Already defined in obj - no double inclusions

Question

I happened to get that already defined in  obj error  This is structure of my project  main cpp  include  quot main h quot    main h  include  lt iostream gt   include  lt string gt   include  lt sstream gt   include  lt boost asio hpp gt   include  lt boost thread thread hpp gt   include  quot client cpp quot   client cpp  ifndef SOCKET CLIENT CLASS  define SOCKET CLIENT CLASS  ifndef BOOST ASIO HPP  include  lt boost asio hpp gt   endif   CLASS DEFINITION HERE    endif  This is what compiler is complaining about   main obj   error LNK2005   quot public  bool   thiscall SocketClient  read int char    quot    read SocketClient  QAE NHPAD Z  already defined in client obj  Note it is complaining about my class  not boost  One interesting thing is  that when I remove  include  lt boost asio hpp gt  from client cpp  I get errors thouhg it is included in main h too  As you can see  I m not double defining including my class  its included exactly once in main h  So what s going on here  I have read this answer  but it was no help  since it expects double inclusions  Take this fact into osideration before voting for duplicate  because this simply means beheading me without mercy

User · Answer

I do recomend doing it in 2 filles   h  cpp  But if u lazy just add inline before the function So it will look something like this inline void functionX         more about inline functions   The inline functions are a C   enhancement feature to increase the execution time of a program  Functions can be instructed to compiler to make them inline so that compiler can replace those function definition wherever those are being called  Compiler replaces the definition of inline functions at compile time instead of referring function definition at runtime  NOTE- This is just a suggestion to compiler to make the function inline  if function is big  in term of executable instruction etc  then  compiler can ignore the    inline    request and treat the function as normal function   more info here

User · Answer

This is one of the method to overcome this issue     Just put the prototype in the header files and include the header files in the  cpp files as shown below         client cpp   ifndef SOCKET CLIENT CLASS  define SOCKET CLIENT CLASS  ifndef BOOST ASIO HPP  include  lt boost asio hpp gt   endif  class SocketClient    Or whatever the name is                   bool read int  char       Or whatever the name is                  endif   client h  bool SocketClient  read int  char            Implementation  goes here        main cpp   include  lt iostream gt   include  lt string gt   include  lt sstream gt   include  lt boost asio hpp gt   include  lt boost thread thread hpp gt   include  client h                     Notice this    main h  int main

User · Answer

This is not a compiler error  the error is coming from the linker  After compilation  the linker will merge the object files resulting from the compilation of each of your translation units   cpp files     The linker finds out that you have the same symbol defined multiple times in different translation units  and complains about it  it is a violation of the One Definition Rule    The reason is most certainly that main cpp includes client cpp  and both these files are individually processed by the compiler to produce two separate object files  Therefore  all the symbols defined in the client cpp translation unit will be defined also in the main cpp translation unit  This is one of the reasons why you do not usually  include  cpp files    Put the definition of your class in a separate client hpp file which does not contain also the definitions of the member functions of that class  then  let client cpp and main cpp include that file  I mean  include   Finally  leave in client cpp the definitions of your class s member functions   client h   ifndef SOCKET CLIENT CLASS  define SOCKET CLIENT CLASS  ifndef BOOST ASIO HPP  include  lt boost asio hpp gt   endif  class SocketClient    Or whatever the name is                   bool read int  char       Or whatever the name is                  endif   client cpp   include  Client h           bool SocketClient  read int  char            Implementation  goes here               add the definitions for all other member functions    main h   include  lt iostream gt   include  lt string gt   include  lt sstream gt   include  lt boost asio hpp gt   include  lt boost thread thread hpp gt   include  client h                     Notice this    main cpp   include  main h

User · Answer

You probably don t want to do this    include  client cpp    A   cpp file will have been compiled by the compiler as part of your build   By including it in other files  it will be compiled again  and again   in every file in which you include it   Now here s the thing   You are guarding it with  ifndef SOCKET CLIENT CLASS  however  each file that has  include  client cpp  is built independently and as such will find SOCKET CLIENT CLASS not yet defined   Therefore it s contents will be included  not  ifdef d out   If it contains any definitions at all  rather than just declarations  then these definitions will be repeated in every file where it s included

[c++] Already defined in .obj - no double inclusions

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