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Allen
class MyClass
{
private:
MyClass(void);
public:
static MyClass* pInstance;
static MyClass& GetInstance();
virtual ~MyClass();
public:
int Add(int op1, int op2);
};
I can use these ways:
c1. MyClass& objRef = MyClass::GetIns tance();
c2. MyClass obj = MyClass::GetIns tance();
The c1 is easy to understand, it uses a reference objRef assignment.
While the c2 is difficult to understand to me.
The constructor of MyClass is private. And C2 uses value assignment.
Who can explaint it?
Thank you!
Nov 22 '06 #1
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9 
1240 
Ondra Holub
c2 makes a copy. Or more precisely it calls operator= (if such operator
is available). I think your class should prohibit usage of operator=
(declare it as private without implementation) and you should use only
c1 version.
Nov 22 '06 #2
Allen
I traced the addresses.
c1. MyClass& objRef = MyClass::GetIns tance();
&objRef is exactly equal to pInstance.
c2. MyClass obj = MyClass::GetIns tance();
&obj is not equal to pInstance.
Therefore, in the case of c2, obj is a new object. But the constructor
of MyClass is private.
Why does assignment is allowed? Can I make a conclusion that assigment
does not need
call constructor?
Nov 22 '06 #3
Allen
"Ondra Holub дµÀ£º
"
c2 makes a copy. Or more precisely it calls operator= (if such operator
is available). I think your class should prohibit usage of operator=
(declare it as private without implementation) and you should use only
c1 version.
Thank you.
Yes, I want to prevent the use of copy method. So I declear these two
methods:
private:
MyClass(const MyClass ©) {};
void operator=(const MyClass ©) {};
But the following lines are still compiled ok.
void MyTestCase::tes tAdd()
{
MyClass obj = MyClass::GetIns tance();
CPPUNIT_ASSERT_ EQUAL( obj.Add(12, 13), 25 );
}
Nov 22 '06 #4
Alf P. Steinbach
* Allen:
class MyClass
{
private:
MyClass(void);public:
static MyClass* pInstance;
static MyClass& GetInstance();
virtual ~MyClass();public:
int Add(int op1, int op2);};
I can use these ways:
c1. MyClass& objRef = MyClass::GetIns tance();
c2. MyClass obj = MyClass::GetIns tance();The c1 is easy to understand, it uses a reference objRef assignment.
While the c2 is difficult to understand to me.
The constructor of MyClass is private. And C2 uses value assignment.
No, C2 uses copy construction.
You have an automatically generated copy constructor, since you haven't
provided or declared one.
You can do
class MyClass
{
private:
MyClass() {}
MyClass( MyClass const& ); // No copy constructor.
MyClass& operator=( MyClass const& ); // No assignment.
public:
static MyClass& theInstance()
{
static MyClass instance;
return instance;
}
int add( int op1, int op2 ) { ... }
};
although a class providing arithmetic operations is seemingly not
consistent with being a singleton.
--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?
Nov 22 '06 #5
Alf P. Steinbach
* Ondra Holub:
c2 makes a copy. Or more precisely it calls operator= (if such operator
is available).
No it doesn't.
Also, please quote some context when you reply (this is Usenet, not a
chat group).
--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?
Nov 22 '06 #6
Allen
I understand it now. Thank you very much.
On 11ÔÂ22ÈÕ, ÏÂÎç5ʱ04·Ö, "Alf P. Steinbach" <a...@start.n o>
wrote:
* Allen:
class MyClass
{
private:
MyClass(void);public:
static MyClass* pInstance;
static MyClass& GetInstance();
virtual ~MyClass();public:
int Add(int op1, int op2);};I can use these ways:
c1. MyClass& objRef = MyClass::GetIns tance();
c2. MyClass obj = MyClass::GetIns tance();The c1 is easy to understand, it uses a reference objRef assignment.
While the c2 is difficult to understand to me.
The constructor of MyClass is private. And C2 uses value assignment.No,C 2 uses copy construction.
You have an automatically generated copy constructor, since you haven't
provided or declared one.You can do
class MyClass
{
private:
MyClass() {}
MyClass( MyClass const& ); // No copy constructor.
MyClass& operator=( MyClass const& ); // No assignment.
public:
static MyClass& theInstance()
{
static MyClass instance;
return instance;
}int add( int op1, int op2 ) { ... }
};although a class providing arithmetic operations is seemingly not
consistent with being a singleton.--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?
Nov 22 '06 #7
Ondra Holub
"Ondra Holub дµÀ£º
"c2 makes a copy. Or more precisely it calls operator= (if such operator
is available). I think your class should prohibit usage of operator=
(declare it as private without implementation) and you should use only
c1 version.
Thank you.Yes, I want to prevent the use of copy method. So I declear these two
methods:private:
MyClass(const MyClass ©) {};
void operator=(const MyClass ©) {};But the following lines are still compiled ok.
void MyTestCase::tes tAdd()
{
MyClass obj = MyClass::GetIns tance();
CPPUNIT_ASSERT_ EQUAL( obj.Add(12, 13), 25 );
}
I was wrong (as Alf P. Steinbach mentioned in other post). It works
this way:
MyClass obj = MyClass::GetIns tance(); // Calls copy constructor
obj = MyClass::GetIns tance(); // Calls operator=
So copy constructor should be also private. Sorry for confusing you.
Nov 22 '06 #8
Allen
It is an interesting question.
I learn a lot from both of you.
Thank you.
"Ondra Holub дµÀ£º
MyClass obj = MyClass::GetIns tance(); // Calls copy constructor
obj = MyClass::GetIns tance(); // Calls operator=So copy constructor should be also private. Sorry for confusing you.
Nov 22 '06 #9
terminator
Ondra Holub wrote:
c2 makes a copy. Or more precisely it calls operator= (if such operator
is available). I think your class should prohibit usage of operator=
(declare it as private without implementation) and you should use only
c1 version.
c2 does not assign(call operator=) it constructs a MyClass via built-in
copy-constructor
you shoud add this to your MyClass if you want c2 not to compile:
private:
MyClass(MyClass &){};
Nov 22 '06 #10
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