在标头而不是源文件（cpp / h）中声明变量之间有什么区别

Question

Are there any difference between declaring variables in a header and declaring variables in a source code ? eg perfomance , securities

Answer 1

There is no difference at all. Except they are best written in such a way that it supports:

• Multiple modules may include the header to share the data structures
• and share the function declarations.

• A header is often written so that it doesn't declare its contents multiple times.

   #ifndef __THIS_HEADER_H #define __THIS_HEADER_H 1 .... (content of header which is protected from multiple insertions) #endif 

A module may include a .c file, but this is rare (and non-recommended) usage:

ac:

 #define PERSONALITY   1
 #include "main_logic.c"

bc:

 #define PERSONALITY   2
 #include "main_logic.c"

main_logic.c:

 #if PERSONALITY == 1
 int main (void)
 {
    printf ("personality 1\n");
 }
 #endif

 #if PERSONALITY == 2
 int main (void)
 {
    printf ("personality 2\n");
 }
 #endif

Answer 2

Declaring variables in the header makes them available to all files that include this header using #include<...> . Declaring it in the cpp file makes them available to the code in this file only.

This is because of the way cpp and h files work together.

• The h file always includes the cpp file. No include statement is necessary here.
• Other files including the h file will get what's in the h file, not the cpp file, therefore variables declared there are out of scope.

The h file contains all the declarations, while the corresponding cpp file should contain the code for these declarations.

Please (also for future readers) note that using only h files in order to avoid a seemingly complex situation is very bad practice, because you would then not be using a very good language feature and it would make it (with the exception of using classes) impossible to separate private from public functions and variables.

Also, on another note: h files are distributed, while their code (which is in the cpp file is compiled into a binary. So for the sake of not reveling your code to everyone and keeping the header files small, you should split this appropriately.