使用#ifndef 时具体定义的是什么

Question

What is "ICT_TOOLS_H__"? Is it a header that I'm defining here, or is it a boolean, or an int of 1 or 0? Does the name have to be the same as the header file, or can it be a custom name? Also, do I use #ifndef if the header is included in multiple .cpp files, or should I use it even if it's only #included once? When the compiler goes through the first time, does "ICT_TOOLS_H__" get defined and on every other pass through it doesn't recompile the header, preventing multiple compilations of the same header?

#ifndef ICT_TOOLS_H__ //my proff said to start the name with "ICT_", its the programming department
#define ICT_TOOLS_H__


//implementation goes here

#endif

Answer 1

no it is a macro used for inclusion guard conditional inclusion so at first always this macro is not defined so the condition succeeds and the content is added to the source/header (where included) and in the second time the condition will fail so the content won't be added again.

this is useful when having multiple source/header files where you can include a header twice or more.

//header.h

#ifndef MY_HEADER_H // the name can be anything 
#define MY_HEADER_H
// code here (some declarations)

int value;

#endif

//source.cpp

#include "header.h"
#include "header.h"

int main()
{
     value = 0; // for example

     return 0;
}

if you remove the inclusion guard then the content of header.h will be added twice so as a result two variables value are there and that is a compile-time-error (redefinition).

Answer 2

They are basically macros and are used for conditional compilation. This is how they work the compile first checks if there is a defined macro called in your case ICT_TOOLS_H__ if it is not defined then the entire code between #ifndef and #endif will compile and as the next line indicates it will define that macro and will be a guard not to compile it again.

If a code compiles more than one time the c++ one definition rule is violated. See here https://en.m.wikipedia.org/wiki/One_Definition_Rule .