C ++ uses [] assignment

class Test 
{
public:
    const int& operator[]( const int& index) 
    {
        std::cout << "[]" << std::endl;
        return 2;
    }
    int& operator[]( int&& index)
    {
        std::cout << "[][]" << std::endl;
        int j = 3;
        return j;
    }
    const int& operator=(const int& index) 
    {
        std::cout << "=" << std::endl;
        return 3;
    }
    friend std::ostream& operator<<(std::ostream& out, const Test&)
    {
        out <<1 << endl;
        return out;
    }
};
int main () 
{
    Test t;
    const int i = 12;
    int k = t[1] = i;
    cout << k << endl;
    cout << t[i] << endl;
  return 0;
} 

Code:

t[1] = i;

Invokes the argument is int && [] overloaded functions, that is,

int& operator[](int&& index)
    {
        std::cout << "[][]" << std::endl;
        int j = 3;
        return j;
    }

Note that this function can not return const int &, because you can not assign to a const.

Meaning of this sentence is the first t [1], and then assign i