Answer.

History of the question:

The const qualifier in C++ was introduced for safety reasons: to mark functions that do not modify the internal state of an object. In the context of classes, const methods are essential for projects requiring strict encapsulation and protection against accidental errors.

Problem:

If getters, helper functions, and checking methods are not marked as const, they cannot be called on const objects. Without const methods, the compiler cannot prevent unwanted data modifications.

Solution:

Add const in the method declaration after the signature (not before it!) This allows calling the method for both normal objects and those declared as const, eliminating state modification errors. Within such a method, class members cannot be changed (except for mutable), only reading is allowed.

Code example:

class Counter {
    int value;
public:
    int getValue() const { return value; } // Can be called for const Counter
};

void print(const Counter& c) {
    std::cout << c.getValue();
}

Key features:

const methods cannot change the object's state (except for mutable fields)
const methods can be called for const objects and const references
const methods can also be called for non-const objects

Tricky questions.

Where should const be placed in a method: before the return type or after the parentheses?

Only after the parentheses: int getValue() const; — it is incorrect to place it before the return type (for example, const int get();).

Can a const method call non-const methods of the class?

No, a const method can only call other const methods if the object is const.

What is the difference between const in a method and const in the return value?

int get() const (the method does not change state) const int get(); (the return value cannot be modified, but the method can change internal data)

Typical mistakes and anti-patterns

Not marking as const methods (getters and checks) that should be
Changing the object's state through const methods via cast or mutable
Placing const before the return type, thinking it makes the method const

Example from life

Negative case

In the class, const is missing from a method that does not change state. It cannot be called for a constant object, which hampers the use of the class in analysis tools and when working with APIs.

Pros:

No need to think about where to place const, code is written faster

Cons:

It becomes impossible to use objects via const reference
Modification of the object is allowed where it was not intended

Positive case

All getters, checks, and helper methods are declared with const, tests cover both const and non-const objects.