c++ - 在Abseil或MSVC中，此absl::StrCat错误在哪里出现？

我从char数组创建string_view

  // creat sv from vector;
  std::vector<char> vec = { 'w', 'h', 'a', 't' };
  char* char_ptr = vec.data();
  size_t sz = vec.size();
  std::string_view sview_obj = std::string_view(char_ptr, sz);

如果您写：

  using namespace std::string_view_literals;

  std::cout << absl::StrCat(sview_obj, "ever")    // Error
            << absl::StrCat(sview_obj, sview_obj) // Error
            << absl::StrCat("what", "ever"sv)     // Error
            << absl::StrCat("what"sv, "ever"sv)   // Error
            << absl::StrCat("what", "ever");      // Okey

MSVC错误：

<source>(16): error C2039: 'string_view_literals': is not a member of 'std'
/opt/compiler-explorer/libs/abseil\absl/numeric/int128.h(231): note: see declaration of 'std'
<source>(16): error C2871: 'string_view_literals': a namespace with this name does not exist
<source>(18): error C2665: 'absl::StrCat': none of the 5 overloads could convert all the argument types
/opt/compiler-explorer/libs/abseil\absl/strings/str_cat.h(310): note: could be 'std::string absl::StrCat(const absl::AlphaNum &,const absl::AlphaNum &)'
<source>(18): note: while trying to match the argument list '(std::string_view, const char [5])'
<source>(19): error C2665: 'absl::StrCat': none of the 5 overloads could convert all the argument types
/opt/compiler-explorer/libs/abseil\absl/strings/str_cat.h(310): note: could be 'std::string absl::StrCat(const absl::AlphaNum &,const absl::AlphaNum &)'
<source>(19): note: while trying to match the argument list '(std::string_view, std::string_view)'
<source>(20): error C3688: invalid literal suffix 'sv'; literal operator or literal operator template 'operator ""sv' not found
<source>(21): error C3688: invalid literal suffix 'sv'; literal operator or literal operator template 'operator ""sv' not found

MSVC中似乎缺少operator""sv
line 18 and 19有一个共同点：

  std::cout << absl::StrCat(sview_obj, "ever")    // Error
            << absl::StrCat(sview_obj, sview_obj) // Error

错误：

<source>(18): error C2665: 'absl::StrCat': none of the 5 overloads could convert all the argument types
/opt/compiler-explorer/libs/abseil\absl/strings/str_cat.h(310): note: could be 'std::string absl::StrCat(const absl::AlphaNum &,const absl::AlphaNum &)'
<source>(18): note: while trying to match the argument list '(std::string_view, const char [5])'

当我在VC BuildTool（使用clang 8目标msvc）上进行编译时，此处提供了其他线索。

In file included from C:/Program Files/C++ Source/abseil-cpp/absl/strings/ascii.cc:15:
C:\Program Files\C++ Source\abseil-cpp\absl/strings/ascii.h(198,10):  error: no matching constructor for initialization of 'absl::string_view' (aka 'basic_string_view<char>')
  return absl::string_view(it, str.end() - it);
         ^                 ~~~~~~~~~~~~~~~~~~
C:\Program Files (x86)\Microsoft Visual Studio\2017\BuildTools\VC\Tools\MSVC\14.13.26128\include\xstring(809,12):  note: candidate constructor not viable: no known conversion from 'std::_String_view_iterator<std::char_traits<char> >' to 'const std::basic_string_view<char, std::char_traits<char> >::const_pointer' (aka 'const char *const') for 1st argument
        constexpr basic_string_view(_In_reads_(_Count) const const_pointer _Cts, const size_type _Count)
                  ^

在当前的MSVC std::_String_view_iterator<std::char_traits<char> >中似乎无法从const std::basic_string_view<char, std::char_traits<char> >::const_pointer转换为const char *const（aka include\xstring）

Clang和gcc对此很满意。

godbolt



更新资料

根据以下AlgirdasPreidžius和Hans Passant的评论，在将Godbolt编译器从MSVC 2017 RTM更改为MSVC Pre 2018之后，sv文字现在都可以使用了。

仍然有上面提到的另一个错误。

std::cout << absl::StrCat(sview_obj, "ever")    // Error

错误

<source>(18): error C2665: 'absl::StrCat': none of the 5 overloads could convert all the argument types
/opt/compiler-explorer/libs/abseil\absl/strings/str_cat.h(310): note: could be 'std::string absl::StrCat(const absl::AlphaNum &,const absl::AlphaNum &)'
<source>(18): note: while trying to match the argument list '(std::string_view, const char [5])'

goldbolt 2

最佳答案

可以从absl::AlphaNum创建absl::string_view。但不是来自std::string_view。您的代码无法构建，因为目前它们是不同的string_view类型。

原因是在编写此答案时，功能测试宏ABSL_HAVE_STD_STRING_VIEW被注释掉了。在该宏下，absl::string_view stands for std::string_view。在宏下，您的代码将生成。因此，可以说“故障”是由于Abesil团队尚未更新库而引起的。

目前，未来的解决方案是使用absl::string_view。

关于c++ - 在Abseil或MSVC中，此absl::StrCat错误在哪里出现？，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/51966426/