?
class
?CTestCopyConstruct
{
public
:
????CTestCopyConstruct()
{
????????TRACE(
"
Enter?CTestCopyConstruct();this?is?%d\n
"
,
this
);
????????strTest?
=
?
"
not?ok
"
;
????????i?
=
?
0
;
????}
????CTestCopyConstruct(
const
?CTestCopyConstruct?
&
src)
{
????????TRACE(
"
Enter?CTestCopyConstruct(const?CTestCopyConstruct?&src);this?is?%d;src?is?%d\n
"
,
this
,
&
src);
????????strTest?
=
?src.strTest;
????????i?
=
?src.i;
????}
????CTestCopyConstruct?
&
?operator?
=
(
const
?CTestCopyConstruct?
&
?src)
{
????????TRACE(
"
Enter?CTestCopyConstruct?&?operator?=(const?CTestCopyConstruct?&?src);this?is?%d;src?is?%d\n
"
,
this
,
&
src);
????????strTest?
=
?src.strTest;
????????i?
=
?src.i;
????????
return
?
*
this
;
????}
????CString?strTest;
????
int
?i;
}
;
CTestCopyConstruct?GetTest()
{
????CTestCopyConstruct?ret1;
????ret1.strTest?
=
?
"
ok
"
;
????ret1.i?
=
?
0
;
????CTestCopyConstruct?ret2;
????
return
?ret1;
}
void
?CTestDlg::OnOK()?
{
????CTestCopyConstruct?var1;
????CTestCopyConstruct?var2?
=
?GetTest();
????TRACE(
"
\nresult?1:\n
"
);
????TRACE(
"
var1?is?%d\n
"
,
&
var1);
????TRACE(
"
var2?is?%d?var2.str?is?%s\n\n
"
,
&
var2,var2.strTest);
????CTestCopyConstruct?var3?
=
?var2;
????CTestCopyConstruct?var4;
????var4?
=
?var2;
????TRACE(
"
\nresult?2:\n
"
);
????TRACE(
"
var3?is?%d?var3.str?is?%s\n
"
,
&
var3,var3.strTest);
????TRACE(
"
var4?is?%d?var2.str?is?%s\n
"
,
&
var4,var4.strTest);
}
代碼如上，調(diào)試窗口輸出如下：
Enter CTestCopyConstruct();this is 1242980
Enter CTestCopyConstruct();this is 1242848
Enter CTestCopyConstruct();this is 1242840
Enter CTestCopyConstruct(const CTestCopyConstruct &src);this is 1242972;src is 1242848
result 1:
var1 is 1242980
var2 is 1242972 var2.str is ok
Enter CTestCopyConstruct(const CTestCopyConstruct &src);this is 1242964;src is 1242972
Enter CTestCopyConstruct();this is 1242956
Enter CTestCopyConstruct & operator =(const CTestCopyConstruct & src);this is 1242956;src is 1242972
result 2:
var3 is 1242964 var3.str is ok
var4 is 1242956 var2.str is ok
分析：
CTestCopyConstruct var1;\\1
CTestCopyConstruct var2 = GetTest();\\2
代碼的執(zhí)行如下：
當(dāng)前堆棧指針(sp) = 1242980
sp -= 8//在堆棧中為var1分配空間
在var1上(1242980 - 1242973)調(diào)用構(gòu)造函數(shù)
sp -= 8//在堆棧中為var2分配空間
sp -= n//保護(hù)當(dāng)前環(huán)境
進(jìn)入了GetTest函數(shù)
當(dāng)前sp = 1242848
sp -= 8//為ret1分配空間
構(gòu)建ret1
sp -= 8//為ret2分配空間
構(gòu)建ret2
......
對(duì)var2(1242972處的堆棧段)調(diào)用拷貝構(gòu)造函數(shù)，以test1(1242848處)為參數(shù)
//析構(gòu)test1 test2等...
sp += n//恢復(fù)運(yùn)行環(huán)境
......
另：
operater = () 和默認(rèn)構(gòu)造函數(shù)不一樣，只重寫=運(yùn)算符而不提供拷貝構(gòu)造函數(shù)，調(diào)用的仍然是默認(rèn)的構(gòu)造函數(shù)。
默認(rèn)構(gòu)造函數(shù)和賦值運(yùn)算符處理的情況不一樣，一個(gè)是在已分配的空間上調(diào)用，一個(gè)是在已構(gòu)造的對(duì)象上調(diào)用。
默認(rèn)拷貝構(gòu)造函數(shù)會(huì)調(diào)用類中各成員的拷貝構(gòu)造函數(shù)。CString 由于提供了拷貝構(gòu)造函數(shù)，所以上面例子中即使去掉拷貝構(gòu)造函數(shù)，var2 仍然會(huì)得到正確的值。
調(diào)試的環(huán)境是vc6.0 debug 默認(rèn)選項(xiàng)。編譯沒有優(yōu)化。
CTestCopyConstruct(
const
?CTestCopyConstruct?
&
src)
????????
{
????????TRACE(
"
Enter?CTestCopyConstruct(const?CTestCopyConstruct?&src);this?is?%d;src?is?%d\n
"
,
this
,
&
src);
????????strTest?
=
?src.strTest;
????????i?
=
?src.i;
????}
與
CTestCopyConstruct(
const
?CTestCopyConstruct?
&
src)
????????:strTest?(src.strTest)
{
????????TRACE(
"
Enter?CTestCopyConstruct(const?CTestCopyConstruct?&src);this?is?%d;src?is?%d\n
"
,
this
,
&
src);
????????i?
=
?src.i;
????}
前者先調(diào)用了CString::CString()再調(diào)用CString::operator =
后者直接調(diào)用了CString::CString(CString & src);
默認(rèn)的賦值運(yùn)算的行為：首先調(diào)用父類的賦值運(yùn)算。
然后會(huì)為自己獨(dú)有的各成員尋找賦值運(yùn)算。如果成員的賦值運(yùn)算符被重寫，則調(diào)用這個(gè)重寫的賦值運(yùn)算符函數(shù)，如果這個(gè)重寫的運(yùn)算符函數(shù)是private，編譯將無(wú)法通過。
默認(rèn)的拷貝構(gòu)造函數(shù)的行為：首先調(diào)用父類的拷貝構(gòu)造函數(shù)。
然后為自己獨(dú)有的各成員尋找拷貝構(gòu)造函數(shù)。如果這個(gè)成員提供拷貝構(gòu)造函數(shù)，則調(diào)用之，如果成員的類提供的拷貝構(gòu)造函數(shù)是private，編譯將無(wú)法通過。
(子類完全可以把父類當(dāng)成自己的一個(gè)成員?)
可以說(shuō)默認(rèn)的賦值運(yùn)算和默認(rèn)的拷貝構(gòu)造函數(shù)是類最常被用到的兩個(gè)函數(shù)了...內(nèi)部卻不是一般的復(fù)雜。