C ++ # define pre-processing command features Evaluation

The Application of C ++ programming language is flexible. Its appearance has brought a lot of help to programmers. So what are the features of other programming languages? Let's start with the features of the C ++ # define pre-processing command and compare it with other languages.

C ++ # define pre-processing commands define so-called symbolic constants, which are directly compiled into binary code. In this case, it is similar to the effect of a constant variable defined by const. ,

But # define also has a function, that is, to change the compiler behavior, for example, decide whether to compile a certain sentence of code based on it.

The following code about the C ++ # define preprocessing command is available in many C ++ source files:

 
 

  
  
# If! Defined (afx_mainfrm_h%0efea8a4_%c_4528 _
B7a8_761f1b02c20c00000000ded _) // If Not Defined
  
  
# Define afx_mainfrm_h1_0efea8a4_1_c_4528_b7a8 _
761f1b02c20c00000000ded _ // is defined
  
  
# If _ MSC_VER> 1000 // if a symbolic constant is greater than 1000
  
  
# Pragma once
  
  
# Endif // _ MSC_VER> 1000
 
 

So how does this behavior behave in. NET?

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1. C # inherits the role of # define as the Compilation instruction.

# Define DEBUG

Note: in C #, you cannot assign values to the defined symbols. For example, the following statement is invalid.

# Define DEBUG = TRUE

That is to say, # define only needs to be defined, and does not need to) specify the value of this symbol

In subsequent code, the # if syntax is used to determine whether a symbol is defined.

2. VB. NET has similar implementations, but it does not use the # define keyword, but the # const keyword.

 
 

  
  
# Const MINI = 1
  
  
# Const VERSION = 1.0
  
  
Const MINI As String = "4"
  
  
# Region "this is a code block"
  
  
Sub Main ()
  
  
# If MINI = 1 Then
  
  
Console. WriteLine ("Hello. world ")
  
  
# End If
  
  
End Sub
  
  
# End Region
 
 

The preceding section describes the differences between the pre-processing commands of C ++ # define and other languages.