Member Initialization List

Member Initialization List
<br />#include <string><br />Class Account<br />{<br />private:<br /> unsigned int _acct_nmbr;<br /> double _balance;<br /> string _name;<br />public:<br /> Account();<br />};<br />//-------------------example 1---------------<br />Account::Account()<br />{<br /> //assignment<br /> _name=""; <br /> _acct_nmbr=0;<br /> _balance<br />};<br /><br />//-------------------example 2---------------<br />Account::Account():_name(string()),_acct_nmbr(0),_balance(0.0)//member initialization <br />{<br />};<br />
常常有人搞不清楚example 1 & example 2的效果有何差異,在大多數的情況下,此兩方法的結果相等,
然而在某些情況下,變不一樣.因此往往也會有暗藏陷阱存在.
對於example 1而言,此方法稱之為在建構式中賦予值(assignment),此時對Class data member來說早已
執行過其construct,因此此時不過是在付予值而已,所以對_name=""變顯得沒必要.
對於example 2而言,此時期Data member尚未逕行Initialization,所以此時寫在:之後的都是指定
成員函數所使用的construct


1.在進行member initialization,其initialiation的順序是根據Class內的member順序而來,而非在
Initialization所指定的順序
2.每一個data member只能Initialization 一次
3.另外,對於兩種資料型態const ,reference 兩種值,因為要在事先必須付予值,所以就必須要採用
member initialization的方式指定值,而不是在事後的construct中賦予值


參考:C++ primer 3/e

Template

<br />//---------------------------------------------------------------------------<br /><br />#include <vcl.h><br />#pragma hdrstop<br /><br />#include "Unit1.h"<br />//---------------------------------------------------------------------------<br />#pragma package(smart_init)<br />#pragma resource "*.dfm"<br />TForm1 *Form1;<br />//---------------------------------------------------------------------------<br />__fastcall TForm1::TForm1(TComponent* Owner)<br /> : TForm(Owner)<br />{<br />}<br />//---------------------------------------------------------------------------<br /><br />void __fastcall TForm1::Button1Click(TObject *Sender)<br />{<br /> int intArray[20]={ 0,5,2,3,4,1,1,5,2,3,<br /> 5,4,2,1,5,2,3,4,1,2};<br /> for(int i=0;i<20;i++)<br /> {<br /> mIntStack.push(intArray[i]);<br /> }<br />}<br />//---------------------------------------------------------------------------<br />void __fastcall TForm1::Button2Click(TObject *Sender)<br />{<br /> while(!mIntStack.empty())<br /> {<br /> OutString.sprintf("%d",mIntStack.top());<br /> Memo1->Lines->Add(OutString);<br /> mIntStack.pop();<br /> }<br />}<br />//---------------------------------------------------------------------------<br />void __fastcall TForm1::Button3Click(TObject *Sender)<br />{<br /> mFloatStack=mIntStack;<br />}<br />//---------------------------------------------------------------------------<br />void __fastcall TForm1::Button4Click(TObject *Sender)<br />{<br /> float floatArray[20]={ 1.5,2.1,3.4,4.1,5.2,2.3,4.0,1.1,2.1,4.0,<br /> 2.1,3.2,4.1,5.5,4.1,4.5,5.7,0.9,1.5,2.3};<br /><br /> for(int i=0;i<20;i++)<br /> {<br /> mFloatStack.push(floatArray[i]);<br /> }<br />}<br />//---------------------------------------------------------------------------<br />void __fastcall TForm1::Button5Click(TObject *Sender)<br />{<br /> while(!mFloatStack.empty())<br /> {<br /> OutString.sprintf("%f",mFloatStack.top());<br /> Memo1->Lines->Add(OutString);<br /> mIntStack.pop();<br /> }<br />}<br />//---------------------------------------------------------------------------<br /><br />void __fastcall TForm1::Button6Click(TObject *Sender)<br />{<br /> mIntStack=mFloatStack;<br />}<br />//---------------------------------------------------------------------------<br /><br />void __fastcall TForm1::FormCreate(TObject *Sender)<br />{<br /><br />}<br />//---------------------------------------------------------------------------<br />