# Define typedef and # ifdef # define # ifndef # endif

1. # difference between define and typedef

# Define is generally used to define constants and statement macro definitions and act as macro replacement during compilation. Of course, you can also complete a small part of the typedef function. Remember that it is only a small part.

Typedef is used to declare a new type name for readability and variable definition.

Example 1:

# Define int // No semicolon !!

Typedef int; // remember that there is no # Before typedef, and this statement is required. The semicolon ends.

In this example, int num; can be implemented in this way, basically no difference.

Example 2:

# Define pint int *

Typedef int * pint;

There is a difference in this example. We need to define int * a, * B; Use define to implement pint a, B; replace int * a, B; it defines pointer A and variable B. In typedef usage, pint A and B define pointer variable A and pointer variable B.

Therefore, typedef must be used to declare a new type name for security purposes.

 

2. # define, # If defined (), # ifndef, # ifdef, # else # elseif # endif

A) # define defines constants. For example, # define debug indicates that debug has been defined.

B) # If defined (Debug), if there is only one condition, it is equivalent to # ifdef debug

Therefore, # If defined () is often used in multi-condition environments. You can use "|, &,!" in the middle of multiple conditions ,&&,!" For example, # If defined (ebug) & defined (win32)

This means that when both Debug and Win32 are defined, the statement under the # If condition is entered.

C) # ifdef If debug is defined, execute the statement under the # ifdef condition. Its intention and # ifndef debug

On the contrary, N in the middle indicates that there is no meaning, that is, if there is no definition ....

D) # endif pairing # If pairing

 

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