Item 3: Use const Whenever Possible

You may add const to almost anything.

• “STL iterators are modeled on pointers.”

• const iterator is like T * const and const_iterator is like const T *.

• operator* should return const.

const Rational operator*(const Rational& lhs, const Rational& rhs);
// This helps compiler catch bug like:
if (a * b = c) { ... }

• “Member function differing only in their constness can be overloaded.”

// Note: You should not return a writable reference
// from a const member function.
const char& operator[](std::size_t position) const { ... }
char& operator[](std::size_t position) { ... }

• const member functions: Compiler checks bitwise constness but you usually want logical constness.

• Logical constness: You may modify the object, but “only in ways that clients cannot detect” (like caching stuff internally).

• You may implement non-const version from const member function using static_cast and const_cast (but not const-version from non-const member function). This is not pretty but better than duplicating code (especially when the member function is not just one-liner).

return const_cast<char&>(static_cast<const Foo&>(*this)[position]);