Why have header files and cpp files

Question

Why does C   have header files and  cpp files

User · Accepted Answer

Well  the main reason would be for separating the interface from the implementation   The header declares  what  a class  or whatever is being implemented  will do  while the cpp file defines  how  it will perform those features   This reduces dependencies so that code that uses the header doesn t necessarily need to know all the details of the implementation and any other classes headers needed only for that  This will reduce compilation times and also the amount of recompilation needed when something in the implementation changes   It s not perfect  and you would usually resort to techniques like the Pimpl Idiom to properly separate interface and implementation  but it s a good start

User · Answer

Often you will want to have a definition of an interface without having to ship the entire code  For example  if you have a shared library  you would ship a header file with it which defines all the functions and symbols used in the shared library  Without header files  you would need to ship the source   Within a single project  header files are used  IMHO  for at least two purposes    Clarity  that is  by keeping the interfaces separate from the implementation  it is easier to read the code Compile time  By using only the interface where possible  instead of the full implementation  the compile time can be reduced because the compiler can simply make a reference to the interface instead of having to parse the actual code  which  idealy  would only need to be done a single time

User · Answer

Because the people who designed the library format didn t want to  waste  space for rarely used information like C preprocessor macros and function declarations   Since you need that info to tell your compiler  this function is available later when the linker is doing its job   they had to come up with a second file where this shared information could be stored   Most languages after C C   store this information in the output  Java bytecode  for example  or they don t use a precompiled format at all  get always distributed in source form and compile on the fly  Python  Perl

User · Answer

Because in C    the final executable code does not carry any symbol information  it s more or less pure machine code   Thus  you need a way to describe the interface of a piece of code  that is separate from the code itself  This description is in the header file

User · Answer

It s the preprocessor way of declaring interfaces  You put the interface  method declarations  into the header file  and the implementation into the cpp  Applications using your library only need to know the interface  which they can access through  include

User · Answer

Because C  where the concept originated  is 30 years old  and back then  it was the only viable way to link together code from multiple files   Today  it s an awful hack which totally destroys compilation time in C    causes countless needless dependencies  because class definitions in a header file expose too much information about the implementation   and so on

User · Answer

C   compilation A compilation in C   is done in 2 major phases   The first is the compilation of  quot source quot  text files into binary  quot object quot  files  The CPP file is the compiled file and is compiled without any knowledge about the other CPP files  or even libraries   unless fed to it through raw declaration or header inclusion  The CPP file is usually compiled into a  OBJ or a  O  quot object quot  file   The second is the linking together of all the  quot object quot  files  and thus  the creation of the final binary file  either a library or an executable     Where does the HPP fit in all this process  A poor lonesome CPP file    The compilation of each CPP file is independent from all other CPP files  which means that if A CPP needs a symbol defined in B CPP  like     A CPP void doSomething        doSomethingElse       Defined in B CPP       B CPP void doSomethingElse           Etc     It won t compile because A CPP has no way to know  quot doSomethingElse quot  exists    Unless there is a declaration in A CPP  like     A CPP void doSomethingElse        From B CPP  void doSomething        doSomethingElse        Defined in B CPP    Then  if you have C CPP which uses the same symbol  you then copy paste the declaration    COPY PASTE ALERT  Yes  there is a problem  Copy pastes are dangerous  and difficult to maintain  Which means that it would be cool if we had some way to NOT copy paste  and still declare the symbol    How can we do it  By the include of some text file  which is commonly suffixed by  h   hxx   h   or  my preferred for C   files   hpp     B HPP  here  we decided to declare every symbol defined in B CPP  void doSomethingElse         A CPP  include  quot B HPP quot   void doSomething        doSomethingElse        Defined in B CPP       B CPP  include  quot B HPP quot   void doSomethingElse           Etc        C CPP  include  quot B HPP quot   void doSomethingAgain        doSomethingElse        Defined in B CPP    How does include work  Including a file will  in essence  parse and then copy-paste its content in the CPP file  For example  in the following code  with the A HPP header     A HPP void someFunction    void someOtherFunction         the source B CPP     B CPP  include  quot A HPP quot   void doSomething           Etc         will become after inclusion     B CPP void someFunction    void someOtherFunction     void doSomething           Etc     One small thing - why include B HPP in B CPP  In the current case  this is not needed  and B HPP has the doSomethingElse function declaration  and B CPP has the doSomethingElse function definition  which is  by itself a declaration   But in a more general case  where B HPP is used for declarations  and inline code   there could be no corresponding definition  for example  enums  plain structs  etc    so the include could be needed if B CPP uses those declaration from B HPP  All in all  it is  quot good taste quot  for a source to include by default its header  Conclusion The header file is thus necessary  because the C   compiler is unable to search for symbol declarations alone  and thus  you must help it by including those declarations  One last word  You should put header guards around the content of your HPP files  to be sure multiple inclusions won t break anything  but all in all  I believe the main reason for existence of HPP files is explained above   ifndef B HPP   define B HPP      The declarations in the B hpp file   endif    B HPP   or even simpler  although not standard   pragma once     The declarations in the B hpp file

User · Answer

It s the preprocessor way of declaring interfaces  You put the interface  method declarations  into the header file  and the implementation into the cpp  Applications using your library only need to know the interface  which they can access through  include

User · Answer

Because C  where the concept originated  is 30 years old  and back then  it was the only viable way to link together code from multiple files   Today  it s an awful hack which totally destroys compilation time in C    causes countless needless dependencies  because class definitions in a header file expose too much information about the implementation   and so on

User · Answer

Responding to MadKeithV s answer      This reduces dependencies so that code that uses the header doesn t   necessarily need to know all the details of the implementation and any   other classes headers needed only for that  This will reduce   compilation times  and also the amount of recompilation needed when   something in the implementation changes    Another reason is that a header gives a unique id to each class   So if we have something like  class A       class B   public A         class C       include A cpp      include B cpp                 We will have errors  when we try to build the project  since A is part of B  with headers we would avoid this kind of headache

User · Answer

It s the preprocessor way of declaring interfaces  You put the interface  method declarations  into the header file  and the implementation into the cpp  Applications using your library only need to know the interface  which they can access through  include

User · Answer

Because C   inherited them from C  Unfortunately

User · Answer

Because in C    the final executable code does not carry any symbol information  it s more or less pure machine code   Thus  you need a way to describe the interface of a piece of code  that is separate from the code itself  This description is in the header file

User · Answer

Responding to MadKeithV s answer      This reduces dependencies so that code that uses the header doesn t   necessarily need to know all the details of the implementation and any   other classes headers needed only for that  This will reduce   compilation times  and also the amount of recompilation needed when   something in the implementation changes    Another reason is that a header gives a unique id to each class   So if we have something like  class A       class B   public A         class C       include A cpp      include B cpp                 We will have errors  when we try to build the project  since A is part of B  with headers we would avoid this kind of headache

User · Answer

Often you will want to have a definition of an interface without having to ship the entire code  For example  if you have a shared library  you would ship a header file with it which defines all the functions and symbols used in the shared library  Without header files  you would need to ship the source   Within a single project  header files are used  IMHO  for at least two purposes    Clarity  that is  by keeping the interfaces separate from the implementation  it is easier to read the code Compile time  By using only the interface where possible  instead of the full implementation  the compile time can be reduced because the compiler can simply make a reference to the interface instead of having to parse the actual code  which  idealy  would only need to be done a single time

User · Answer

C   compilation A compilation in C   is done in 2 major phases   The first is the compilation of  quot source quot  text files into binary  quot object quot  files  The CPP file is the compiled file and is compiled without any knowledge about the other CPP files  or even libraries   unless fed to it through raw declaration or header inclusion  The CPP file is usually compiled into a  OBJ or a  O  quot object quot  file   The second is the linking together of all the  quot object quot  files  and thus  the creation of the final binary file  either a library or an executable     Where does the HPP fit in all this process  A poor lonesome CPP file    The compilation of each CPP file is independent from all other CPP files  which means that if A CPP needs a symbol defined in B CPP  like     A CPP void doSomething        doSomethingElse       Defined in B CPP       B CPP void doSomethingElse           Etc     It won t compile because A CPP has no way to know  quot doSomethingElse quot  exists    Unless there is a declaration in A CPP  like     A CPP void doSomethingElse        From B CPP  void doSomething        doSomethingElse        Defined in B CPP    Then  if you have C CPP which uses the same symbol  you then copy paste the declaration    COPY PASTE ALERT  Yes  there is a problem  Copy pastes are dangerous  and difficult to maintain  Which means that it would be cool if we had some way to NOT copy paste  and still declare the symbol    How can we do it  By the include of some text file  which is commonly suffixed by  h   hxx   h   or  my preferred for C   files   hpp     B HPP  here  we decided to declare every symbol defined in B CPP  void doSomethingElse         A CPP  include  quot B HPP quot   void doSomething        doSomethingElse        Defined in B CPP       B CPP  include  quot B HPP quot   void doSomethingElse           Etc        C CPP  include  quot B HPP quot   void doSomethingAgain        doSomethingElse        Defined in B CPP    How does include work  Including a file will  in essence  parse and then copy-paste its content in the CPP file  For example  in the following code  with the A HPP header     A HPP void someFunction    void someOtherFunction         the source B CPP     B CPP  include  quot A HPP quot   void doSomething           Etc         will become after inclusion     B CPP void someFunction    void someOtherFunction     void doSomething           Etc     One small thing - why include B HPP in B CPP  In the current case  this is not needed  and B HPP has the doSomethingElse function declaration  and B CPP has the doSomethingElse function definition  which is  by itself a declaration   But in a more general case  where B HPP is used for declarations  and inline code   there could be no corresponding definition  for example  enums  plain structs  etc    so the include could be needed if B CPP uses those declaration from B HPP  All in all  it is  quot good taste quot  for a source to include by default its header  Conclusion The header file is thus necessary  because the C   compiler is unable to search for symbol declarations alone  and thus  you must help it by including those declarations  One last word  You should put header guards around the content of your HPP files  to be sure multiple inclusions won t break anything  but all in all  I believe the main reason for existence of HPP files is explained above   ifndef B HPP   define B HPP      The declarations in the B hpp file   endif    B HPP   or even simpler  although not standard   pragma once     The declarations in the B hpp file

User · Answer

Because the people who designed the library format didn t want to  waste  space for rarely used information like C preprocessor macros and function declarations   Since you need that info to tell your compiler  this function is available later when the linker is doing its job   they had to come up with a second file where this shared information could be stored   Most languages after C C   store this information in the output  Java bytecode  for example  or they don t use a precompiled format at all  get always distributed in source form and compile on the fly  Python  Perl

User · Answer

Because C   inherited them from C  Unfortunately

User · Answer

Because C  where the concept originated  is 30 years old  and back then  it was the only viable way to link together code from multiple files   Today  it s an awful hack which totally destroys compilation time in C    causes countless needless dependencies  because class definitions in a header file expose too much information about the implementation   and so on

User · Answer

C   compilation A compilation in C   is done in 2 major phases   The first is the compilation of  quot source quot  text files into binary  quot object quot  files  The CPP file is the compiled file and is compiled without any knowledge about the other CPP files  or even libraries   unless fed to it through raw declaration or header inclusion  The CPP file is usually compiled into a  OBJ or a  O  quot object quot  file   The second is the linking together of all the  quot object quot  files  and thus  the creation of the final binary file  either a library or an executable     Where does the HPP fit in all this process  A poor lonesome CPP file    The compilation of each CPP file is independent from all other CPP files  which means that if A CPP needs a symbol defined in B CPP  like     A CPP void doSomething        doSomethingElse       Defined in B CPP       B CPP void doSomethingElse           Etc     It won t compile because A CPP has no way to know  quot doSomethingElse quot  exists    Unless there is a declaration in A CPP  like     A CPP void doSomethingElse        From B CPP  void doSomething        doSomethingElse        Defined in B CPP    Then  if you have C CPP which uses the same symbol  you then copy paste the declaration    COPY PASTE ALERT  Yes  there is a problem  Copy pastes are dangerous  and difficult to maintain  Which means that it would be cool if we had some way to NOT copy paste  and still declare the symbol    How can we do it  By the include of some text file  which is commonly suffixed by  h   hxx   h   or  my preferred for C   files   hpp     B HPP  here  we decided to declare every symbol defined in B CPP  void doSomethingElse         A CPP  include  quot B HPP quot   void doSomething        doSomethingElse        Defined in B CPP       B CPP  include  quot B HPP quot   void doSomethingElse           Etc        C CPP  include  quot B HPP quot   void doSomethingAgain        doSomethingElse        Defined in B CPP    How does include work  Including a file will  in essence  parse and then copy-paste its content in the CPP file  For example  in the following code  with the A HPP header     A HPP void someFunction    void someOtherFunction         the source B CPP     B CPP  include  quot A HPP quot   void doSomething           Etc         will become after inclusion     B CPP void someFunction    void someOtherFunction     void doSomething           Etc     One small thing - why include B HPP in B CPP  In the current case  this is not needed  and B HPP has the doSomethingElse function declaration  and B CPP has the doSomethingElse function definition  which is  by itself a declaration   But in a more general case  where B HPP is used for declarations  and inline code   there could be no corresponding definition  for example  enums  plain structs  etc    so the include could be needed if B CPP uses those declaration from B HPP  All in all  it is  quot good taste quot  for a source to include by default its header  Conclusion The header file is thus necessary  because the C   compiler is unable to search for symbol declarations alone  and thus  you must help it by including those declarations  One last word  You should put header guards around the content of your HPP files  to be sure multiple inclusions won t break anything  but all in all  I believe the main reason for existence of HPP files is explained above   ifndef B HPP   define B HPP      The declarations in the B hpp file   endif    B HPP   or even simpler  although not standard   pragma once     The declarations in the B hpp file

User · Answer

C   compilation A compilation in C   is done in 2 major phases   The first is the compilation of  quot source quot  text files into binary  quot object quot  files  The CPP file is the compiled file and is compiled without any knowledge about the other CPP files  or even libraries   unless fed to it through raw declaration or header inclusion  The CPP file is usually compiled into a  OBJ or a  O  quot object quot  file   The second is the linking together of all the  quot object quot  files  and thus  the creation of the final binary file  either a library or an executable     Where does the HPP fit in all this process  A poor lonesome CPP file    The compilation of each CPP file is independent from all other CPP files  which means that if A CPP needs a symbol defined in B CPP  like     A CPP void doSomething        doSomethingElse       Defined in B CPP       B CPP void doSomethingElse           Etc     It won t compile because A CPP has no way to know  quot doSomethingElse quot  exists    Unless there is a declaration in A CPP  like     A CPP void doSomethingElse        From B CPP  void doSomething        doSomethingElse        Defined in B CPP    Then  if you have C CPP which uses the same symbol  you then copy paste the declaration    COPY PASTE ALERT  Yes  there is a problem  Copy pastes are dangerous  and difficult to maintain  Which means that it would be cool if we had some way to NOT copy paste  and still declare the symbol    How can we do it  By the include of some text file  which is commonly suffixed by  h   hxx   h   or  my preferred for C   files   hpp     B HPP  here  we decided to declare every symbol defined in B CPP  void doSomethingElse         A CPP  include  quot B HPP quot   void doSomething        doSomethingElse        Defined in B CPP       B CPP  include  quot B HPP quot   void doSomethingElse           Etc        C CPP  include  quot B HPP quot   void doSomethingAgain        doSomethingElse        Defined in B CPP    How does include work  Including a file will  in essence  parse and then copy-paste its content in the CPP file  For example  in the following code  with the A HPP header     A HPP void someFunction    void someOtherFunction         the source B CPP     B CPP  include  quot A HPP quot   void doSomething           Etc         will become after inclusion     B CPP void someFunction    void someOtherFunction     void doSomething           Etc     One small thing - why include B HPP in B CPP  In the current case  this is not needed  and B HPP has the doSomethingElse function declaration  and B CPP has the doSomethingElse function definition  which is  by itself a declaration   But in a more general case  where B HPP is used for declarations  and inline code   there could be no corresponding definition  for example  enums  plain structs  etc    so the include could be needed if B CPP uses those declaration from B HPP  All in all  it is  quot good taste quot  for a source to include by default its header  Conclusion The header file is thus necessary  because the C   compiler is unable to search for symbol declarations alone  and thus  you must help it by including those declarations  One last word  You should put header guards around the content of your HPP files  to be sure multiple inclusions won t break anything  but all in all  I believe the main reason for existence of HPP files is explained above   ifndef B HPP   define B HPP      The declarations in the B hpp file   endif    B HPP   or even simpler  although not standard   pragma once     The declarations in the B hpp file

User · Answer

Because in C    the final executable code does not carry any symbol information  it s more or less pure machine code   Thus  you need a way to describe the interface of a piece of code  that is separate from the code itself  This description is in the header file

User · Answer

Because C  where the concept originated  is 30 years old  and back then  it was the only viable way to link together code from multiple files   Today  it s an awful hack which totally destroys compilation time in C    causes countless needless dependencies  because class definitions in a header file expose too much information about the implementation   and so on

User · Answer

Often you will want to have a definition of an interface without having to ship the entire code  For example  if you have a shared library  you would ship a header file with it which defines all the functions and symbols used in the shared library  Without header files  you would need to ship the source   Within a single project  header files are used  IMHO  for at least two purposes    Clarity  that is  by keeping the interfaces separate from the implementation  it is easier to read the code Compile time  By using only the interface where possible  instead of the full implementation  the compile time can be reduced because the compiler can simply make a reference to the interface instead of having to parse the actual code  which  idealy  would only need to be done a single time

User · Answer

It s the preprocessor way of declaring interfaces  You put the interface  method declarations  into the header file  and the implementation into the cpp  Applications using your library only need to know the interface  which they can access through  include

User · Answer

Because the people who designed the library format didn t want to  waste  space for rarely used information like C preprocessor macros and function declarations   Since you need that info to tell your compiler  this function is available later when the linker is doing its job   they had to come up with a second file where this shared information could be stored   Most languages after C C   store this information in the output  Java bytecode  for example  or they don t use a precompiled format at all  get always distributed in source form and compile on the fly  Python  Perl

User · Answer

Because the people who designed the library format didn t want to  waste  space for rarely used information like C preprocessor macros and function declarations   Since you need that info to tell your compiler  this function is available later when the linker is doing its job   they had to come up with a second file where this shared information could be stored   Most languages after C C   store this information in the output  Java bytecode  for example  or they don t use a precompiled format at all  get always distributed in source form and compile on the fly  Python  Perl

User · Answer

Often you will want to have a definition of an interface without having to ship the entire code  For example  if you have a shared library  you would ship a header file with it which defines all the functions and symbols used in the shared library  Without header files  you would need to ship the source   Within a single project  header files are used  IMHO  for at least two purposes    Clarity  that is  by keeping the interfaces separate from the implementation  it is easier to read the code Compile time  By using only the interface where possible  instead of the full implementation  the compile time can be reduced because the compiler can simply make a reference to the interface instead of having to parse the actual code  which  idealy  would only need to be done a single time

User · Answer

Because in C    the final executable code does not carry any symbol information  it s more or less pure machine code   Thus  you need a way to describe the interface of a piece of code  that is separate from the code itself  This description is in the header file

[c++] Why have header files and .cpp files?

Examples related to c++

Examples related to header-files