# Define usage

# Define usage

1.Simple define Definition

# Define maxtime1000

A simple maxtime is defined. It represents 1000. If you write

If (I <maxtime ){.........}

The compiler replaces maxtime with 1000 before processing this code.

This definition looks similar to a normal constant definition const, but it is also different, because the definition of define is more like a simple text replacement, rather than being used as a volume, this problem is particularly highlighted below.

 

2. Define"Function Definition"

Define can accept some parameters as the function does, as follows:

# Define max (x, y) (x)> (y )? (X) :( y );

This definition will return the big one in two numbers. Have you seen it? Because this "function" does not have a type check, it is like a function template. Of course, it is definitely not as secure as a template. It can be used as a simple template.

However, there are hidden risks. The example is as follows:

# Define add (A, B) A + B;

In general use, there is no problem, but if you encounter problems such as: C * Add (a, B) * D, the purpose of the algebraic formula is that a + B is then multiplied by C and D, but because define is used (it is just a simple replacement), the formula is actually changed

C * A + B * d

 

Another example is as follows:

# Define pin (int *);

Pin A, B;

The intention is that A and B are both int-type pointers, but they actually become int * a, B;

A is an int pointer while B is an int variable.

This should use typedef instead of define, so that both A and B are int type pointers.

Therefore, when defining it, we should develop a good habit. We recommend that you add brackets to all layers.

3.Macro single row Definition

# Define a (x) T _ # x

# Define B (X) # @ x

 

# Define C (x) # x

Suppose: x = 1, there are:

A (1) ------> T_1

B (1) ------> '1'

C (1) ------> "1"

 

(For more information, see hustli)

 

4. DefineMulti-line definition

 

Define can replace multiple lines of code, such as the macro definition in MFC (very classic, although disgusting)

 

# Define macro (arg1, arg2) do {\

/* Declarations */\

Stmt1 ;\

Stmt2 ;\

/*...*/\

} While (0)/* (no trailing ;)*/

The key is to add "\" to each line feed "\"

 

Excerpted from the http://www.blog.edu.cn/user1/16293/archives/2005/115370.shtml fix several bugs

 

5.In large-scale development, especially for cross-platform and system software, Conditional compilation is the most important function of define.

 

That is:

# Ifdef windows

......

......

# Endif

# Ifdef Linux

......

......

# Endif

 

You can use # define to set the compiling environment during compilation.

 

6.How to define and cancel macros

// Definition macro

# Define [macroname] [macrovalue]

// Cancel the macro

# UNDEF [macroname]

Normal macro

# Define Pi (3.1415926)

Macro with Parameters

# Define max (A, B) (a)> (B )? (A), (B ))

The key is that errors are easily generated, including differences in Machine understanding and human understanding.

7. Conditional compilation

# Ifdef XXX... (# Else )... # Endif

For example, # ifdef dv22_aux_input

# Define aux_mode 3

# Else

# Define auy_mode 3

# Endif

# Ifndef XXX... (# Else )... # Endif

 

8. The header file (. h) can be included by the header file or the c file;

Duplicate include (repeated definition)

Because header files can be nested, the C file may contain the same header file multiple times, and the definition may be repeated.