# Define Summary (online data collection)

1. Define a simple constant: Define a constant to facilitate modification. (do not add a semicolon to the end !) # Define n 1000 is equivalent to const int n = 1000, but slightly different, define is just a simple replacement, rather than using it as a quantity. 2. defining simple functions: note that brackets are often used.

Define can accept some parameters like the function, 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.

so when we define, make a good habit. We recommend that you add brackets to all layers.

3. define a single row macro: There are three main usage types. 1) The prefix ##or Postfix ## is used as a part of a valid identifier. note: It is not a string. macro definition that is used for multiple rows. for example: # define a (x) T _ # X, int A (1) = 10; // equivalent to int t_1 = 10; # define a (x) TX ##__then int A (1) = 10; // equivalent to int T1 _ = 10; 2) Add # @ before to convert the mark to the corresponding character, note: only valid for single tag conversion (incorrect understanding ?) # Define B (X) # @ X then B (a) is 'A', B (1) is '1 '. B (ABC) is not valid. 3) add # to convert the tag to a string. # define C (x) # X then C (1 + 1) is "1 + 1 ". 4. define multi-row macros: note the use of the slash. the last row cannot use a slash. # define declare_rtti (thisclass, superclass) \ virtual const char * getclassname () const \ {return # thisclass;} \ static int istypeof (const char * type) \ {\ If (! Strcmp (# thisclass, type) \ return 1; \ return superclass: istypeof (type); \ return 0 ;\}\ virtual int ISA (const char * type) \{\ return thisclass: istypeof (type) ;\}\ static thisclass * safedowncast (ditkobject * O) \ {\ If (O & O-> ISA (# thisclass) \ return static_cast <thisclass *> (o); \ return NULL ;\} 5. used for Conditional compilation: (common form) # ifndef _ aaa_h # DEFINE _ aaa_h // C/C ++ Code # Endif

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.

5. 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.

6. 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

7. 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.

Use the Conditional compilation switch to avoid repeated inclusion (repeated definition)

For example

# Ifndef _ headerfilexxx __

# DEFINE _ headerfilexxx __

...

File Content

...

# Endif

8. Some Considerations: 1) the definition cannot be repeated unless the definition is exactly the same. # define a (x )... And # define a are repeated definitions.

2) You can only define symbols without defining values. For example, # define aaa