Cannot call a method of const reference parameter in C++

Question

class A
{
public:
    A(){};
    ~A(){};
    void method(){};

};

void call(const A &a)
{
    a.method();   // I cannot call this method here if I use "const" but I can call it if not using "const"
}

int main()
{
    A a;
    call(a);
    return 0;
}

In this case, the error is: "passing const A as this argument of void A::method() discards qualifiers [-fpermissive]|"

In function call, if I use const, I get the error, but if I get rid of it, it works.

Can anyone explain it for me?

Answer 1

You can't call non-const member functions via const references. You can fix this by making the member function const:

void method() const {};
              ^^^^^

This indicates that calling the member does not mutate the object it is called on^*

^{* Conceptually. In practice it can mutate members marked mutable}