Why can we access a.x in void A::f(A a)?

Question

I am new to C++, and I don't know why this piece of code can pass.

#include<iostream>
using namespace std;
class A
{
    public:
        static void f(A a);
    private:
        int x;
};
void A::f(A a)
{
   
    cout<<a.x<<endl; 
}

int main(int argc, char const *argv[])
{
    A a;
    a.f(A());
    return 0;
}

Here x is a private member of class A, so why can we access a.x in void A::f(A a)?

I hope to understand how this works.