1.1.2.1 Translation unit

• Each .cpp is a translation unit. Basically, you should put each class into a single .cpp file, and make sure each .cpp file has appropriate .h file.
• A translation unit roughly consists of a source file after it has been processed by the C preprocessor, meaning that header files listed in #include directives are literally included, sections of code within #ifdef may be included, and macros have been expanded.
• If you just want a function or variable visible to only current translation unit. You can declare it as static. You don’t need put it into a header file. But in C++, you have to put function definition or variable before it use. Different with C language, In C language, compiler will guess a function prototype from it’s usage, It’s not good sometimes.
• C/C++ is separate compilation. If you need to use a function or variable in two different files, you should put them in the #include .h file. (add extern keyword before variable name). It will keep declaration just once, DO NOT copy declaration to the other positions in the .cpp file. It will lead to two or more copies, it’s bad smell of code.
• A better suggestion is to have a single global.h file for a complex system. Then put some common type, define, constant , global function in this single global.h file. So it will help to reduce duplication, just keep once appearance.
• use #progma once to include guard, It’s not standard, but It has been supported by many compiler. Including gcc and MSVC.
• Three rules about header file:
  1. Put your local/private header file in front of system header file. . Why? There are two advantage, 1) You can know what header file should be included, It’s helpful to achieve the goal of demand 2. 2) Some times, if you have you own fun with same name as system or library, It can give you a compile error; Below example will give you a compile error. But if you put <cmath> before myHead.h. Then, main will use acos in cmath, and your acos will be override.
     #include "myHead.h"  //double acos(double) 
     #include <cmath> 
     main{ 
     acos(0.5); 
     }
  2. You will need to put he minimal set of #include statements that are needed to make the header compilable when your local/private header is included on the first place.
  3. Removing (or not adding in the first place) useless include statements. If all you have are references or pointers to a class, you don’t need to include that class’s header file; a forward reference will do nicely and much more efficiently.
• Change order of including order in .cpp file to avoid include another header file in current head file. (This is only ad-hoc way, not a good code style.)
  //a1.h 
  typedef struct Name{... 
  } NAME; 
   
  //a2.h 
  fun1(NAME a); 
   
  //a.cpp 
  #include "a1.h" 
  #include "a2.h" // put a1.h in front of a2.h
• In .h file, you can include template and inline function. In fact, you have to put template into .h file. and put a semicolon in the end of head file is good suggestion. You also need semicolon after declare class. In .cpp file, no semicolon after each function definition.