为什么我在以下代码段中收到UMR（未初始化的内存读取）错误

Question

Why I am getting UMR(unInitialized Memory Read)Error in following code snippet. I am getting UMR error while calling copy constructor of derived class

class base{
public :

    base(int);
    int id;

    int getid()
    {
        return id;
    }
};

base::base(int a)
{
    id = a;
}

class derived:public base{
public :
    derived(derived &);
    int inum;

    int getnum()
    {
        return inum;
    }

};

derived::derived(derived &d):base(d.getnum()) // UMR Error
{
    inum = id;
}

Can anybody have any idea what is wrong with the implementation?

Answer 1

You derived class never initialize inum variable. Error message said it to you. d.getnum() returns uninitialized inum variable.

Answer 2

I don't think you can compile your code, a few issues dressed below:

class derived:class base{
               ^^^-> public

should be:

class derived : public base

change int getnum(); to int getnum() const;

derived::derived(derived &d):base(d.getnum) // UMR Error
                                         ^^^ function call? 

why initialize base::id with derived::inum?

should be:

   derived::derived(const derived &d) : base(d) // initialize base with base
   {
                    ^^^  copy constructor has const 
      inum = d.inum;  // initialize derived member
      // inum = id;  why you assign id back to inum again?
   }