如何强制模板替换失败，除非T::answer是整型

How to force template substitution failure unless T::answer is in integral type?

本文关键字：answer 整型 除非 何强制 替换 失败      更新时间：2023-10-16

我有以下代码:

#include <type_traits>
struct SA {};
struct SB { static const int answer = 42; };
const int SB::answer;
template <typename T>
int F() {
  return T::answer;
}
int main(int argc, char **argv) {
  (void)argc; (void)argv;
  // F<SA>();  // I want to make this a template substitution failure.
  return F<SB>();  // This should still work.
}

我想使调用F<SA>()成为模板替换失败。我已经尝试将int F更改为typename std::enable_if<std::is_integral<T::answer>::value, int>::type F，但我得到了以下错误:

$ g++ --versiong++ (Ubuntu 4.8.2-19ubuntu1) 4.8.2
$ g++ -W -Wall -Wextra -Werror -std=c++0x -fno-diagnostics-show-caret ei.cc && ./a.out
ei.cc:8:55: error: to refer to a type member of a template parameter, use ‘typename T:: answer’ [-fpermissive]
ei.cc: In function ‘int main(int, char**)’:
ei.cc:16:20: error: no matching function for call to ‘F()’
ei.cc:16:20: note: candidate is:
ei.cc:8:76: note: template<class T> typename std::enable_if<std::is_integral<typename T::answer>::value, int>::type F()
ei.cc:8:76: note:   template argument deduction/substitution failed:
ei.cc: In substitution of ‘template<class T> typename std::enable_if<std::is_integral<typename T::answer>::value, int>::type F() [with T = SB]’:
ei.cc:16:20:   required from here
ei.cc:8:76: error: no type named ‘answer’ in ‘struct SB’

这是试图在struct SB中寻找名为answer的类型，但我想让它在struct SB中寻找名为answer的积分字段。

我发现将enable_if检查放在模板参数中使您的界面更清晰:

template <typename T,
          std::enable_if_t<std::is_integral<decltype(T::answer)>::value>* = nullptr>
int F() {
  return T::answer;
}

或者更清楚，使用R. Martinho Fernandes的Remastered enable_if.

namespace detail {
    enum class enabler {};
}
template <typename Condition>
using EnableIf = std::enable_if_t<Condition::value, detail::enabler>;
template <typename T, EnableIf<std::is_integral<decltype(T::answer)>>...>
int F() {
  return T::answer;
}

添加decltype(...)似乎可以工作:将int F更改为typename std::enable_if<std::is_integral<decltype(T::answer)>::value, int>::type F

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