[Dune] Warnings about FieldVector::size

[Jö Fahlke]

Am Tue, 16. Aug 2011, 16:43:04 +0200 schrieb Martin Nolte:
> Using a static const member variable can lead to linker errors, if a reference
> to this constant is passed by the compiler.
> I assume that this situation would be far worse than the unnerving warning we
> currently have.

I suppose you mean a situation where the static const member has been declared
but not defined [1].  A static member variable needs to be defined *outside*
the class body if you want to take its adress, even when it has been given an
initializer together with its declaration *inside* the class body.

Even if we don't define FieldVector::size there is no problem.  Up until now
it was an enumerator, so it was never possible to take its adress.  So if we
declare it as a static const member now, this won't break anything that wasn't
broken before.

Bye,
Jö.

[1] Consider the following code:

    === test.hh ==========================================================
    template<unsigned i>
    struct A {
      static const unsigned size = i;
    };

    const unsigned &frob1();
    === test.cc ==========================================================
    #include "test.hh"

    const unsigned &frob1() {
      return A<1>::size;
    }
    === main.hh ==========================================================
    #include <iostream>
    #include "test.hh"

    int main() {
      std::cout << "direct: " << &A<1>::size << std::endl;
      std::cout << "via frob: " << &frob1() << std::endl;
    }
    ======================================================================

    When compiled using

      g++ -Wall -g -O0 test.cc main.cc -o test

    This will yield the folloing error:
    ======================================================================
    cd /tmp/ && g++ -Wall -g -O0 test.cc main.cc -o test
    /tmp/ccOALiAR.o: In function `frob1()':
    /tmp/test.cc:4: undefined reference to `A<1u>::size'
    /tmp/ccsrWApH.o: In function `main':
    /tmp/main.cc:5: undefined reference to `A<1u>::size'
    collect2: ld returned 1 exit status
    ======================================================================

    The problem is (of course [2]) that A<i>::size has been declared but not
    defined.  If you insert the definition

      template<unsigned i>
      const unsigned A<i>::size;

    after the class definition in test.hh, everything works fine.

[2] (eyes rolling) C++, you are so simple...

--
If God had intended Man to Smoke, He would have set him on Fire.
-- fortune
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