A few days ago
, Encounter a problem in the program
When I checked the program error
In the STL implementation, I found the following code.
:
#include
Using Namespace STD
;
Class A
{
public
:
A
(
Const string
& S
) {
cout
<<
.c_str
() << Endl
;}};
int
main
() {
Const
charr
* PSZ
=
"ABC"
;
A a a
(PSZ
);
Return
0
}
Here
I feel very strange
, Because I have never done this
How can I point a reference to a type different from the reference itself?
?then
, I sent a post online.
,quickly
Someone extracted the following from the CPL 3RD as a reply
:
// BeginInitialization of a reference is trivial when the initializer is an
Lvalue
(An Object Whose Address You Can Take
See §
4.9.6
). The
Initializer
For a
'Plain' T
& Must be an Lvalue of Type T
. The
Initializer
For a
Const T
& Need
NOT BE An Lvalue
OR EVEN OF TYPE T
.
In Such Cases
(David
'' s Notes
:
And Only in Such Cases
), [
1
] First
, Implicit Type Conversion To T IS Applie
IF Necessary
(See §C
.6
[
2
] THEN
, Resulting Value Is Placed in a Temporary Variable of Type T
;
and
[
3
] Finally
,
This Temporary Variable is buy as the value of the initializer
.
Consider
:
Double
& DR
=
1
;
// error: LValue Needed
Const
Double
& CDR
=
1
;
// Okthe Interpretation of
THIS Last Initialization Might BE
:
Double Temp
=
Double
(
1
);
// first create a Temporary with the right value
Const
Double
& CDR
= TEMP
;
// then use the temporary as the initializer for cdra temporary created to hold a reason
The end of its reference's scope
.
References to variables
And References to Constants Are Distinguished
Because the Introduction of a Temporary in the
Case of the variable
IS Highly ErrorProne
;
An Assignment to the Variable Would Become An Assignment To The - Soon to Disappear - Temporary
No Such Problem EXISTS
For References
To constants
,
And References to Constants Are Offen Important AS
Function Arguments
(§
11.6
).
// end
Means of
, As long as there is a feasible conversion path
The compiler will create a temporary variable as the reference type type information when constructing.
,then
, Point to this temporary variable with reference
.at the same time
The above text also gave us the following information.
:
1. Non
Const Reference will not enjoy such special treatments
The following code is wrong
:
Float F
=
0
;
Double
& D
= F
;
charr
* PSZ
=
"ABC"
;
String
& RSTR
= PSZ
;(Note
: VC6 reports an incorrect message with actual inconsistency when compiling each other
: A Reference That IS
Not to
'Const' Cannot Be Bound To a Non
-LValue
Here, F and PSZ are clearly left value
)
2.
Const Reference will make the function more applicable
But the price may introduce a large overhead when performing type conversion
, Lose the efficiency of the reference transfer
.
of course
In addition to this
,
Const is just a compile time concept
,with
public
/
protected
/
Private, etc.
, Just tell the compiler
We need to add some additional descriptions to things after these indications.
Please distinguish between treatments
These descriptions
PRIVATE
,says
: If you find non-book or
Friend visited these members
(method
), Report errors
;for
CONST
It is said that
: If you find any action pairs when you have a syntax check?
CONST modified objects have been modified
, Report error
.
After the syntax check
These modifications should be quietly retired.
.
