如何在c++中定义编译时三元文字?

Question

In第十九章4在c++程序设计语言的版本中，有一个使用模板技术定义三进制数字字面量的示例，但该示例无法编译。我试着以我认为正确的方式修复它，但它仍然无法编译。

#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;
}

叮当声给出以下错误:

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.

如何正确定义它?

Answer 1

目前，当tail只有1个字符时(当使用用户定义文字的最后一个数字'0'调用时)，它可以调用base3的过载

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

没有理由偏爱其中一个而不喜欢另一个，因此它是二义性的。

你需要第二个重载不能只使用一个参数，所以你可以确保它至少使用两个参数:

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...>();
}

或者使用SFINAE:

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...>();
}

或者将其更改为0个字符的基本大小写:

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...>();
}