cc-6QQcwm.o
"boost::program_options::abstract_variables_map::operator[](std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&) const", referenced from:
boost::program_options::variables_map::operator[](std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&) const in cc-6QQcwm.o
"boost::program_options::detail::cmdline::set_additional_parser(boost::function1<std::__1::pair<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > >, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&>)", referenced from:
boost::program_options::basic_command_line_parser<char>::extra_parser(boost::function1<std::__1::pair<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > >, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&>) in cc-6QQcwm.o
"boost::program_options::detail::cmdline::cmdline(std::__1::vector<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, std::__1::allocator<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > > > const&)", referenced from:
boost::program_options::basic_command_line_parser<char>::basic_command_line_parser(int, char const* const*) in cc-6QQcwm.o
"boost::program_options::to_internal(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&)", referenced from:
std::__1::vector<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, std::__1::allocator<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > > > boost::program_options::to_internal<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > >(std::__1::vector<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, std::__1::allocator<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > > > const&) in cc-6QQcwm.o
"boost::program_options::invalid_option_value::invalid_option_value(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&)", referenced from:
void boost::program_options::validate<int, char>(boost::any&, std::__1::vector<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, std::__1::allocator<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > > > const&, int*, long) in cc-6QQcwm.o
"boost::program_options::validation_error::validation_error(boost::program_options::validation_error::kind_t, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&)", referenced from:
std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const& boost::program_options::validators::get_single_string<char>(std::__1::vector<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, std::__1::allocator<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > > > const&, bool) in cc-6QQcwm.o
"boost::program_options::value_semantic_codecvt_helper<char>::parse(boost::any&, std::__1::vector<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, std::__1::allocator<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > > > const&, bool) const", referenced from:
vtable for boost::program_options::typed_value<int, char> in cc-6QQcwm.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
使用 MacPorts 安装的 g++4.7 编译/链接的相同代码:
The same code compiled/linked with g++4.7 installed with MacPorts:
$ g++-mp-4.7 -std=c++0x -I/opt/local/include -L/opt/local/lib -lboost_program_options -o first first.cpp
...工作正常.与在没有 libc++ 的情况下使用 clang 一样:
... works fine. As does using clang without libc++:
clang++ -std=c++0x -I/opt/local/include -L/opt/local/lib -lboost_program_options first.cpp -o first
怎么了?为什么 boost::program_options 和 libc++ 不能一起工作?
What's wrong? Why does boost::program_options and libc++ not work together?
推荐答案
您需要使用 clang++ -stdlib=libc++ 重建 boost.
You need to rebuild boost using clang++ -stdlib=libc++.
libc++ 与 gcc 的 libstdc++ 不是二进制兼容的(除了一些低级的东西，比如 operator new).例如，gcc 的 libstdc++ 中的 std::string 被引用计数，而在 libc++ 中它使用短字符串优化".如果您不小心在同一个程序中混合了这两个字符串(并将它们误认为是相同的数据结构)，则不可避免地会导致运行时崩溃.
libc++ is not binary compatible with gcc's libstdc++ (except for some low level stuff such as operator new). For example the std::string in gcc's libstdc++ is refcounted, whereas in libc++ it uses the "short string optimization". If you were to accidentally mix these two strings in the same program (and mistake them for the same data structure), you would inevitably get a run time crash.
这起事故正是你的情况.
This accident is exactly what has occurred in your case.
为了把这个运行时崩溃变成链接时错误，libc++使用了一个叫做inline namespace的C++11语言特性来改变std::string 的 API.也就是说，对你来说 std::string 看起来是一样的.但是对于链接器来说，std::string 被破坏了，就好像它在命名空间 std::__1 中一样.因此链接器知道 std::basic_string 和 std::__1::basic_string 是两种不同的数据结构(前者来自 gcc 的 libstdc++，后者来自 libc++).
In order to turn this run time crash into a link time error, libc++ uses a C++11 language feature called inline namespace to change the ABI of std::string without impacting the API of std::string. That is, to you std::string looks the same. But to the linker, std::string is being mangled as if it is in namespace std::__1. Thus the linker knows that std::basic_string and std::__1::basic_string are two different data structures (the former coming from gcc's libstdc++ and the latter coming from libc++).
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