constcorrectness

Const correctness is a design principle in programming, especially in C and C++, that requires code to respect immutability guarantees conveyed by const qualifiers. Declaring data as const and marking functions as const when they do not mutate their object helps the compiler enforce immutability, catch unintended side effects, and improve code reliability. It also enables potential optimizations by allowing the compiler to assume that certain data will not change.

The concept involves several interrelated ideas. Top-level const vs low-level const distinguishes whether the object or the data it points to is immutable; pointer and reference qualifiers differentiate what may be changed. A function parameter declared as const T& promises not to modify the passed object. A member function declared as const promises not to alter the observable state of the object (aside from members declared mutable). In C, const can apply to pointers to data, yielding distinctions like pointer-to-const data versus const pointer (the pointer itself being immutable).

Common examples include: void print(const std::string& s); which guarantees s will not be changed, or int getValue() const; inside a class to indicate the method does not modify the object. Code examples also demonstrate various forms of pointer constness, such as const int* p (pointer to const int) and int* const p (const pointer to int).

Pitfalls include using const_cast to remove constness, which is only safe if the original object was not const. Violating const correctness can lead to undefined behavior. Best practices advocate designing APIs with appropriate constness, providing const and non-const overloads where needed, and keeping internal data encapsulated behind const-qualified accessors. History-wise, const qualifiers appeared with early C and matured in C++; today const correctness remains central to robust C/C++ design.