Why is `const int operator[](const int index) const` preferred over `int operator[](const int index) const` for non-class types in C ?

The Significance of Returning Const for Non-Class Types

Question:

In C , why do we need to use const int operator[](const int index) const instead of int operator[](const int index) const?

Answer:

For non-class types, top-level const qualifiers on return types are ignored. This means that the return type of both:

int foo() {}

and

const int foo() {}

is interpreted as int. However, when returning a reference, the const becomes non-top-level and makes a significant difference:

int& operator[](int index);

and

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

are distinct.

Similarly, for return values of class types, returning T const prevents the caller from calling non-const functions on the returned value:

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

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