简单的std :: regex_search()代码不能用Apple clang ++ -std = c ++ 14编译

Question

这是MCVE:

#include <iostream>
#include <regex>

std::string s()
{
    return "test";
}

int main()
{
    static const std::regex regex(R"(\w)");
    std::smatch smatch;

    if (std::regex_search(s(), smatch, regex)) {
        std::cout << smatch[0] << std::endl;
    }

    return 0;
}

编译很好:

$ clang ++ -std = c ++ 11 main.cpp

但不是:

$ clang ++ -std = c ++ 14 main.cpp

后一种情况下的错误消息(使用-std = c ++ 14):

main.cpp:14:9: error: call to deleted function 'regex_search'
    if (std::regex_search(s(), smatch, regex)) {
        ^~~~~~~~~~~~~~~~~
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/regex:5998:1: note: 
      candidate function [with _ST = std::__1::char_traits<char>, _SA = std::__1::allocator<char>,
      _Ap = std::__1::allocator<std::__1::sub_match<std::__1::__wrap_iter<const char *> > >, _Cp =
      char, _Tp = std::__1::regex_traits<char>] has been explicitly deleted
regex_search(const basic_string<_Cp, _ST, _SA>&& __s,
^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/regex:2876:5: note: 
      candidate function [with _ST = std::__1::char_traits<char>, _SA = std::__1::allocator<char>,
      _Ap = std::__1::allocator<std::__1::sub_match<std::__1::__wrap_iter<const char *> > >, _Cp =
      char, _Tp = std::__1::regex_traits<char>]
    regex_search(const basic_string<_Cp, _ST, _SA>& __s,
    ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/regex:2851:5: note: 
      candidate template ignored: deduced conflicting types for parameter '_Bp'
      ('std::__1::basic_string<char>' vs. 'std::__1::match_results<std::__1::__wrap_iter<const char
      *>, std::__1::allocator<std::__1::sub_match<std::__1::__wrap_iter<const char *> > > >')
    regex_search(_Bp, _Bp, const basic_regex<_Cp, _Tp>&,
    ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/regex:2857:5: note: 
      candidate template ignored: could not match 'const _Cp *' against 'std::string' (aka
      'basic_string<char, char_traits<char>, allocator<char> >')
    regex_search(const _Cp*, const _Cp*,
    ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/regex:2863:5: note: 
      candidate template ignored: could not match 'const _Cp *' against 'std::string' (aka
      'basic_string<char, char_traits<char>, allocator<char> >')
    regex_search(const _Cp*, match_results<const _Cp*, _Ap>&, const basic_regex<_Cp, _Tp>&,
    ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/regex:2869:5: note: 
      candidate template ignored: could not match 'basic_regex' against 'match_results'
    regex_search(const basic_string<_Cp, _ST, _SA>& __s,
    ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/regex:5963:1: note: 
      candidate template ignored: could not match 'const _CharT *' against 'std::string' (aka
      'basic_string<char, char_traits<char>, allocator<char> >')
regex_search(const _CharT* __str, const basic_regex<_CharT, _Traits>& __e,
^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/regex:2839:5: note: 
      candidate function template not viable: requires at least 4 arguments, but 3 were provided
    regex_search(_Bp, _Bp, match_results<_Bp, _Ap>&, const basic_regex<_Cp, _Tp>&,
    ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/regex:2845:5: note: 
      candidate function template not viable: requires at least 4 arguments, but 3 were provided
    regex_search(const _Cp*, const _Cp*, match_results<const _Cp*, _Ap>&,
    ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/regex:2884:5: note: 
      candidate function template not viable: requires at least 4 arguments, but 3 were provided
    regex_search(__wrap_iter<_Iter> __first,
    ^
1 error generated.

编译器版本信息:

$ clang++ -v
Apple LLVM version 7.0.0 (clang-700.0.72)
Target: x86_64-apple-darwin15.0.0
Thread model: posix

那么,怎么了？

Answer 1

从C++ 11到C++ 14的变化std::regex_search不再允许采用r值

template< class STraits, class SAlloc,
          class Alloc, class CharT, class Traits >
bool regex_search( const std::basic_string<CharT,STraits,SAlloc>&&,
                   std::match_results<
                       typename std::basic_string<CharT,STraits,SAlloc>::const_iterator, 
                       Alloc>&,
                   const std::basic_regex<CharT, Traits>&,
                   std::regex_constants::match_flag_type flags =
                       std::regex_constants::match_default ) = delete;

这被添加为需要的重载 const std::string&

禁止接受临时字符串,否则此函数会将match_results m填充为立即变为无效的字符串迭代器.

因此,您不能再将临时文件传递给std::regex_searchC++ 14

要修复代码,我们只需将返回值存储s()到main中的变量中,然后使用它来调用std::regex_search.

#include <iostream>
#include <regex>

std::string s()
{
    return "test";
}

int main()
{
    static const std::regex regex(R"(\w)");
    std::smatch smatch;

    auto search = s();
    if (std::regex_search(search, smatch, regex)) {
        std::cout << smatch[0] << std::endl;
    }

    return 0;
}

Live Example

Answer 2

这在C++ 11和C++ 14之间发生了变化.如果我们转到std :: regex_search的cppreference部分,我们可以看到自从C++ 14以来删除了带右值引用的重载:

template< class STraits, class SAlloc,
          class Alloc, class CharT, class Traits > bool regex_search( const std::basic_string<CharT,STraits,SAlloc>&&,
                   std::match_results<
                       typename std::basic_string<CharT,STraits,SAlloc>::const_iterator,
                       Alloc
                   >&,
                   const std::basic_regex<CharT, Traits>&,
                   std::regex_constants::match_flag_type flags =
                       std::regex_constants::match_default ) = delete;

由于LWG问题2329而改变了:带有match_results的regex_match()/ regex_search()应该禁止临时字符串(强调我的):

请考虑以下代码:

const regex r(R"(meow(\d+)\.txt)");
smatch m;
if (regex_match(dir_iter->path().filename().string(), m, r)) {
  DoSomethingWith(m[1]);
}

这偶尔会崩溃.问题是dir_iter-> path().filename().string()返回一个临时字符串,因此match_results包含一个被破坏的临时字符串中的无效迭代器.

regex_match/regex_search(str,reg)接受临时字符串很好,因为它们只返回bool.但是,使用match_results的重载应该禁止临时字符串.

事实上,如果我们使用非临时的:

std::string s1 = s() ;

if (std::regex_search(s1, smatch, regex)) {
//...
}

它编译(看到它直播),不再表现出未定义的行为.

有趣的是,gcc/libstdc ++在C++ 11模式下删除了这个重载,并且看到了它.由于这是未定义的行为,它似乎是一个很好的解决方案.

这个问题还会出现在库的其他区域,请参阅Visual Studio regex_iterator Bug？它处理同样的问题,但有regex_iterator/regex_token_iterator.