Monday, January 30, 2012

"this" (C++)

this是一個被C++保留的詞彙,即他有特殊意義,不能拿來當作變數的名字。

  • 用在:某個class裡面
  • 意義:指向自己這個物件的指標
  • 注意:他是指標,並非說該物件中又包含該物件自己的遞迴,而只是儲存自己的記憶體位置
故讀取自己class中的變數,有以下幾種方法:
  1. 直接讀取;如Class (C++)中的studentName
  2. 讀取指標內的值;如this->studentName
  3. 先讀取指標指向的物件,再找尋物件中的值;如(*this).studentName
第1種方法是最簡單的,然而在遇到以下列舉的情況時,我們必須透過第2或3的方法才能解決:
  • local variable、function argument variable通常優先權高於class variable,即有同名的local variable、function argument variable出現時,就沒辦法用該名字讀到class variable,此時,可以加寫this找尋回class variable
  • 串列寫法,如果我們需要回傳物件自己,就會需要用到return *this。
    • 串列寫法:myClass.setA(1).setB(2).setC(3)
    • setA, setB, setC需要回傳*this方可以讓下一個"."符號串起來


myClass.h
class MyClass{
 public:
  MyClass(int = -1, int = -1, int = -1);
  // Return the reference of this object
  MyClass &setA(int);
  MyClass &setB(int);
  MyClass &setC(int);
  void display();
 private:
  int a;
  int b;
  int c;
};

myClass.cpp
#include <iostream>
#include "myClass.h"

using namespace std;

// constructor
MyClass::MyClass(int new_a, int new_b, int new_c){
 a = new_a;
 b = new_b;
 c = new_c;
}

// setA
MyClass &MyClass::setA(int new_a){

 cout << "function argument new_a = " << new_a << endl;

 a = new_a;
 return *this;
}

// setB
MyClass &MyClass::setB(int b){

 cout << "function argument b = " << b << endl;
 cout << "this.b = " << this->b << endl;

 this->b = b;
 return *this;
}

// setC
MyClass &MyClass::setC(int c){

 cout << "function argument c = " << c << endl;
 cout << "this.c = " << (*this).c << endl;
 // *this.c is a wrong usage since "this.c" isn't a pointer

 (*this).c = c;
 return *this;
}

// display the member values(a, b, c)
void MyClass::display(){
 cout << "a=" << a << "\tb=" << b << "\tc" << c << endl;
}

main.cpp
#include <iostream>
#include "myClass.h"

using namespace std;

int main(){

 // create a new class instance
 MyClass myClass;

 // setup
 myClass.setA(10).setB(3).setC(2);

 // display it
 myClass.display();

 return 0;
}

1 comment:

  1. Thanks for all the helpful insights. we are going to be change this post with new info. This is very a beautiful post. I appreciate your exertions. Keep posting new updates with us.

    Dedicated hosting

    ReplyDelete