索引运算符constness

Question

为什么我们需要两个？在哪种情况下调用以下每个operator []？

  class X {
      public:
        //...
        int &operator [](int index);
        const int &operator [](int index) const;
    };

Answer 1

foo( X x )
{
  x[0];  // non-const indexer is called
}

bar ( const X x )
{
  x[0]; //const indexer is called
}