C++ basic tutorial

Pre processor directive

define, undef, include, if, ifdef, ifndef, else, elif, elifdef, elifndef
endif, line, error, pragma

Source file translation.
- Conditionality: #if, #ifdef, #ifndef, #else, #elif, #elifdef, #elifndef (since C++23), and #endif
- replace: #undef #define
- include: #include other files.

The following preprocessor can be controlled
implementation defined: #pragma
file and line information: #line

###define

it is towrite macro.  
multiline macro can be written using \ at the end;  
even semicolon can be put in macro then using that will not require semicolon  
it just replace the word with whatever defined in #define.  
define sum(num1,num2) num1+num2; this works too. with two para  

###if endif elif else

in main if endif can change the code compilation based on condition  
some unncessary compilation will be not compiled  

ifdef endif else

if not defined -> ifndef  
if defined -> ifdef  
even code error will be ignored if condition doesnt matche.  

undef

this is just undefining previous defined macro.

line

__LINE__  current line number.  
__FILE__ current file name  
__DATE__ when the program was compiled  
__TIME__ compile time  
line 100 "text.txt"  
this will change file and line number