Method add8

tests/test_methods_and_attributes.cpp:54–54

passing by reference

    void add6(int other) { value += other; }        // passing by value
    void add7(int &other) { value += other; }       // passing by reference
    void add8(const int &other) { value += other; } // passing by const reference
    // NOLINTNEXTLINE(readability-non-const-parameter) Deliberately non-const for testing
    void add9(int *other) { value += *other; }        // passing by pointer
    void add10(const int *other) { value += *other; } // passing by const pointer