使用代码示例比较不同语言中的编程概念
更新：2007 年 11 月
下面是无法用一个关键字概述的基本编程概念的代码示例。有关更多信息，请参见 不同语言中的关键字的比较。
提供了下列任务的代码示例：
赋值语句
注释
条件语句
声明变量
For 循环
泛型类型
隐藏基类成员
初始化值类型
通过引用传递参数
通过值传递参数
选择语句
将对象引用设置为空
结构化异常处理
While 循环
声明变量
Visual Basic
Dim x As Integer
Public x As Integer = 10
C++
int x;
int x = 10;
C#
int x;
int x = 10;
JScript
var x : int;
var x : int = 10;
var x = 10;
Visual FoxPro
LOCAL x
X = 10
注释
Visual Basic
' comment
x = 1 ' comment
Rem comment
C++
// comment
/* multiline
comment */
C#
// comment
/* multiline
comment */
JScript
// comment
/* multiline
comment */
Visual FoxPro
* full line
USE && end of line
NOTE multiline ;
comment
赋值语句
Visual Basic
nVal = 7
C++
nVal = 7;
C#
nVal = 7;
JScript
nVal = 7;
Visual FoxPro
nVal = 7
– 或 –
STORE 7 to nVal
条件语句
Visual Basic
If nCnt <= nMax Then
nTotal += nCnt ' Same as nTotal = nTotal + nCnt.
nCnt += 1 ' Same as nCnt = nCnt + 1.
Else
nTotal += nCnt
nCnt -= 1
End If
C++
if(nCnt < nMax) {
nTotal += nCnt;
nCnt++;
}
else {
nTotal += nCnt;
nCnt --;
};
C#
if (nCnt <= nMax)
{
nTotal += nCnt;
nCnt++;
}
else
{
nTotal +=nCnt;
nCnt--;
}
JScript
if(nCnt < nMax) {
nTotal += nCnt;
nCnt ++;
}
else {
nTotal += nCnt;
nCnt --;
};
Visual FoxPro
IF nCnt < nMax
nTot = nTot + nCnt
nCnt = nCnt + 1
ELSE
nTot = nTot + nCnt
nCnt = nCnt - 1
ENDIF
选择语句
Visual Basic
Select Case n
Case 0
MsgBox ("Zero")
' Visual Basic exits the Select at the end of a Case.
Case 1
MsgBox ("One")
Case 2
MsgBox ("Two")
Case Else
MsgBox ("Default")
End Select
C++
switch(n) {
case 0:
printf_s("Zero\n");
break;
case 1:
printf_s("One\n");
break;
case 2:
printf_s("Two\n");
break;
default:
printf_s("?\n");}
C#
switch(n)
{
case 0:
Console.WriteLine("Zero");
break;
case 1:
Console.WriteLine("One");
break;
case 2:
Console.WriteLine("Two");
break;
default:
Console.WriteLine("?");
break;
}
JScript
switch(int(n)) {
case 0 :
print("Zero")
break
case 1 :
print("One")
break
case 2 :
print("Two")
default :
print("Default")
}
Visual FoxPro
DO CASE
CASE n = 0
? 'Zero'
CASE n > 0
? 'Pos'
OTHERWISE
? 'Neg'
ENDCASE
For 循环
Visual Basic
For n = 1 To 10
MsgBox("The number is " & n)
Next
For Each prop In obj
prop = 42
Next prop
C++
for(int n=1; n<11; n++)
printf_s("%d\n",n);
C#
for (int i = 1; i <= 10; i++)
Console.WriteLine("The number is {0}", i);
foreach (int i in testArr)
{
Console.WriteLine(i);
}
JScript
for (var n = 0; n < 10; n++) {
print("The number is " + n)
}
for (var prop in obj)
obj[prop] = 42
Visual FoxPro
FOR n = 1 TO 10
? n
ENDFOR
隐藏基类成员
Visual Basic
Public Class BaseCls
Public Z As Integer = 100 ' The element to be shadowed
public Sub Test()
System.Console.WriteLine("Test in BaseCls")
End Sub
End Class
Public Class DervCls
Inherits BaseCls
Public Shadows Z As String = "*" ' The shadowing element.
public Shadows Sub Test()
System.Console.WriteLine("Test in DervCls")
End Sub
End Class
Public Class UseClasses
Dim BObj As BaseCls = New DervCls() ' DervCls widens to BaseCls.
Dim DObj As DervCls = New DervCls() ' Access through derived class.
Public Sub ShowZ()
System.Console.WriteLine("Accessed through base class: " & BObj.Z)
System.Console.WriteLine("Accessed through derived class: " & DObj.Z)
BObj.Test()
DObj.Test()
End Sub
End Class
C++
#using <mscorlib.dll>
#include <stdio.h>
public __gc class BaseCls
{
public:
int Z; // The element to be hidden
void Test()
{
printf_s("Test in BaseCls\n");
}
};
public __gc class DervCls : public BaseCls
{
public:
char Z; // The hiding element
void Test()
{
printf_s("Test in DervCls\n");
}
};
public __gc class UseClasses
{
public:
BaseCls * BObj; // DervCls widens to BaseCls
DervCls * DObj; // Access through derived class
void ShowZ()
{
BObj = new DervCls;
BObj->Z = 100;
DObj = new DervCls;
DObj->Z = '*';
printf_s("Accessed through base class: %d\n", BObj->Z);
printf_s("Accessed through derived class: %c\n", DObj->Z);
BObj->Test();
DObj->Test();
}
};
C#
public class BaseCls
{
public int Z = 100; // The element to be hidden
public void Test()
{
System.Console.WriteLine("Test in BaseCls");
}
}
public class DervCls : BaseCls
{
public new string Z = "*"; // The hiding element
public new void Test()
{
System.Console.WriteLine("Test in DervCls");
}
}
public class UseClasses
{
BaseCls BObj = new DervCls(); // DervCls widens to BaseCls
DervCls DObj = new DervCls(); // Access through derived class
public void ShowZ()
{
System.Console.WriteLine("Accessed through base class: {0}", BObj.Z);
System.Console.WriteLine("Accessed through derived class: {0}", DObj.Z);
BObj.Test();
DObj.Test();
}
}
JScript
class BaseCls
{
function get Z() : int { return 100 } // The element to be hidden
}
class DervCls extends BaseCls
{
hide function get Z() : String { return "*" } // The element to be hidden
}
class UseClasses
{
var BObj : BaseCls = new DervCls(); // DervCls widens to BaseCls
var DObj : DervCls = new DervCls(); // Access through derived class
function ShowZ()
{
print("Accessed through base class: " + BObj.Z);
print("Accessed through derived class: " + DObj.Z);
}
}
new UseClasses().ShowZ()
Visual FoxPro
oBase = newobject ("BaseCls")
oDerived = newobject ("DerivedCls")
?oBase.Z
?oDerived.Z
DEFINE CLASS BaseCls AS SESSION
PROTECTED Z && The element to be hidden outside of class
Z = 100
ENDDEFINE
DEFINE CLASS DerivedCls AS BaseCls
HIDDEN Z && The hiding element from subclass
Z = "*"
ENDDEFINE
While 循环
Visual Basic
While n < 100 ' Test at start of loop.
n += 1 ' Same as n = n + 1.
End While '
C++
while(int n < 100)
n++;
C#
while (n < 100)
n++;
JScript
while (n < 100)
n++;
Visual FoxPro
DO WHILE n < 100
n = n + n
ENDDO
通过值传递参数
Visual Basic
Public Sub ABC(ByVal y As Long) ' The argument Y is passed by value.
' If ABC changes y, the changes do not affect x.
End Sub
ABC(x) ' Call the procedure.
' You can force parameters to be passed by value, regardless of how
' they are declared, by enclosing the parameters in extra parentheses.
ABC((x))
C++
testMethod(i,j);
C#
/* Note that there is no way to pass reference types (objects) strictly by value.
You can choose to either pass the reference (essentially a pointer),
or a reference to the reference (a pointer to a pointer).*/
// The method:
void ABC(int x)
{
...
}
// Calling the method:
ABC(i);
JScript
ABC(i)
Visual FoxPro
ABC(x)
– 或 –
DO ABC WITH (x)
通过引用传递参数
Visual Basic
Public Sub ABC(ByRef y As Long)
' The parameter y is declared by by referece:
' If ABC changes y, the changes are made to the value of x.
End Sub
ABC(x) ' Call the procedure.
C++
// Prototype of ABC that takes a pointer to integer.
int ABC(long *py);
ABC(&VAR);
// Prototype of ABC that takes a reference to integer.
int ABC(long &y);
ABC(VAR);
C#
/* Note that there is no way to pass reference types (objects) strictly by value.
You can choose to either pass the reference (essentially a pointer),
or a reference to the reference (a pointer to a pointer).*/
/* Note also that unsafe C# methods can take pointers just like C++ methods.
For details, see <MSHelp:link keywords="vclrfUnsafe" TABINDEX="0">unsafe</MSHelp:link>. */
// The method:
void ABC(ref int x)
{
...
}
// Calling the method:
ABC(ref i);
JScript
/* Reference parameters are supported for external objects, but not
internal JScript functions. Use '&' to call by reference */
testObject.ByRefMethod(&x);
Visual FoxPro
ABC(@x)
– 或 –
DO ABC WITH x
结构化异常处理
Visual Basic
Try
If x = 0 Then
Throw New Exception("x equals zero")
Else
Throw New Exception("x does not equal zero")
End If
Catch err As System.Exception
MsgBox("Error: " & Err.Description)
Finally
MsgBox("Executing finally block.")
End Try
C++
__try{
if (x == 0)
throw new Exception ("x equals zero");
else
throw new Exception ("x does not equal zero");
}
__catch(Exception e)
{
Console.WriteLine("Caught Exception");
}
__finally
{
Console.WriteLine("Executing finally block");
}
C#
// try-catch-finally
try
{
if (x == 0)
throw new System.Exception ("x equals zero");
else
throw new System.Exception ("x does not equal zero");
}
catch (System.Exception err)
{
System.Console.WriteLine(err.Message);
}
finally
{
System.Console.WriteLine("executing finally block");
}
JScript
try {
if (x == 0)
throw "x equals zero"
else
throw "x does not equal zero"
}
catch(e) {
print("Error description: " + e)
}
finally {
print("Executing finally block.")
}
Visual FoxPro
TRY
IF x = 0
THROW "x equals zero"
ELSE
THROW "x does not equal zero"
ENDIF
CATCH TO err
MESSAGEBOX("Error description: " + err.UserValue)
FINALLY
MESSAGEBOX("Executing finally block.")
ENDTRY
将对象引用设置为空
Visual Basic
o = Nothing
C++
o = nullptr; // when compiling with /clr
C#
o = null;
JScript
o = undefined;
Visual FoxPro
TestObj = null
– 或 –
TestObj = .NULL.
初始化值类型
Visual Basic
Dim dt as New System.DateTime(2001, 4, 12, 22, 16, 49, 844)
C++
System::DateTime dt = System::DateTime(2001, 4, 12, 22, 16, 49, 844);
C#
System.DateTime dt = new System.DateTime(2001, 4, 12, 22, 16, 49, 844);
JScript
var dt : System.DateTime = new System.DateTime(2001, 4, 12, 22, 16, 49, 844);
Visual FoxPro
LOCAL ltDateTime AS DateTime
ltDateTime = DATETIME(2001, 4, 12, 22, 16, 49)
泛型类型
Visual Basic
' Define a generic type
Public Class classMaker(Of t)
' Constrain a type parameter
Public Class classMaker(Of t As IComparable)
' Create an object from a generic type
Dim integerClass As New classMaker(Of Integer)
C++
// Define a generic type
generic <typename T> ref class testList { ... };
// Constrain a type parameter
generic <typename T> where T : IComparable<T>
ref class testList { ... };
// Create an object from a generic type
testList<int>^ list = gcnew testList<int>();
C#
// Define a generic type
public class testList<T>{...}
// Constrain a type parameter
public class testList<T> where T : IComparable<T>
// Create an object from a generic type
testList<int> list = new testList<int>();
JScript
// Define a generic type
// Constrain a type parameter
// Create an object from a generic type
Visual FoxPro
* Define a generic type
* Constrain a type parameter
* Create an object from a generic type