Detailed description of C/C ++ Conditional compilation

 

1

 

# Ifdef _ XXXX

 

... Procedure 1...

 

# Else

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... Procedure 2...

 

# Endif

 

This indicates that if the identifier _ XXXX has been defined by the # define command, the program segment 1 will be compiled; otherwise, the program segment 2 will be compiled.

 

2

 

# Ifndef _ XXXX

 

... Procedure 1...

 

# Else

 

... Procedure 2...

 

# Endif

 

# Ifndef is used here, indicating if not def. of course, it is the opposite of # ifdef (if the identifier _ XXXX is not defined, the execution segment 1; otherwise, the execution Segment 2 ).

 

3

 

# If constant

 

... Procedure 1...

 

# Elif <constant expression 2>

 

... Procedure 2...

 

# Elif <constant expression 3>

 

... Program Section 3...

 

......

 

# Else

 

... Procedure 2...

 

# Endif

 

It indicates that if the constant is true (not 0, whatever number, as long as it is not 0), the program segment 1 will be executed; otherwise, the program segment 2 will be executed.

 

For example:

 

1.

 

# If defined (_ PC) & defined (_ SSE)

 

# Ifdef DEBUG

 

# Define _ PC_VERIFY_ALIGNMENT __

 

# Endif

 

# Ifdef _ PC_VERIFY_ALIGNMENT __

 

# Define PC_VERIFY_ALIGN_ASSERT (ptr )\

 

{\

 

If (INT) ptr) % 16 )! = 0 )\

 

{\

 

Debugf (NAME_Critical, TEXT ("Unaligned PC data (0x % X)"), ptr );\

 

DebugBreak ();\

 

}\

 

}

 

# Else

 

# Define PC_VERIFY_ALIGN_ASSERT (ptr)

 

# Endif

 

# Else

 

# Define PC_VERIFY_ALIGN_ASSERT (ptr)

 

# Endif

 

2.

 

# Define ABC 3

 

Main ()

 

{

 

# If ABC> 0

 

Int a = 1;

 

Printf ("% d \ n", );

 

# Elif ABC <0

 

Int B =-1;

 

Printf ("% d \ n", B );

 

# Else

 

Int c = 0;

 

Printf ("% d \ n", c );

 

# Endif

 

}

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