template redefinition

This is a discussion on template redefinition within the C++ Programming forums, part of the General Programming Boards category; Hi! I will try to explain my problem in the simplest way. I have a template class Queue, as follows: ...

  1. #1
    Registered User Mortissus's Avatar
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    template redefinition

    Hi!

    I will try to explain my problem in the simplest way. I have a template class Queue, as follows:
    Code:
    template<class T>
    class Queue{
    
     public:
    
      explicit Queue();
    
      void add(T);
      T get();
    
     private:
    
      sem_t sem_struct;
      sem_t sem_count;
      list<T> elements;
      
      Queue(Queue&);
      Queue& operator=(const Queue&);
    };
    // definitions...
    The definitions are inside this file. Other two files include this header. There is other file that includes these two files. I compile each file separately. The two files that include the template compile succesfully. But, when I try to compile the third file, I have compiler error:

    In file included from ether.h:15,
    from host.cpp:2:
    fila.h:8: redefinition of `class Queue<T>'
    fila.h:8: previous definition of `class Queue<T>'

    The general structure of the files are:

    ether.h includes queue.h
    application.h includes queue.h
    host.cpp includes application.h and ether.h


    Thanks any help!

  2. #2
    vae victus! skorman00's Avatar
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    header files need to be protected to make sure that they aren't getting linked more than once, which in this case it's being linked twice. At the top of your header file, place the following two lines:

    Code:
    #ifndef _THIS_STRING_DOESNT_MATTER_AS_LONG_AS_ITS_UNIQUE
    #define _THIS_STRING_DOESNT_MATTER_AS_LONG_AS_ITS_UNIQUE
    and place
    Code:
    #endif
    at the bottom of the header. When your compiler is going through and linking everything, it will come to this file, see that _THIS_STRING_DOESNT_MATTER_AS_LONG_AS_ITS_UNIQUE is not yet defined, then define and then define your class using your declaration. Every time after that, it tries to link to the header, sees that _THIS_STRING_DOESNT_MATTER_AS_LONG_AS_ITS_UNIQUE is now defined, and does try to define it again.

  3. #3
    Registered User Dogtree's Avatar
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    >>_THIS_STRING_DOESNT_MATTER_AS_LONG_AS_ITS_UNIQ UE
    It does matter because a leading underscore in this context is reserved by the implementation.

  4. #4
    Registered User Mortissus's Avatar
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    I was not sure if this would work! Thanks!

  5. #5
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    an advice
    Do void add(const T&);
    Your implementation creates a new T var that is passed to the method, which causes a overhead. Using const T& only a pointer is passed to the class. No copy of the passed var is created, therefore it's more efficient. In the end 'elements[some_index]=passed_var'. So either way you'll have a copy of the original object in the list.

  6. #6
    Registered User Mortissus's Avatar
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    Quote Originally Posted by xErath
    an advice
    Do void add(const T&);
    Your implementation creates a new T var that is passed to the method, which causes a overhead. Using const T& only a pointer is passed to the class. No copy of the passed var is created, therefore it's more efficient. In the end 'elements[some_index]=passed_var'. So either way you'll have a copy of the original object in the list.
    Yes! Very good advice! Thank you very much!

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