# If 1 ......

Today, when I was reading a program, I found a lot

# If 1

......

# Endif

# If 0

......

# Endif

It is a bit inexplicable because it has never been used. Search for answers online. Finally, I understand that it is used to annotate the compilation content. It means:

# If 1 requires the compiler to compile the following content;

# If 0 compiler does not compile the following content;

# If 0 # endifAnnotation nesting!!!"

For example:

# Include <iostream>
# Include <tchar. h>
# Include <stdlib. h>
Using namespace STD;

 

Int _ tmain (INT, char **, char **)
{
# If 1
Int A = 0;
# Endif
# If 0
Int B = 0;
# Endif
# If 1
Int C = 0;
# Endif
Cout <A <B <C <Endl;
Return 0;
}

The compilation fails.