David
:
During this time, the old information is
See some articles
Although it is a small problem
Not very likely to use
But it is also a knowledge point
I specialize in sharing with everyone.
Some understandings of the author of the article related to this are wrong
, I wrote this article
It is also some wrong understanding to correct for the author.
.of course
If my understanding is wrong
, I also invite everyone to criticize
.
C
Although it is B
.
A new language
, But it has a thousand connection with C.
Although B
.
Repeatedly insisted
But I am still willing to put C
is considered to be C
.
We should use the Convention in C.
union
This is what I want to give this article.
Although C
makes we can expand some new things in
,but
I suggest you don't do it.
After reading this article
I think you are probably thinking.
.
C because there is no concept
All types can act as a combination of basic types
,Thus, in
Union contains
Struct is also a natural thing.
, To c
After
Since it is generally considered C
Struct
Class basic equivalent
So, then
Is there a class member in Union?
Let's take a look at the following code.
:
Struct Testunion
{
Testunion
() {}};
Typedef Union
{
Testunion Obj
UT
;
int
main
(
Void
) {
Return
0
}
Compile this procedure
We will be told
:
Error C2620
:
union
'__unnamed'
: MEMBER
'Obj' HAS User
-defined constructor
OR NON
-Trivial
Default Constructionor
And if you remove the constructor, there is no dry constructor
Other OK
.
Why does the compiler do not allow our
UNION member has a constructor?
I can't find the interpretation of the comparative authority about this issue.
, For this issue
My explanation is
:
If c
standard allows us
Union has a constructor
So, then
Do you want this constructor when you are allocated?
? If the answer is YES
, Then some memory allocation operations are included in the Testunion constructor
Or other modifications to the entire Application status
So, then
If I want to use Obj in the future
Things may be relatively reasonable
But if I don't use the OBJ member at all.
• Because OBJ's introduction caused by system status is obviously unreasonable
;on the contrary
If the answer is NO
So once we will choose OBJ in the future
, All information is not initialized
(If it is ordinary
Struct
,no problem
,but
If there is a virtual function?
?). Further
, Suppose now our
Union is not only one testunion Obj
And a testunion2 Obj2
, Both have constructor
And all execute some memory allocation in the constructor
(Even many other things
), Then
If the OBJ is constructed first
After construction OBJ2
, The result of execution is almost certainly caused by the leak of memory.
.
In view of the above troubles
(May have more trouble
)
Union
The compiler is only responsible for allocation space
Not responsible for performing additional initialization work
, In order to simplify work
As long as we provide a constructor
, Will receive the above Error
.
Concrete
, In addition to impovering functions
Destructor
/ Copy constructor
/ Assignment operator is not available
.
In addition
If we contain anything in our class
Virtual function
, Compile
We will receive the following error message
:
Error C2621
:
union
'__unnamed'
: MEMBER
'obj' HAS COPY CONSTRUCTOR
and so
Dispel
Contains constructor in Union
/ Destructor / copy constructor
/ Assignment operator
/ The idea of the class member variable of virtual functions
, Old man practical your C style
Struct
!
but
Defining a normal member function is OK
Because this will not make
Class with C style
Struct has any essential difference
, You can completely
Class is understood as a C style
Struct
N all-course functions
.
right now
, Then look at the interior in the class
What is different when union?
Take a look at the procedure below
, Please pay attention to reading program prompts
:
Class testunion
{
Union Dataunion
{
Dataunion
(
Const
charr
*);
Dataunion
(
Long
);
Const
charr
* Ch_
;
Long L_
} DATA_
;
public
:
Testunion
(
Const
charr
* Ch
);
Testunion
(
Long L
};
Testunion
:: Testunion
(
Const
charr
* Ch
: DATA_
(CH
)
// if you want to use initialzing list to initiate a nested-union member, The Union Must Not Be Anonymous and Must Have a Construction.
{}
Testunion
:: Testunion
(
Long L
: DATA_
(l
) {}
Testunion
:: Dataunion
:: Dataunion
(
Const
charr
* Ch
: Ch_
(CH
) {}
Testunion
:: Dataunion
:: Dataunion
(
Long L
: L_
(l
) {}
int
main
(
Void
) {
Return
0
}
As shown above
, C
Union can also include constructor
,but
Although this is supported by the language
But it is really a poor programming habit
,therefore
I don't intend to excessive instructions on the above program.
I recommend the following programming style
:
Class testunion
{
Union Dataunion
{
Const
charr
* Ch_
;
Long L_
} DATA_
;
public
:
Testunion
(
Const
charr
* Ch
);
Testunion
(
Long L
};
Testunion
:: Testunion
(
Const
charr
* Ch
) {
Data_
.ch_
= CH
}
Testunion
:: Testunion
(
Long L
) {
Data_
.l_
= L
}
int
main
(
Void
) {
Return
0
}
It is completely C style
.
and so
Accept this conclusion
:
Please follow the CONVENTION in C.
union
Don't try to use any C as much as possible
additional characteristics
.
