why r-value is being assigned to variable inside array subscript

I'm having a problem in push function.Variable inside array subscript(top) is being initialized with r-value(item).

1.when I do the same operation with top=0 it works fine.

2.problem occurs only when top=-1.

3.I have printed the top value each time to ease up debugging.

    class stack
{
         int top;
         public:
         int arr[500];
    stack()
    {
        top=-1;
    }

    void push(int);

};

    void stack::push(int item)
{


    arr[top++]=item;                        //problem here
    cout<<"\n"<<item<<"pushed in stack";
    cout<<"\n top value is"<<top;
}


   int main()
{
    stack s;
    s.push(12);
    s.push(19);
    s.push(31);
    return 0;
}

I expected the output as"12 pushed in stack and top value =-1, but the actual output is 12 pushed in stack and top value is 12