constexpr 变量必须由常量表达式初始化

Question

I'm programming with C++Builder 10.3 from Embarcadero Technology which uses a c++17 compiler. I did not write the code below and it is a little more complex than I am used to working with. I have never used constexpr before.

At the bottom of the code below when the compiler hits, "constexpr array g_PERT_S1" it gives the error "constexpr variable 'g_PERT_S1' must be initialized by a constant expression. non-constexpr function 'data' cannot be used in a constant expression array(226): declared here" Can you recommend a code change to resolve this error.

struct AAPLUS_EXT_CLASS ELPMPP02PertubationsCoefficient
{
  double m_S;
  double m_C;
  array<int, 13> m_I;
};

constexpr array<ELPMPP02PertubationsCoefficient, 2> g_PERT_S3_4
{ {
 { -5.458720424980e-07,  2.801517894073e-07, {   0,   0,   2,   0,   0, -18,  16,   0,   0,   0,   0,   0,   0 } },
 { -5.121329506146e-07, -2.627345838573e-07, {   0,   0,   0,   0,   0,  18, -16,   0,   0,   0,   0,   0,   0 } }
} };


struct AAPLUS_EXT_CLASS ELPMPP02Pertubations
{
  const ELPMPP02PertubationsCoefficient* m_pTable;
  size_t m_nTableSize;
};


/*constexpr variable 'g_PERT_S1' must be initialized by a constant expression
non-constexpr function 'data' cannot be used in a constant expression array(226): declared here*/

constexpr array<ELPMPP02Pertubations, 4> g_PERT_S1  /*ERROR HERE*/
{ {
  { g_PERT_S1_1.data(), g_PERT_S1_1.size() },
  { g_PERT_S1_2.data(), g_PERT_S1_2.size() },
  { g_PERT_S1_3.data(), g_PERT_S1_3.size() },
  { g_PERT_S1_4.data(), g_PERT_S1_4.size() }
} };

Answer 1

It's quite explicit indeed ( I assume "array" is indeed "std::array"
https://en.cppreference.com/w/cpp/container/array ,

and that you have a using namespace std somewhere...

the constexpr g_PERT_S1 can only accept something build with constexpr.

so one of the g_PERT_S1_1 to g_PERT_S1_4 is not a constexpr.

But as pointed in the comment, your example is not complete as we do not have those 4 definitions.

The size() is and data() member function of std::array are constexpr in C++17 but obviously they will return a constexpr only if the array is a constexpr.