鏗鏘mac os x C++ 11鏈接器問題

Question

我有一個問題在mac os x 10.8.3下使用xcode 4.6.2中的clang ++編譯帶「-std = C++ 11 -stdlib = libC++」的程序。當我嘗試使用std :: mem_fn（）或（不推薦）std :: mem_fun_ref（）時，我得到鏈接器錯誤「找不到符號」。相同的代碼（使用std :: mem_fun_ref代替std :: mem_fn）在C++ 03標準下編譯和鏈接時沒有任何問題。

如果我通過mem_fn或mem_fun_ref引用對象上的同一個成員函數，程序編譯並運行時沒有任何問題。它是一個鏗鏘+ +問題，一個mac os問題，或者我做錯了什麼？

代碼：

#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; 
} 

編譯日誌：

$ 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) 

Answer 1

這是鏗鏘的錯誤。雖然我找不到它的錯誤報告。一個錯誤報告將不勝感激。

問題是，string::empty標記爲「always_inline」，但string模板標記爲extern，這意味着它已經爲您實例化。

由於某些原因，當我們有這種組合，並且您構成了always_inline成員的成員函數指針時，clang拒絕概述它，以便它可以指向它。如果實例化被標記爲extern，它只會拒絕。

如果您禁用extern或always_inline，那麼代碼將起作用。您可以通過在「C++其他標誌」中包括此來禁用前者：

-D'_LIBCPP_EXTERN_TEMPLATE(...)='