虚拟函数和缺省实参 ----来自C ++ Primer

虚拟函数和缺省实参

#include <iostream>

class base
{
public:
    virtual int foo( int ival = 1024 )
    {
        cout < "base::foo() -- ival: " < ival < endl;
        return ival;
    }
    // ...
};

class derived : public base
{
public:
    virtual int foo( int ival = 2048 )
    {
        cout << "derived::foo() -- ival: " << ival << endl;
        return ival;
    }
    // ...
};

int main() {     derived *pd = new derived;     base *pb = pd;     int val = pb->foo();     cout << "main() : val through base: "          << val << endl;     val = pd->foo();     cout << "main() : val through derived: "          << val << endl; }

编译运行上面的代码，将输出：
derived::foo() -- ival: 1024
main() : val through base: 1024
 
derived::foo() -- ival: 2048
main() : val through derived: 2048

 在这两个调用中,foo() 的派生类实例被正确调用,这是因为foo() 调用的真正实例是
在运行时刻根据pd和pb指向的实际类型决定的;然而传递给foo() 的缺省实参不是在运行
时刻决定的,而是在编译时刻根据被调用函数的对象的类型决定的,当通过pb调用foo() 时,
缺省实参中base::foo()的声明决定为1024, 当通过pd调用foo() 时,缺省实参由derived::foo()
的声明决定为2048