Why should I add # ifndef # define # endif to the header file?

When you use the VC menu to add a class, you will find that the automatically generated code is always like the following:

# If! Defined (afx_xxxx1_included _)

# Define afx_xxxx1_encoded _

Code

# Endif

This is to prevent the header file from being repeatedly contained. Repeated inclusion can be illustrated in the following example: for example, there is a header file. h, which contains a function FA and another header file B. h. The function FB is provided. If fa is required for FB implementation, B. h must contain. h; if a function in the CPP file needs to use FA and Fb, it must contain. H and B. h,. h. Compile the program with the following error:

Error c2084: function 'bool _ cdecl FA () 'already has a body

The solution is to add the following in a. h:

# Ifndef

# Define

Original code

# Endif

 

The sample source code is as follows:

// A. h

# Ifndef

# Define

 

Bool aorb (bool)

{

Return;

}

 

# Endif

 

// B. H

# Include "A. H"

 

Bool cord (bool)

{

Return aorb ();

}

 

// A. cpp

# Include "A. H"

# Include "B. H"

 

Int main ()

{

Bool a = 0;

Bool B = aorb ();

Bool c = cord (B );

 

Getchar ();

Return 0;

}