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Item 3: Use const whenever possible

Concept​

const communicates semantic constraints to compilers and other programmers. It can apply to:

  • Pointers (const char* vs char* const)
  • Iterators (const_iterator)
  • Function return types and parameters
  • Member functions (logical constness via mutable)

Code Example​

// const with pointers
const char* p1 = "Hello";    // pointer to const data
char* const p2 = buf;        // const pointer to non-const data

// const member functions
class TextBlock {
public:
    const char& operator[](std::size_t pos) const {
        return text[pos];
    }
    char& operator[](std::size_t pos) {
        return const_cast<char&>(
            static_cast<const TextBlock&>(*this)[pos]
        );
    }
private:
    std::string text;
};

Full source code

Things to Remember​

  • Declaring something const helps compilers detect usage errors.
  • Compilers enforce bitwise constness, but you should program using logical constness.
  • When const and non-const member functions have essentially identical implementations, the non-const version can call the const version to avoid duplication.
  • Item 2 — Prefer consts to #defines
  • Item 21 — Don't try to return a reference when you must return an object