UpdatedAug 4, 2026

Const Correctness

Question Variations

"Why should getters usually be marked `const`?"
"What is the difference between `const int*` and `int* const`?"
"Can a const object call a non-const member function?"
"When is `mutable` appropriate?"

Why This Is Asked

Const correctness is a core C++ API-design tool. This question tests whether you can express read-only intent, understand how const affects member functions and pointer declarations, and avoid weakening type guarantees with unnecessary casts.

Key Concepts

A const member function cannot modify non-mutable members through this.
Const and non-const overloads can return appropriately qualified references.
const T*, T* const, and const T* const qualify different parts of a pointer declaration.
const prevents modification through that access path; it does not necessarily mean the underlying object is immutable.

Question Variations

“Why should getters usually be marked const?”
“What is the difference between const int* and int* const?”
“Can a const object call a non-const member function?”
“When is mutable appropriate?”

Answers by Technology

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Expected Answer (C++23)

Const correctness makes mutation permissions part of an API. A const member function receives a pointer to const and cannot modify ordinary members. This allows callers with a const object or reference to use read-only operations safely.

For pointer declarations, read from the identifier outward: const T* points to a const T; T* const is a const pointer to mutable T; and const T* const is a const pointer to const T.

#include <string>

class User {
public:
    explicit User(std::string name) : name_(std::move(name)) {}

    const std::string& name() const { return name_; }
    void rename(std::string name) { name_ = std::move(name); }

private:
    std::string name_;
};

int main() {
    const User user("Ada");
    return user.name() == "Ada" ? 0 : 1;
}

Why It Matters

Const-correct APIs prevent accidental state changes and make code easier to reuse with read-only inputs. They also enable overloads and compiler diagnostics that expose incorrect ownership or mutation assumptions early.

Common Mistakes

Omitting const from read-only member functions: Const objects then cannot use otherwise safe operations.
Returning a mutable reference from a const getter: This lets callers modify an object through a read-only interface.
Using const_cast to bypass a real const object: Modifying an object originally declared const is undefined behavior.

Follow-up Questions

When is mutable appropriate? (Answer: For implementation details such as a cache or mutex that do not change the object’s logical value.)
Can a non-const object call a const member function? (Answer: Yes; const member functions are callable on both const and non-const objects.)