void function returning value?

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  • Jochen Zeischka
    #1

    void function returning value?

    I'm puzzled. When compiling this:

    template<class ValRes, class Val1, class Val2>
    Veld<ValRes>& mult(Veld<ValRe s>& res, const Veld<Val1>& v1, const
    Veld<Val2>& v2) {
    // something
    return res;
    }

    the compiler says:

    error C2562: 'mult' : 'void' function returning a value
    c:\fe dev\lin lag 9\veld.h(64) : see declaration of 'mult'

    which is the following line within the 'Veld' class:

    template<class ValRes, class Val1, class Val2>
    friend Veld<ValRes>& mult(Veld<ValRe s>& res, const Veld<Val1>& v1, const
    Veld<Val2>& v2);

    forward declaration of the friend function was done as follows:

    template<class Val> class Veld;

    template<class ValRes, class Val1, class Val2>
    Veld<ValRes>& mult(Veld<ValRe s>& res, const Veld<Val1>& v1, const
    Veld<Val2>& v2);

    Why is the compiler talking about a 'void' function? To me it seems that a
    'Veld<ValRes>&' is returned...

    Thanks a lot for any help!

    Jochen Zeischka


    Tags: None
    • tom_usenet
      #2
      Re: void function returning value?

      On Wed, 25 Feb 2004 12:11:17 +0100, "Jochen Zeischka"
      <jochen.zeischk a@rug.ac.be> wrote:
      [color=blue]
      >I'm puzzled. When compiling this:
      >
      >template<cla ss ValRes, class Val1, class Val2>
      >Veld<ValRes> & mult(Veld<ValRe s>& res, const Veld<Val1>& v1, const
      >Veld<Val2>& v2) {
      > // something
      > return res;
      >}
      >
      >the compiler says:
      >
      >error C2562: 'mult' : 'void' function returning a value
      >c:\fe dev\lin lag 9\veld.h(64) : see declaration of 'mult'
      >
      >which is the following line within the 'Veld' class:
      >
      > template<class ValRes, class Val1, class Val2>
      > friend Veld<ValRes>& mult(Veld<ValRe s>& res, const Veld<Val1>& v1, const
      >Veld<Val2>& v2);
      >
      >forward declaration of the friend function was done as follows:
      >
      >template<cla ss Val> class Veld;
      >
      >template<cla ss ValRes, class Val1, class Val2>
      >Veld<ValRes> & mult(Veld<ValRe s>& res, const Veld<Val1>& v1, const
      >Veld<Val2>& v2);
      >
      >Why is the compiler talking about a 'void' function? To me it seems that a
      >'Veld<ValRes>& ' is returned...[/color]

      To me too. This compiled fine for me on 3 compilers:

      template<class Val>
      class Veld;

      template<class ValRes, class Val1, class Val2>
      Veld<ValRes>& mult(Veld<ValRe s>& res, const Veld<Val1>& v1, const
      Veld<Val2>& v2);

      template<class Val>
      class Veld
      {
      template<class ValRes, class Val1, class Val2>
      friend Veld<ValRes>& mult(Veld<ValRe s>& res, const Veld<Val1>& v1,
      const Veld<Val2>& v2);
      //private var to test friendship
      Val v;
      };

      template<class ValRes, class Val1, class Val2>
      Veld<ValRes>& mult(Veld<ValRe s>& res, const Veld<Val1>& v1, const
      Veld<Val2>& v2)
      {
      res.v = v1.v + v2.v;
      return res;
      }

      int main()
      {
      Veld<double> v1;
      Veld<int> v2;
      Veld<short> v3;
      Veld<double> v = mult(v1, v2, v3);
      }

      What compiler are you using? Can you post a complete program
      exhibiting the error?

      Tom
      --
      C++ FAQ: http://www.parashift.com/c++-faq-lite/
      C FAQ: http://www.eskimo.com/~scs/C-faq/top.html

        Comment

        • Jochen Zeischka
          #3
          Re: void function returning value?

          Strange! When I use your code (which seems perfectly correct to me) I get
          the following message:

          c:\test\templat e mult\main.cpp(2 8) : error C2440: 'initializing' : cannot
          convert from 'void' to 'class Veld<double>'

          line 28 is:

          Veld<double> v = mult(v1, v2, v3);

          So, again, MS VC 6 sees mult as returning void...

          Jochen


          "tom_usenet " <tom_usenet@hot mail.com> wrote in message
          news:lbcp305seu nikm6qfje4faco3 gld0a1ki5@4ax.c om...[color=blue]
          > On Wed, 25 Feb 2004 12:11:17 +0100, "Jochen Zeischka"
          > <jochen.zeischk a@rug.ac.be> wrote:
          >[color=green]
          > >I'm puzzled. When compiling this:
          > >
          > >template<cla ss ValRes, class Val1, class Val2>
          > >Veld<ValRes> & mult(Veld<ValRe s>& res, const Veld<Val1>& v1, const
          > >Veld<Val2>& v2) {
          > > // something
          > > return res;
          > >}
          > >
          > >the compiler says:
          > >
          > >error C2562: 'mult' : 'void' function returning a value
          > >c:\fe dev\lin lag 9\veld.h(64) : see declaration of 'mult'
          > >
          > >which is the following line within the 'Veld' class:
          > >
          > > template<class ValRes, class Val1, class Val2>
          > > friend Veld<ValRes>& mult(Veld<ValRe s>& res, const Veld<Val1>& v1, const
          > >Veld<Val2>& v2);
          > >
          > >forward declaration of the friend function was done as follows:
          > >
          > >template<cla ss Val> class Veld;
          > >
          > >template<cla ss ValRes, class Val1, class Val2>
          > >Veld<ValRes> & mult(Veld<ValRe s>& res, const Veld<Val1>& v1, const
          > >Veld<Val2>& v2);
          > >
          > >Why is the compiler talking about a 'void' function? To me it seems that[/color][/color]
          a[color=blue][color=green]
          > >'Veld<ValRes>& ' is returned...[/color]
          >
          > To me too. This compiled fine for me on 3 compilers:
          >
          > template<class Val>
          > class Veld;
          >
          > template<class ValRes, class Val1, class Val2>
          > Veld<ValRes>& mult(Veld<ValRe s>& res, const Veld<Val1>& v1, const
          > Veld<Val2>& v2);
          >
          > template<class Val>
          > class Veld
          > {
          > template<class ValRes, class Val1, class Val2>
          > friend Veld<ValRes>& mult(Veld<ValRe s>& res, const Veld<Val1>& v1,
          > const Veld<Val2>& v2);
          > //private var to test friendship
          > Val v;
          > };
          >
          > template<class ValRes, class Val1, class Val2>
          > Veld<ValRes>& mult(Veld<ValRe s>& res, const Veld<Val1>& v1, const
          > Veld<Val2>& v2)
          > {
          > res.v = v1.v + v2.v;
          > return res;
          > }
          >
          > int main()
          > {
          > Veld<double> v1;
          > Veld<int> v2;
          > Veld<short> v3;
          > Veld<double> v = mult(v1, v2, v3);
          > }
          >
          > What compiler are you using? Can you post a complete program
          > exhibiting the error?
          >
          > Tom
          > --
          > C++ FAQ: http://www.parashift.com/c++-faq-lite/
          > C FAQ: http://www.eskimo.com/~scs/C-faq/top.html[/color]


            Comment

            • John Harrison
              #4
              Re: void function returning value?


              "Jochen Zeischka" <jochen.zeischk a@rug.ac.be> wrote in message
              news:c1ibm1$b2b $1@gaudi2.UGent .be...[color=blue]
              > Strange! When I use your code (which seems perfectly correct to me) I get
              > the following message:
              >
              > c:\test\templat e mult\main.cpp(2 8) : error C2440: 'initializing' : cannot
              > convert from 'void' to 'class Veld<double>'
              >
              > line 28 is:
              >
              > Veld<double> v = mult(v1, v2, v3);
              >
              > So, again, MS VC 6 sees mult as returning void...
              >
              > Jochen
              >[/color]

              VC++ 6 has lots of problems with templates. Make sure you have the latest
              service pack (SP5 I think) if that doesn't fix it then try an VC group,
              maybe they'll know a workaround.

              john


                Comment

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