如何在c＆＃x2b;＆＃x2b;中定义编译时间三元字面字面？

Question

在第19章的第4章 c ++编程语言书籍，有一个使用模板技术来定义三元编号的示例，但是该示例没有编译。我试图以对我看上去正确的方式修复它，但仍然没有编译。

#include <cstdint>
#include <iostream>

using namespace std;

constexpr uint64_t ipow(uint64_t x, uint64_t n)
{
  return n > 0 ? x * ipow(x, n - 1) : 1;
}

template <char c>
constexpr uint64_t base3()
{
  static_assert(c >= '0' && c <= '2', "Not a ternary digit.");
  return c - '0';
}

template <char c, char... tail>
constexpr uint64_t base3()
{
  static_assert(c >= '0' && c <= '2', "Not a ternary digit.");
  return ipow(3, sizeof...(tail)) * (c - '0') + base3<tail...>();
}

template <char... chars>
constexpr uint64_t operator""_b3()
{
  return base3<chars...>();
}

int main()
{
  cout << 1000_b3 << endl;
  return 0;
}

clang 给出以下错误：

error: call to 'base3' is ambiguous
  return ipow(3, sizeof...(tail)) * (c - '0') + base3<tail...>();
                                                ^~~~~~~~~~~~~~
<source>:22:49: note: in instantiation of function template specialization 'base3<'0', '0'>' requested here
<source>:22:49: note: in instantiation of function template specialization 'base3<'0', '0', '0'>' requested here
<source>:28:10: note: in instantiation of function template specialization 'base3<'1', '0', '0', '0'>' requested here
  return base3<chars...>();
         ^
<source>:33:15: note: in instantiation of function template specialization 'operator""_b3<'1', '0', '0', '0'>' requested here
  cout << 1000_b3 << endl;
              ^
<source>:12:20: note: candidate function [with c = '0']
constexpr uint64_t base3()
                   ^
<source>:19:20: note: candidate function [with c = '0', tail = <>]
constexpr uint64_t base3()
                   ^
1 error generated.

定义它的正确方法是什么？

Answer 1

当前，当tail只有1个字符（当用用户定义的文字定义的最后一个数字调用时，它可以调用base3 base3 base3 < /code>

template <char c>
constexpr uint64_t base3()  // With c as '0'
template <char c, char... tail>
constexpr uint64_t base3()  // With c as '0' and tail as an empty parameter pack

没有理由比另一个更喜欢一个，因此这是模棱两可的。

您需要第二个过载以无法完全处理1个参数，因此您可以确保它至少需要2个参数：

template <char c, char second, char... tail>
constexpr uint64_t base3()
{
  static_assert(c >= '0' && c <= '2', "Not a ternary digit.");
  return ipow(3, 1+sizeof...(tail)) * (c - '0') + base3<second, tail...>();
}

或使用Sfinae进行：

template <char c, char... tail>
constexpr
typename std::enable_if<sizeof...(tail) != 0, uint64_t>::type
base3()
{
  static_assert(c >= '0' && c <= '2', "Not a ternary digit.");
  return ipow(3, sizeof...(tail)) * (c - '0') + base3<tail...>();
}

或将其更改为0个字符的基本案例：

template <typename = void>  // Can be called with an empty set of template args
constexpr uint64_t base3()
{
  return 0;
}

template <char c, char... tail>
constexpr uint64_t base3()
{
  static_assert(c >= '0' && c <= '2', "Not a ternary digit.");
  return ipow(3, sizeof...(tail)) * (c - '0') + base3<tail...>();
}