问题描述
我在使用xcode 4.6.2中的clang ++在mac os x 10.8.3下使用 -std = c ++ 11 -stdlib = libc ++编译程序时遇到问题。
I have a problem compiling a program with "-std=c++11 -stdlib=libc++" under mac os x 10.8.3 using clang++ from xcode 4.6.2.
当我尝试使用std :: mem_fn（）或（不推荐使用）std :: mem_fun_ref（）时，出现链接器错误找不到符号。相同的代码（使用std :: mem_fun_ref代替std :: mem_fn）在c ++ 03标准下编译和链接没有任何问题。
When I try to use std::mem_fn() or (deprecated) std::mem_fun_ref(), I get linker error "symbol(s) not found". The same code (with std::mem_fun_ref instead of std::mem_fn) compiles and links without any issues under the c++03 standard.
如果我调用相同的代码无需通过mem_fn或mem_fun_ref引用该对象上的成员函数，该程序即可编译并运行，不会出现任何问题。是clang ++问题，mac OS问题还是我做错了事？
If I call the same member function on an object without referring to it via mem_fn or mem_fun_ref, the program compiles and runs without any problems. Is it a clang++ problem, a mac os problem, or am I doing something wrong?
代码：
#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
using namespace std;
int main(int argc, char **arvc)
{
/* The beginning of the function compiles without any errors */
string str = "It's a test)";
if (str.empty())
{
cout << "String is empty!!!" << endl;
}
vector<string> v1;
v1.push_back("str1");
v1.push_back("str2");
v1.push_back("");
v1.push_back("str4");
v1.push_back("");
v1.push_back("str6");
/* The code after this point leads to linker error */
vector<string>::iterator it = remove_if(v1.begin(), v1.end(), mem_fn(&string::empty));
v1.erase(it, v1.end());
for (it = v1.begin(); it < v1.end(); ++it)
{
cout << " '" << *it << "'" << endl;
}
return 0;
}
编译日志：
$ xcrun clang++ -v -std=c++11 -stdlib=libc++ ../src/test_cxx.cpp
Apple LLVM version 4.2 (clang-425.0.28) (based on LLVM 3.2svn)
Target: x86_64-apple-darwin12.3.0
Thread model: posix
"/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang" -cc1 -triple x86_64-apple-macosx10.8.0 -emit-obj -mrelax-all -disable-free -disable-llvm-verifier -main-file-name test_cxx.cpp -pic-level 2 -mdisable-fp-elim -masm-verbose -munwind-tables -target-cpu core2 -target-linker-version 136 -v -resource-dir /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/clang/4.2 -isysroot / -fmodule-cache-path /var/folders/nz/0x05d6p14gddzxxn8ymnn74c0000gn/T/clang-module-cache -stdlib=libc++ -std=c++11 -fdeprecated-macro -fdebug-compilation-dir /Volumes/Work/projects/web/sp3/build -ferror-limit 19 -fmessage-length 80 -stack-protector 1 -mstackrealign -fblocks -fobjc-runtime=macosx-10.8.0 -fobjc-dispatch-method=mixed -fobjc-default-synthesize-properties -fcxx-exceptions -fexceptions -fdiagnostics-show-option -fcolor-diagnostics -o /var/folders/nz/0x05d6p14gddzxxn8ymnn74c0000gn/T/test_cxx-kAQ3wk.o -x c++ ../src/test_cxx.cpp
clang -cc1 version 4.2 based upon LLVM 3.2svn default target x86_64-apple-darwin12.3.0
ignoring nonexistent directory "/usr/include/c++/v1"
#include "..." search starts here:
#include <...> search starts here:
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/c++/v1
/usr/local/include
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/clang/4.2/include
/usr/include
/System/Library/Frameworks (framework directory)
/Library/Frameworks (framework directory)
End of search list.
"/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/ld" -demangle -dynamic -arch x86_64 -macosx_version_min 10.8.0 -syslibroot / -o a.out /var/folders/nz/0x05d6p14gddzxxn8ymnn74c0000gn/T/test_cxx-kAQ3wk.o -lc++ -lSystem /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/clang/4.2/lib/darwin/libclang_rt.osx.a
Undefined symbols for architecture x86_64:
"std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >::empty() const", referenced from:
_main in test_cxx-kAQ3wk.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
推荐答案
这是clang中的错误。我找不到关于它的错误报告。一个错误报告将不胜感激。
This is a bug in clang. I can't find a bug report on it though. A bug report would be much appreciated.
问题是 string :: empty 被标记为 always_inline，但 string 模板也标记为extern，这意味着它已经为您实例化。
The problem is that string::empty is marked "always_inline", but also the string template is marked extern, meaning that it has been instantiated already for you.
由于某种原因当我们有了这种组合，并且您形成了指向always_inline成员的成员函数指针时，clang拒绝对其进行轮廓描述，以便它可以指向它。
For some reason when we have this combination, and you form a member function pointer to the always_inline member, clang refuses to outline it so that it can point to it. It only refuses if the instantiation is marked as extern.
如果禁用extern或always_inline，则代码将起作用。您可以通过在 C ++其他标志中添加前者来禁用前者：
If you disable either the extern, or the always_inline, then the code will work. You can disable the former by including this in "C++ Other Flags":
-D'_LIBCPP_EXTERN_TEMPLATE(...)='
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