const method specifier stop the method from writing to class members. It throws compile time error for writes.

Example Link to heading

class cls
{

public:
    int x;
    void func() const
    {
        x = 0;
    }
};
int main()
{
}

const.cc:8:11: error: assignment of member ‘cls::x’ in read-only object
    8 |         x = 0;
      |         ~~^~~

Note that const methods can only call constant methods.

Work around const-ness Link to heading

To force write inside const method, const_cast can be used to remove const-ness.

#include <iostream>
using namespace std;
class cls
{

public:
    int x;
    void func() const
    {
        // x = 0;
        const_cast<int &>(x) = 3;
        std::cout << x << endl;
    }
};
int main()
{
    cls c;
    c.func();
}