用户定义的文字参数不是constexpr？ [英] User defined literal arguments are not constexpr?

问题描述

我测试用户定义的文字。我想让 _fac 返回数字的阶乘。

让它调用 constexpr 函数工作，但它不让我做模板，因为编译器抱怨的参数不是，不能 constexpr 。 / p>

我很困惑这 - 不是字面量常量表达式？在 5_fac 中的 5 始终是一个可以在编译时评估的文字，所以为什么不能使用

  constexpr int factorial_function（int x）{
 return（x> 0）？ x * factorial_function（x-1）：1; 
} 
 
 constexpr int operator_fac（unsigned long long x）{
 return factorial_function（x）; // this works 
} 
  

第二种方法：

  template< int N> struct factorial {
 static const unsigned int value = N * factorial< N-1> :: value; 
}; 
模板<> struct factorial< 0> {
 static const unsigned int value = 1; 
}; 
 
 constexpr int operator_fac（unsigned long long x）{
 return factorial_template< x> :: value; //不工作 -  x不是constexpr 
} 
  

解决方案

这是我最后做的：

  template< typename t> 
 constexpr t pow（t base，int exp）{
 return（exp> 0）？ base * pow（base，exp-1）：1; 
}; 
 
 template< char ...> struct literal; 
模板<> struct literal<> $ {
 static const unsigned int to_int = 0; 
}; 
 template< char c，char ... cv> struct literal< c，cv ...> {
 static const unsigned int to_int =（c  - '0'）* pow（10，sizeof ...（cv））+ literal< cv ...& 
}; 
 
 template< int N> struct factorial {
 static const unsigned int value = N * factorial< N-1> :: value; 
}; 
模板<> struct factorial< 0> {
 static const unsigned int value = 1; 
}; 
 
 template< char ... cv> 
 constexpr unsigned int operator_fac（）
 {
 return factorial< cv ...> :: to_int> :: value; 
} 
  

非常感谢KerrekSB！

I'm testing out user defined literals. I want to make _fac return the factorial of the number.

Having it call a constexpr function works, however it doesn't let me do it with templates as the compiler complains that the arguments are not and cannot be constexpr.

I'm confused by this - aren't literals constant expressions? The 5 in 5_fac is always a literal that can be evaluated during compile time, so why can't I use it as such?

First method:

constexpr int factorial_function(int x) {
  return (x > 0) ? x * factorial_function(x - 1) : 1;
}

constexpr int operator "" _fac(unsigned long long x) {
  return factorial_function(x); // this works
}

Second method:

template <int N> struct factorial {
  static const unsigned int value = N * factorial<N - 1>::value;
};
template <> struct factorial<0> {
  static const unsigned int value = 1;
};

constexpr int operator "" _fac(unsigned long long x) {
  return factorial_template<x>::value; // doesn't work - x is not a constexpr
}

解决方案

This is how I ended up doing it:

template <typename t>
constexpr t pow(t base, int exp) {
  return (exp > 0) ? base * pow(base, exp-1) : 1;
};

template <char...> struct literal;
template <> struct literal<> {
  static const unsigned int to_int = 0;
};
template <char c, char ...cv> struct literal<c, cv...> {
  static const unsigned int to_int = (c - '0') * pow(10, sizeof...(cv)) + literal<cv...>::to_int;
};

template <int N> struct factorial {
  static const unsigned int value = N * factorial<N - 1>::value;
};
template <> struct factorial<0> {
  static const unsigned int value = 1;
};

template <char ...cv>
constexpr unsigned int operator "" _fac()
{
  return factorial<literal<cv...>::to_int>::value;
}

Huge thanks to KerrekSB!

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