Question

// The constant base "a" that is being used to compute f_{ut}.
constexpr float A_CONST = 6.76;

// The max number of ratings by any given user on a given date. This
// was found by create_f_u_t.py.
constexpr int MAX_NUM_RAT_USER_DATE = 2651;

// The maximum possible value for f_{ut} is the floor of the log base
// "a" of the maximum number of ratings by any user on a given date.
auto BB = std::floor(std::log(MAX_NUM_RAT_USER_DATE)/std::log(A_CONST));


constexpr int MAX_F_U_T = BB;

它给了我错误！当我编译时，它

说：错误：constexpr变量＆＃39; MAX_F_U_T＆＃39;必须由a初始化不断表达 constexpr int MAX_F_U_T = BB;

Answer 1

您可以在GCC中获得constexpr版std::floor和std::log，但我不认为它是ISO C ++。另外，请不要忘记BB constexpr。

#include <cmath>

int main()
{
  // The constant base "a" that is being used to compute f_{ut}.
  constexpr float A_CONST = 6.76;

  // The max number of ratings by any given user on a given date. This
  // was found by create_f_u_t.py.
  constexpr int MAX_NUM_RAT_USER_DATE = 2651;

  // The maximum possible value for f_{ut} is the floor of the log base
  // "a" of the maximum number of ratings by any user on a given date.
  constexpr auto BB = std::floor(std::log(MAX_NUM_RAT_USER_DATE)/std::log(A_CONST));

  constexpr int MAX_F_U_T = BB;
}

Demo on Wandbox

Answer 2

constexpr变量必须满足以下要求：


其类型必须是LiteralType。

必须立即初始化

其初始化的完整表达式，包括所有隐式转换，构造函数调用等，必须是常量表达式

检查constexpr了解详情

错误！ constexpr变量必须由常量表达式constexpr初始化

2 个答案: