简单的 std：：regex_search() 代码无法使用 Apple clang++ -std=c++14 编译

Simple std::regex_search() code won't compile with Apple clang++ -std=c++14

本文关键字：Apple clang++ 编译 c++14 -std 代码 regex std search 简单      更新时间：2023-10-16

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;
}

使用

可以很好地编译

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

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

后一种情况下的错误信息(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

怎么了?

从c++ 11到c++ 14, std::regex_search不再允许取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;

这是作为接受const std::string&

的重载添加的。

禁止接受临时字符串，否则该函数将使用立即失效的字符串迭代器填充match_results m。

所以你不能再传递一个临时的std::regex_search作为c++ 14

要修复您的代码，我们只需将s()的返回值存储到main中的一个变量中，并使用该变量调用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;
}
<<p> 生活例子/kbd>

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，可以看到从c++ 14:

开始，接受右值引用的重载被删除了。
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而更改:regex_match()/regex_search() with match_results应该禁止临时字符串(强调mine):

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，somatch_results包含销毁类的无效迭代器临时字符串 .

regex_match/regex_search(str, reg)接受临时是可以的字符串，因为它们只返回bool值。然而，重载正在发生match_results应该禁止临时字符串。

，如果我们使用非临时的:

std::string s1 = s() ;
if (std::regex_search(s1, smatch, regex)) {
//...
}

它编译(查看实时)并且不再显示未定义行为。

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

这个问题也出现在库的其他区域参见Visual Studio regex_iterator Bug?它处理相同的问题，但与regex_iterator/regex_token_iterator。

这不是bug，而是预期的行为。原因是s()返回临时字符串，regex_search使用regex_match，因此，如果使用临时字符串，匹配结果将包含指向不再存在的字符串的迭代器。这是未定义的行为。因此，委员会在c++ 14中取消了regex_search重载。

您也可以在标准28.4 Header摘要中确认[re.syn]:

template <class ST, class SA, class Allocator, class charT, class traits>
bool regex_search(const basic_string<charT, ST, SA>&&,
match_results<
typename basic_string<charT, ST, SA>::const_iterator,
Allocator>&,
const basic_regex<charT, traits>&,
regex_constants::match_flag_type =
regex_constants::match_default) = delete;

可以看到，将右值赋给basic_string的重载标记为已删除

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