I try following code

#include "iostream"

class Cls {
public:
    // Assume Cls has an array-like structure
    int operator[](int idx) {
        // Provide access to an element
        std::cout << idx << std::endl;
        return 0;
    }

    void operator[]= (int idx, int newV) {

    }
};

int main() {
    // Create an instance of Cls
    auto cls = Cls();

    // Assign a new value to the i-th element
    int i = 5; // Example index
    int newV = 10; // Example new value
    cls[i] = newV;

    // Now cls[i] will have the value of newV
    return 0;
}

void operator[]= (int idx, int newV) { raise err ‘operator[]’ cannot be the name of a variable or data member