C++ 面试题与答案
相关差异对比
问题 21. Define a constructor - What it is and how it might be called
(2 methods).
Answer1
constructor is a member function of the class, with the name of the function being the same as the class name. It also specifies how the object should be initialized.
Ways of calling constructor:
1) Implicitly: automatically by complier when an object is created.
2) Calling the constructors explicitly is possible, but it makes the code unverifiable.
Answer2
class Point2D{
int x; int y;
public Point2D() : x(0) , y(0) {} //default (no argument) constructor
};
main(){
Point2D MyPoint; // Implicit Constructor call. In order to allocate memory on stack, the default constructor is implicitly called.
Point2D * pPoint = new Point2D(); // Explicit Constructor call. In order to allocate memory on HEAP we call the default constructor.
问题 22. What is the difference between class and structure?
Answer1
1.) ?new and delete? are preprocessors while ?malloc() and free()? are functions. [we dont use brackets will calling new or delete].
2.) no need of allocate the memory while using ?new? but in ?malloc()? we have to use ?sizeof()?.
3.) ?new? will initlize the new memory to 0 but ?malloc()? gives random value in the new alloted memory location [better to use calloc()]
Answer2
new() allocates continous space for the object instace
malloc() allocates distributed space.
new() is castless, meaning that allocates memory for this specific type,
malloc(), calloc() allocate space for void * that is cated to the specific class type pointer.
问题 23. What is RTTI?
Runtime type identification (RTTI) lets you find the dynamic type of an object when you have only a pointer or a reference to the base type. RTTI is the official way in standard C++ to discover the type of an object and to convert the type of a pointer or reference (that is, dynamic typing). The need came from practical experience with C++. RTTI replaces many Interview Questions - Homegrown versions with a solid, consistent approach.
问题 24. What is encapsulation?
Packaging an object?s variables within its methods is called encapsulation.
问题 25. Explain term POLIMORPHISM and give an example using eg. SHAPE object: If I have a base class SHAPE, how would I define DRAW methods for two objects CIRCLE and SQUARE
Answer1
POLYMORPHISM : A phenomenon which enables an object to react differently to the same function call.
in C++ it is attained by using a keyword virtual
Example
public class SHAPE
{
public virtual void SHAPE::DRAW()=0;
}
Note here the function DRAW() is pure virtual which means the sub classes must implement the DRAW() method and SHAPE cannot be instatiated
public class CIRCLE::public SHAPE
{
public void CIRCLE::DRAW()
{
// TODO drawing circle
}
}
public class SQUARE::public SHAPE
{
public void SQUARE::DRAW()
{
// TODO drawing square
}
}
now from the user class the calls would be like
globally
SHAPE *newShape;
When user action is to draw
public void MENU::OnClickDrawCircle(){
newShape = new CIRCLE();
}
public void MENU::OnClickDrawCircle(){
newShape = new SQUARE();
}
the when user actually draws
public void CANVAS::OnMouseOperations(){
newShape->DRAW();
}
Answer2
class SHAPE{
public virtual Draw() = 0; //abstract class with a pure virtual method
};
class CIRCLE{
public int r;
public virtual Draw() { this->drawCircle(0,0,r); }
};
class SQURE
public int a;
public virtual Draw() { this->drawRectangular(0,0,a,a); }
};
Each object is driven down from SHAPE implementing Draw() function in its own way.
4) What is encapsulation? 5) Explain term POLIMORPHISM and give an example using eg. SHAPE object: If I have a base class SHAPE, how would I define DRAW methods for two objects CIRCLE and SQUARE " />
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