Subscript operator should be "const"-overloaded

Usually, it is the container data structures that define operator[] to mimic the traditional C-array interface. The subscript operator looks up an element in the container. It can return the element by value or by (optionally const) reference. You need a non-const subscript operator returning non-const reference to the element to be able to modify it. You need a const subscript operator to make a const-qualified instance of the container useful and enable the read-access with a consistent lookup interface.

This rule raises an issue on a class that has a non-const operator[] overload that does not modify *this object but lacks a const overload.

Noncompliant Code Example

template<typename T>
class MyVector {
public:
  T& operator[] (unsigned index); // Noncompliant, missing const overload
};

Compliant Solution

template<typename T>
class MyArray {
public:
  const T& operator[] (unsigned index) const; // Compliant
  // const-only operator and the compiler will tell you
  // when you try to use it in a non-const context and need an overload
};
template<typename T>
class MyArray {
public:
  T const& operator[] (unsigned index) const;
  T& operator[] (unsigned index); // Compliant, const-overloaded
};

Exceptions

If operator[] might insert a new key, as it is done in std::map, or modifies the container in another externally-visible manner, there is no intuitive const overload for it.

See

• Isocpp - What's the deal with "const-overloading"?