class template member function - compilation error

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  • subramanian100in@yahoo.com, India
    #1

    class template member function - compilation error

    consider the following program

    #include <iostream>

    using namespace std;

    class Rec
    {
    public:
    Rec(int arg = 10) : val(arg) { }

    private:
    int val;
    };

    template <class Tclass Test
    {
    private:
    T t;

    public:
    void print( ) const { cout << t << endl; }
    };

    int main( )
    {
    Test<Recr;

    // r.print( );

    return 0;
    }

    This program compiles fine with g++ and VC++2005 Express Edition.

    However if I remove the comment in the line
    // r.print( ),
    I get compilation error because operator<<( ) is not defined for Rec.

    Why doesn't the compiler report this error when it tries to generate
    (that is, instantiate) a class declaration for Test<Recitself? . Does
    it mean the that the template member function definition is generated
    only when the corresponding function is used(Test<T>::p rint( ) in this
    case) ?

    Kindly explain.

    Thanks
    V.Subramanian

    Tags: None
    • Neelesh Bodas
      #2
      Re: class template member function - compilation error

      On Nov 2, 11:33 am, "subramanian10. ..@yahoo.com, India"
      <subramanian10. ..@yahoo.comwro te:
      consider the following program
      >
      #include <iostream>
      >
      using namespace std;
      >
      class Rec
      {
      public:
      Rec(int arg = 10) : val(arg) { }
      >
      private:
      int val;
      >
      };
      >
      template <class Tclass Test
      {
      private:
      T t;
      >
      public:
      void print( ) const { cout << t << endl; }
      >
      };
      >
      int main( )
      {
      Test<Recr;
      >
      // r.print( );
      >
      return 0;
      >
      }
      >
      This program compiles fine with g++ and VC++2005 Express Edition.
      >
      However if I remove the comment in the line
      // r.print( ),
      I get compilation error because operator<<( ) is not defined for Rec.
      >
      Why doesn't the compiler report this error when it tries to generate
      (that is, instantiate) a class declaration for Test<Recitself? . Does
      it mean the that the template member function definition is generated
      only when the corresponding function is used(Test<T>::p rint( ) in this
      case) ?
      >
      Yes, when the template member function is non-virtual.

      14.7.1.(9) from the C++ standard: "An implementation shall not
      implicitly instantiate a function template, a member template, a
      nonvirtual member function, a member class or a static data member of
      a class template that does not require instantiation"

      -N

        Comment

        • nick4ng@googlemail.com
          #3
          Re: class template member function - compilation error

          On 2 Nov, 08:57, Neelesh Bodas <neelesh.bo...@ gmail.comwrote:
          On Nov 2, 11:33 am, "subramanian10. ..@yahoo.com, India"
          >
          >
          >
          <subramanian10. ..@yahoo.comwro te:
          consider the following program
          >
          #include <iostream>
          >
          using namespace std;
          >
          class Rec
          {
          public:
          Rec(int arg = 10) : val(arg) { }
          >
          private:
          int val;
          >
          };
          >
          template <class Tclass Test
          {
          private:
          T t;
          >
          public:
          void print( ) const { cout << t << endl; }
          >
          };
          >
          int main( )
          {
          Test<Recr;
          >
          // r.print( );
          >
          return 0;
          >
          }
          >
          This program compiles fine with g++ and VC++2005 Express Edition.
          >
          However if I remove the comment in the line
          // r.print( ),
          I get compilation error because operator<<( ) is not defined for Rec.
          >
          Why doesn't the compiler report this error when it tries to generate
          (that is, instantiate) a class declaration for Test<Recitself? . Does
          it mean the that the template member function definition is generated
          only when the corresponding function is used(Test<T>::p rint( ) in this
          case) ?
          >
          Yes, when the template member function is non-virtual.
          >
          14.7.1.(9) from the C++ standard: "An implementation shall not
          implicitly instantiate a function template, a member template, a
          nonvirtual member function, a member class or a static data member of
          a class template that does not require instantiation"
          >
          -N
          AFAIK it is not possible to define template virtual method. In general
          as you have reported:
          14.7.1.(9) from the C++ standard: "An implementation shall not
          implicitly instantiate a function template,...".
          This is a feature used for example in the policy pattern.

          Cheers
          n

            Comment

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