1.predefined symbol(we can view it as some can deirectly be used constants)
for example,
2.#define define constants
Now we supplement some knowledges.
(1)The storage device on the computer
1.memory 2.hard disk 3.register(on the cpu,reading speed very fast)
A code --> register int num;//meaning -- advice put the value of num in the register.
(we often use it when this variable need to be used repeatedly but whether use it depend on the complier judgement)
(2)for(;;) if the judgement write nothing,representing constantly true,which is dead circle.
(3)#define .... At last better not to add ;
A #define using example.
3.#define define macro
format -- #define name(parameter) stuff // do not need";" and the name and "(" need fit closely.
function-- parameter do not calculate and direct put in stuff.
for example，
#undef can cancel a #define
or
so we should remind add () when we use #define
4.side effect
b = a++
a++ is side effect.
when a macro parameter have side effect,it also will make some surprise situations.
5.Some positive and negative between marco and function
Characteristic
The string content will not be search.
Positive
(1)marco performance more better amd running more fast than function because marco just replace the code but function need stack creating to run.
(2) marco perameter need not type but function need.
(3)marco can directly deliver type.
Negative
(1)marco just direct replace the code,so it will add coding length.
(2)marco can not debug because the switch it is running in preprocessing progress but debug just test the program code.
(3)The operator precedence also have question.
(4)marco can not recurse.Can #define itself.
6.
7.#
make parameter in the string that can not be change be the object signal and make it stringify.
8.##
Creating a function mold
A using situation example
9.marco name always all capitalize while function name always not all capitalize
10.conditional comliation directive
1.ifdef
2.#if
do not use variable because this coding all processing in preprocessing but in this time just constant having and variable is not created.
3.branch
4.#if defined(M) if M is defined dy #define ,it will be true which is at the same of #ifdef
5.#if !defined(M) if M is not defined by #define ,it will be true which is at the same of #ifndef
5.head document including
#include <...> standard head document
#include "..." customize head document
#include "..." first will find the path of the source file's directory(文件所在目录的路径)
if not find ,it will find the standard library haed document.
#include <...> just will find the standard library haed document.
6.File nesting
A head document possbility will be used many times.
So we should have some idea to reduce repeat use.
1.#ifndef TEST_H
//first time will do in it
#define TEST_H//put this twe codes in head document head
//when define it,the last time can not put in it
....
#endif // put this code in the document last
2.#program once
using it can directly make this document just using once.
At last I tell something I found.
A variable just a digital.Like a int * p.The p essence is a digital.In pointer, it will be viewed as a address.
In pointer,it save just a address but by it type,it will control the address memory by the type manner.
for example，
//指针是带有有色眼镜的，它看内存的方式只和她的类型有关。
Like it ,using 0 to be the struct start address to calculate it offset.we can not visit the value but we can using pointer to find the struct member address.