c++ - 错误! constexpr 变量必须由常量表达式 constexpr 初始化

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;

它给我错误!当我编译时，它

says: error: constexpr variable 'MAX_F_U_T' must be initialized by a constant expression constexpr int MAX_F_U_T = BB;

Answer 1

您可以在 GCC 中获得 std::floor 和 std::log 的 constexpr 版本，但我不认为它是ISO C++。也不要忘记将 BB 也设为 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