Why use ifndef CLASS H and define CLASS H in h file but not in cpp

Question

I have always seen people write  class h   ifndef CLASS H  define CLASS H    blah blah blah   endif   The question is  why don t they also do that for the  cpp file that contain definitions for class functions   Let s say I have  main cpp  and main cpp includes class h  The class h file does not include anything  so how does main cpp know what is in the class cpp

User · Answer

main cpp doesn t have to know what is in class cpp  It just has to know the declarations of the functions classes that it goes to use  and these declarations are in class h   The linker links between the places where the functions classes declared in class h are used and their implementations in class cpp

User · Answer

That s done for header files so that the contents only appear once in each preprocessed source file  even if it s included more than once  usually because it s included from other header files   The first time it s included  the symbol CLASS H  known as an include guard  hasn t been defined yet  so all the contents of the file are included  Doing this defines the symbol  so if it s included again  the contents of the file  inside the  ifndef  endif block  are skipped   There s no need to do this for the source file itself since  normally  that s not included by any other files   For your last question  class h should contain the definition of the class  and declarations of all its members  associated functions  and whatever else  so that any file that includes it has enough information to use the class  The implementations of the functions can go in a separate source file  you only need the declarations to call them

User · Answer

its because of Headerfiles define what the class contains  Members  data-structures  and cpp files implement it   And of course  the main reason for this is that you could include one  h File multiple times in other  h files  but this would result in multiple definitions of a class  which is invalid

User · Answer

That s the distinction between declaration and definition   Header files typically include just the declaration  and the source file contains the definition    In order to use something you only need to know it s declaration not it s definition   Only the linker needs to know the definition   So this is why you will include a header file inside one or more source files but you won t include a source file inside another    Also you mean  include and not import

User · Answer

It doesn t - at least during the compilation phase   The translation of a c   program from source code to machine code is performed in three phases    Preprocessing - The Preprocessor parses all source code for lines beginning with   and executes the directives  In your case  the contents of your file class h is inserted in place of the line   include  class h  Since you might be includein your header file in several places  the  ifndef clauses avoid duplicate declaration-errors  since the preprocessor directive is undefined only the first time the header file is included  Compilation - The Compiler does now translate all preprocessed source code files to binary object files   Linking - The Linker links  hence the name  together the object files  A reference to your class or one of its methods  which should be declared in class h and defined in class cpp  is resolved to the respective offset in one of the object files  I write  one of your object files  since your class does not need to be defined in a file named class cpp  it might be in a library which is linked to your project    In summary  the declarations can be shared through a header file  while the mapping of declarations to definitions is done by the linker

User · Answer

It is generally expected that modules of code such as  cpp files are compiled once and linked to in multiple projects  to avoid unnecessary repetitive compilation of logic  For example  g   -o class cpp would produce class o which you could then link from multiple projects to using g   main cpp class o   We could use  include as our linker  as you seem to be implying  but that would just be silly when we know how to link properly using our compiler with less keystrokes and less wasteful repetition of compilation  rather than our code with more keystrokes and more wasteful repetition of compilation     The header files are still required to be included into each of the multiple projects  however  because this provides the interface for each module  Without these headers the compiler wouldn t know about any of the symbols introduced by the  o files   It is important to realise that the header files are what introduce the definitions of symbols for those modules  once that is realised then it makes sense that multiple inclusions could cause redefinitions of symbols  which causes errors   so we use include guards to prevent such redefinitions

User · Answer

cpp files are not included  using  include  into other files  Therefore they don t need include guarding  Main cpp will know the names and signatures of the class that you have implemented in class cpp only because you have specified all that in class h - this is the purpose of a header file   It is up to you to make sure that class h accurately describes the code you implement in class cpp   The executable code in class cpp will be made available to the executable code in main cpp thanks to the efforts of the linker

User · Answer

The CLASS H is an include guard  it s used to avoid the same header file being included multiple times  via different routes  within the same CPP file  or  more accurately  the same translation unit   which would lead to multiple-definition errors   Include guards aren t needed on CPP files because  by definition  the contents of the CPP file are only read once   You seem to have interpreted the include guards as having the same function as import statements in other languages  such as Java   that s not the case  however  The  include itself is roughly equivalent to the import in other languages

User · Answer

First  to address your first inquiry   When you see this in  h file    ifndef FILE H  define FILE H         Declarations etc here          endif   This is a preprocessor technique of preventing a header file from being included multiple times  which can be problematic for various reasons   During compilation of your project  each  cpp file  usually  is compiled   In simple terms  this means the compiler will take your  cpp file  open any files  included by it  concatenate them all into one massive text file  and then perform syntax analysis and finally it will convert it to some intermediate code  optimize perform other tasks  and finally generate the assembly output for the target architecture   Because of this  if a file is  included multiple times under one  cpp file  the compiler will append its file contents twice  so if there are definitions within that file  you will get a compiler error telling you that you redefined a variable   When the file is processed by the preprocessor step in the compilation process  the first time its contents are reached the first two lines will check if FILE H has been defined for the preprocessor   If not  it will define FILE H and continue processing the code between it and the  endif directive   The next time that file s contents are seen by the preprocessor  the check against FILE H will be false  so it will immediately scan down to the  endif and continue after it   This prevents redefinition errors   And to address your second concern   In C   programming as a general practice we separate development into two file types   One is with an extension of  h and we call this a  header file    They usually provide a declaration of functions  classes  structs  global variables  typedefs  preprocessing macros and definitions  etc   Basically  they just provide you with information about your code   Then we have the  cpp extension which we call a  code file    This will provide definitions for those functions  class members  any struct members that need definitions  global variables  etc   So the  h file declares code  and the  cpp file implements that declaration   For this reason  we generally during compilation compile each  cpp file into an object and then link those objects  because you almost never see one  cpp file include another  cpp file    How these externals are resolved is a job for the linker   When your compiler processes main cpp  it gets declarations for the code in class cpp by including class h   It only needs to know what these functions or variables look like  which is what a declaration gives you    So it compiles your main cpp file into some object file  call it main obj    Similarly  class cpp is compiled into a class obj file   To produce the final executable  a linker is invoked to link those two object files together   For any unresolved external variables or functions  the compiler will place a stub where the access happens   The linker will then take this stub and look for the code or variable in another listed object file  and if it s found  it combines the code from the two object files into an output file and replaces the stub with the final location of the function or variable   This way  your code in main cpp can call functions and use variables in class cpp IF AND ONLY IF THEY ARE DECLARED IN class h   I hope this was helpful

[c++] Why use #ifndef CLASS_H and #define CLASS_H in .h file but not in .cpp?

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