Autodesk的c++题.doc

1. What and why is explicit constructor and explicit copy constructor?2. What the difference between Struct and Class?3. What is vitrual function and why we need vtable?1.Explicit constructor and copy constructor is used to preventing implicit type cast.An explicit constructor can not take part in implicit conversions. It can only be used to explicit construct an objectConsider the Class below:class Apublic:A();A(int i);A(A& a);int i;virtual A();The following program will success to compile :A a = 10;orA a;a = 10;because A has a constructor that accept a Int as a argumentsThose codes above are equal toA tmp(10); /constructora(tmp); /copy constructororA tmp(10) ; /constructora = tmp; / operator =Note: explicit on a constructor with mutiple arguments has no effect, unless all but one of the arguments has a default value !2: differences between struct and class?3 difference between struct and class1)members in struct are by default public while members in class are by default private.2)inhert from a struct is by default public , inhert from a class is by default private.For examplestruct A;struct B : (public)A ;class A;class B: (private)A;3)class can be used as typename in template while struct can nottemplate(class T) /righttemplate(struct T) /wrong3: What is virtual function and why we need vtable?1) a virtual function is a function member of a class decleared with a virtual keyword2) it usually has a different functionality in the derived class3) A function is resolved at run-timeThe main difference between a non_virtual function and a virtual function is, the non_virtual function is resolved at compile time. This mechanism is called static_binding. As the virtual function is resolved at run time, This mechanism is know as dynamic_bindingv_table is kind of lookup table.It contains pointers to the virtual functions provided by a class, and each object of the class contains a pointer to its v_table(or vtables, in some multiple-inheritance situations)A v_table is created at compile time, so whether a object is referenced by any pointers