C++ How to use variables in a dynamic library? [duplicate]

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• C++ static const class members initialization 3 answers

API.h (in dll & in the test.exe)

    class Math
    {
    public:
        // Pi || π = 3.14159265358979 
        static __declspec(dllexport) double Pi;

        // Returns the square root of a specified number.
        static __declspec(dllexport) double sqrt(double value);
        // Returns the cube root of a specified number.
        static __declspec(dllexport) double cbrt(double value);
    };

API.cpp (in the dll)

    #include <iostream>
    #include <math.h>

    double Math::Pi = 3.14159265358979;

    double Math::sqrt(double value)
    {
      double root = std::sqrt(value);
      return root;
    }
    double Math::cbrt(double value)
    {
     double root = std::cbrt(value)
     return root;
    }

Test.CPP

    #include <iostream>
    #include "API.h"
    #pragma comment(lib, "test.lib") //for test.dll
    using namespace std;

    int main()
    {
     cout << Math::cbrt(125); //works fine
     cout << Math::Pi; // Error 1   error LNK2001:
     getchar();
    }

Error 1 error LNK2001: unresolved external symbol "public: static double Math::Pi" (?Pi@Math@@2NA)