1. ByVal passes value: a way to pass parameter values instead of address to the process, which allows the process to access a copy of the variable. As a result, the process cannot change the true value of the variable.
2. ByRef passes value: a way to pass parameter addresses instead of values to the process, which allows the process to access the actual variables. As a result, the process can change the true value of the variable. Pass parameters by address unless otherwise specified.
3、System defaultWhat isByRefPass the value。
example:
Copy the codeThe code is as follows:
<SCRIPT LANGUAGE="vbScript">
dim a
a=0
"a=0"
"<br/>sub change(byref ar)<br/>"
change a
a
a=0
"<br/>sub change2(ByVal ar)<br/>"
change2 a
a
a=0
"<br/>sub change3( ar)<br/>"
change3 a
a
a=0
"<br/>function change4(byref ar)<br/>"
change4 a
a
a=0
"<br/>function change5(ByVal ar)<br/>"
change5 a
a
a=0
"<br/>function change6( ar)<br/>"
change6 a
a
a=0
sub change(byref ar)
ar=111
end sub
sub change2(ByVal ar)
ar=222
end sub
sub change3( ar)
ar=333
end sub
function change4(byref ar)
ar=444
end function
function change5(ByVal ar)
ar=555
end function
function change6( ar)
ar=666
end function
</SCRIPT>
=======================
result:
a=0
sub change(byref ar)
111
sub change2(ByVal ar)
0
sub change3( ar)
333
function change4(byref ar)
444
function change5(ByVal ar)
0
function change6( ar)
666
Instruction vbs is byRef by default, which is the same as VB, by address.
Let me give you another small example, and everyone will run it to see the results!
Copy the codeThe code is as follows:
<%
Dim i,j,p,m
i = 10
j = 12
i&"******"&j&"<br>"
Call Fun2 (i,j)
i&"******"&j&"<br>"
i = 10
j = 12
Call Fun (i,j)
i&"*******"&j&"<br>"
Function Fun2 (a,b)
a = 5
b = 6
Fun2 = 0
End Function
Function Fun (ByVal a,ByRef b)
a = 5
b = 6
Fun = 0
End Function
%>
Through the example above you can find:
1. The value of ByVal pass does not change the value of the global variable.
2. ByRef passes the value to change the value of the global variable.
3. The system defaults to pass the value by ByRef.
As for when should it be used? It depends on your actual situation!
ByVal transfers a copy of the parameter memory to the callee. In other words, the value that is directly pushed into the stack is the passed value.
ByRef transfers the actual address of the parameter memory to the callee. In other words, what is pressed into the stack is the address of the actual content. The callee can directly change the contents in the address.
ByVal is the value passed. The source data will not be modified.
You can use this value as your own local variable
ByRef is the delivery address, the source data may be modified
Your operation on this variable will have an impact on the variable you pass in, just like the feeling of a pointer
Description of vb6
Nothing said, continue with the question:
Copy the codeThe code is as follows:
Function Test(a)
a = a + 1
End Function
c = 1
'------------------------------------
'question:
' Please tell me the result of calling the following four methods separately?
'------------------------------------
'Method One
Test c
'Method 2
'Test (c)
'Method Three
'Test (c+1)
'Method 4
'Call Test(c)
msgbox c
Correct answers: 2, 1, 1, 2
Have you answered it correctly? If you answer all the answer correctly, you can return it directly. If you don’t answer correctly, continue reading.
Trace back
Almost all programming languages have different values and references in function transmission. Our VB man is no exception. Moreover, she chose to pass references as its default method. What's more, she is not picky about all variable types, and all use the method of passing references by default, including integer variables (int). This is also the most fundamental reason for the defeat in the Huashan Sword Contest.
You can use the ByVal (pass value) and ByRef (pass reference) keywords before the parameter to indicate the method of passing the parameter:
'The value is passed, the value c remains unchanged
Copy the codeThe code is as follows:
Function Test(ByVal a)
a = a + 1
End Function
c = 1
Test c
'c = 1
'The value is passed, the value c remains unchanged
Copy the codeThe code is as follows:
Function Test(ByRef a)
a = a + 1
End Function
c = 1
Test c
'c = 2
Understand
Generally, there are two ways to call Function function in VB6:
Func Params
As method one: Test c
Call Func(Params)
For example, method 4: Call Test(c)
Therefore, the method one and method four in the swordsmanship are essentially the same. Parameters c are passed through the default reference method. After executing the function Test, the value of c will change accordingly. Therefore, the results of Methods 1 and Method 4 are: 2
However, there seems to be such a calling method circulating in the world:
Func(Expression)
For example, method 2: Test(c) and method 3: Test(c+1)
Noted that the content in the brackets is not Params, but Expression, which is an expression. The evaluation result of the expression will be saved in a temporary variable and passed into the function body. After the function call is completed, the temporary variable will be destroyed. Therefore, when calling method two, the calculation result of expression (c) is 1, and is saved to a temporary variable and passed into the function Test. After the function is executed, the original c value does not change. Similarly, the call to method three is more intuitive. The (c+1) expression operation result is saved to a temporary variable and passed into Test, and the original c value is not sent to change. Therefore, the results of Method 2 and Method 3 are: 1
Get out of the army
The summary is as follows:
Function parameters in VB6 and VBS are of type ByRef by default.
When the Test(c) method is called, VB will think that what you are passing is not a variable, but an expression: (c) The result of this expression operation is the same as the value of c, but it is stored in a temporary variable. The change of this temporary variable will not affect the original variable c.