深入C++混乱地带 001 函数参数默认值

#include <iostream>#include <ctime>externtime_ttime(time_t*=nullptr);
intmain()
{
std::cout<<time() <<std::endl;
// usuallystd::cout<<time(nullptr) <<std::endl;
return0;
}

#include <iostream>usingnamespacestd;
voidshow_point(intx, inty, intz)
{
cout<<x<<" "<<y<<" "<<z<<endl;
}
externvoidshow_point(intx, inty, intz=0);
externvoidshow_point(intx, inty=0, intz);
externvoidshow_point(intx=0, inty, intz);
intmain()
{
show_point(1, 2, 3);
show_point(1);
show_point(1, 2);
return0;
}

#include <iostream>usingnamespacestd;
classA{
public:
virtualvoidrun(intx=0)
    {
cout<<"this is A x="<<x<<endl;
    }
virtualvoidrun_x0()
    {
run();
    }
virtual~A() =default;
};
classB : publicA{
public:
virtualvoidrun(intx=1) override    {
cout<<"this is B x="<<x<<endl;
    }
};
intmain()
{
Bb;
B*pb=&b;
A*pa=pb;
pb->run(); // this is B x=1pa->run(); // this is B x=0 （用A的默认参数调用了B的run）pb->run_x0(); // this is B x=0 （同pa->run())pa->run_x0(); // this is B x=0 （同pa->run())return0;
}