爲什麼沒有在棧更新值eventough其參考

我有我有同一個問題，下面的結構和類定義：爲什麼沒有在棧更新值eventough其參考

struct customer{ 
    string fullname; 
    double payment; 
}; 

class Stack{ 
private: 
    int top; 
    customer stack[10]; 
    bool full; 
    double sum; 
public: 
    Stack(){ 
     top=0; 
     full=false; 
     double sum=0.0; 
    } 

    bool isFull(){ 
     return full; 
    } 

    void push(customer &c){ 
     if(!full) 
      stack[top++]=c; 
     else 
      cout << "Stack full!" << endl; 
    } 

    void pop(){ 
     if(top>0){ 
      sum+=stack[--top].payment; 
      cout << "Cash status: $" << sum << endl; 
     } 
     else 
      cout << "Stack empty!" << endl; 
    } 
};

我在主運行下面的代碼：

int main(){ 
    customer c1 = {"Herman", 2.0}; 
    customer c2 = {"Nisse", 3.0}; 
    Stack stack = Stack(); 
    stack.push(c1); 
    stack.push(c2); 
    c2.payment=10.0; 
    cout << c2.payment << endl; 
    stack.pop(); 
    stack.pop(); 
    return 0; 
}

爲什麼總和不等於12？我指定推構造器是：void push(customer &c)。代碼的輸出是：