无法创建字符串文字类型

Question

我想创建一个我可以用作模板参数的字符串文字.它将编译器抛入某种无限循环.有什么问题并修复？

template <char...> struct slit { };

template <typename ...A>
constexpr auto make_slit(char const* const s, A const ...args)
{
  return *s ? make_slit(s + 1, *s, args...) : slit<args...>();
}

int main()
{
  auto const tmp_(make_slit("slit"));

  return 0;
}

强制性错误(带clang++ -std=c++1y):

t.cpp:4:16: fatal error: recursive template instantiation exceeded maximum depth of 256
constexpr auto make_slit(char const* const s, A const ...args)
               ^
t.cpp:6:15: note: in instantiation of function template specialization 'make_slit<char, char, char, char, char, char, char, char, char, char, char, char, char, char, char,
      char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char,
      char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char,
      char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char,
      char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char,
      char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char,
      char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char,
      char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char,
      char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char,
      char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char, char>' requested here
  return *s ? make_slit(s + 1, *s, args...) : slit<args...>();

Answer 1

抛开循环实例化不谈，你无法实现你想要的，因为函数参数不能用作常量表达式，而这是模板参数所必需的。这意味着以下内容也是不允许的：

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template <typename... A>\nconstexpr auto make_slit(A const... args)\n{\n  return slit<args...>();\n}\n\n// error\nmake_slit(\'a\');\n

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如果您对此感到惊讶，请记住 constexpr 函数是一项允许某些函数在常量表达式中使用的功能。不过，你的一般是\xe2\x80\x99t：

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char c;\nstd::cin >> c;\n// what is the result type?\nmake_slit(c);\n

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然而，我应该注意，在设计文字字符串运算符期间，建议允许使用函数模板形式（很像它们用于整数和浮点文字），这将完全实现您所需要的：

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// allowed...\ntemplate<char... Cs>\nconstexpr slit<Cs...> operator"" _slit()\n{ return {}; }\n\n// ...but only for\nauto constexpr s = 123_slit;\nauto constexpr t = 12.3_slit;\n\n// ... and not\nauto constexpr u = "abc"_slit;\n

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这个缺失的功能最近在 2013 年由 Richard Smith\xe2\x80\x99s n3599: Literal Operator templates for strings提出。不幸的是我不知道该功能的当前状态是什么。

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