Why is `const int operator[](const int index) const` preferred over `int operator[](const int index) const` when returning objects?

const Objects as Return Values: Beyond the Trivial

While it's widely accepted to use const in return types whenever possible, the question arises as to why const int operator[](const int index) const is preferred over int operator[](const int index) const. To delve into this, let's first explore the subtleties of const in return types.

As mentioned by "Effective C " Item 03, top-level const qualifiers on non-class return types are indeed ignored. Despite writing int const foo();, the return type remains int. However, this is not the case for reference return types. The difference between int& operator[](int index); and int const& operator[](int index) const; is crucial.

When returning class types, the const qualifier plays a significant role. If T const is returned, the caller is restricted from invoking non-const member functions on the returned object. Consider the following example:

<code class="cpp">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</code>

In this scenario, the const qualifier attached to gg() constrains the caller from accessing non-const member functions, such as f(), ensuring that the returned object's integrity is preserved.

Therefore, when returning const objects, the const qualifier on the return type serves a purpose beyond the trivial case of non-reference return types. It ensures that the caller interacts with the returned object in a consistent manner, preserving its internal state in accordance with the intent of the class design.

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