How to initialize a static array of pointer to function method c++

Question

I have a class that hold public static method corresponding to mathematical operations. The class is implemented like such

class test
{
    public:
        test(){};
        ~test(){};
        typedef int(test::*calcul)(int a,int b);
        /*STATIC METHOD*/
        static int add(int a, int b);
        static int divide(int a, int b);
        static int multiply(int a, int b);
        static int substract(int a, int b);
        static calcul _calcul[128];
    private:
        /* data */
};

I also added an array of pointer to function so that I can store these functions into that array and use them for later. However I have an error when I try to initialize the static array in the implementation file. Here is the error I get:

test.cpp:34:14: error: conflicting declaration ‘int (test::* test::_calcul [43])(int, int)’
   34 | test::calcul test::_calcul['+'] = test::add;
      |              ^~~~
test.cpp:15:23: note: previous declaration as ‘int (test::* test::_calcul [128])(int, int)’
   15 |         static calcul _calcul[128];

To initialize one of the indexex of the array here what I wrote:

test::calcul test::_calcul['+'] = test::add;

My assumption of the above code sample is that the _calcul array at index '+' will be initialized with the function pointer of the add static method however it is not what is actually happening.

Here is the full program:

#include <cstdlib>
#include <iostream>

class test
{
    public:
        test(){};
        ~test(){};
        typedef int(*calcul)(int a,int b);
        /*STATIC METHOD*/
        static int add(int a, int b);
        static int divide(int a, int b);
        static int multiply(int a, int b);
        static int substract(int a, int b);
        static calcul _calcul[128];
    private:
        /* data */
};

/*----------------------------------STATIC MEMBER FUNCTION----------------------------------*/
int test::add(int a, int b){return a + b;};
int test::substract(int a, int b){return a - b;};
int test::multiply(int a, int b){return a * b;};
int test::divide(int a, int b)
{
    if (b == 0)
    {
        std::cerr << "Error\n";
        exit(1);
    }
    return a / b;
};
/*----------------------------------STATIC MEMBER FUNCTION----------------------------------*/
test::calcul test::_calcul['+'] = test::add;

int main(void)
{
    test d;
}

Answer 1

What you wrote is a definition of an array, not an assignment to an array member. Like the error says, you can't declare and define the same array with different bounds.

C++ does not support designated initializers for arrays, so your best bet would be to use a std::array<calcul, 128> instead of a raw array and write a helper function or lambda that can be used to initialize the static member:

class test
{
public:
    using calcul = int(*)(int, int);
    //...
    static int add(int a, int b);
    static inline const std::array<calcul, 128> _calcul = []() {
        std::array<test::calcul, 128> ret;
        ret['+'] = &calcul::add;
        //...
        return ret;
    }();
    //...
};

Demo

Here I've used an immediately-invoked lambda instead of a separate helper function, but the latter would work as well.