Question

In Effective C++ Item 03, Use const whenever possible.

class Bigint
{
  int _data[MAXLEN];
  //...
public:
  int& operator[](const int index) { return _data[index]; }
  const int operator[](const int index) const { return _data[index]; }
  //...
};

const int operator[] does make difference from int& operator[].

But what about:

int foo() { }

and

const int foo() { }

Seems like that they’re the same.

My question is, why we use const int operator[](const int index) const instead of int operator[](const int index) const ?

Answer 1

Top level cv-qualifiers on return types of non class type are ignored.
Which means that even if you write:

int const foo();

The return type is int. If the return type is a reference, of course,
the const is no longer top level, and the distinction between:

int& operator[]( int index );

and

int const& operator[]( int index ) const;

is significant. (Note too that in function declarations, like the above,
any top level cv-qualifiers are also ignored.)

The distinction is also relevant for return values of class type: if you
return T const, then the caller cannot call non-const functions on the
returned value, e.g.:

class Test
{
public:
    void f();
    void g() const;
};

Test ff();
Test const gg();

ff().f();             //  legal
ff().g();             //  legal
gg().f();             //  **illegal**
gg().g();             //  legal