Question

Here is the code which causes a problem:

class Base
{
public:
    virtual void fun()
    {
        cout<<"Base";
    }
};
class Der:public Base
{
    Base &pb;
public:
    Der(Base&b):pb(b){}
    virtual void fun()
    {
        cout<<"Der...";
        pb.fun();
    }
};
int main()
{
    Der(Der(Base())).fun();
    return 0;
}

Run this code,and the result shows “Der…Base…”! This is so amazing, I can’t figure it out why the result is not “Der…Der…Base” which is logically right?!
Then I replace the member in class Der Base&pb with Base*pb and change the code into legal, finnaly the output is right which is “Der…Der…Base”!
I debug the code and find that when I use Base&pb, the constructor of Der only ran once while use Base*pb, the constructor ran twice correctly!
Any one who can explain to me what had happened and why?

Answer 1

In Der(Der(Base())).fun() expression the inner Der(Base()) yields an rvalue – the compiler optimizes the code by using copy elision and removes unnecessary copying of objects.