Template friend function injection

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  • Victor Bazarov
    #16
    Re: Template friend function injection

    Victor Bazarov wrote:
    H9XLrv5oXVNvHiU I@spambox.us wrote:
    >On Jul 11, 7:53 pm, Victor Bazarov <v.Abaza...@com Acast.netwrote:
    >>H9XLrv5oXVNvH ...@spambox.us wrote:
    >>>[..]
    >>>From what I know a function gets inlined only when declaration and
    >>>definition occur in the same place, which is not true for my code.
    >>Is that code different from what you posted?
    >>>
    >>
    >I don't understand the question. I'm talking about the code I posted,
    >which doesn't compile as well as my real code I'm working on.
    >
    I wasn't sure whether you meant the code you posted or some other code
    in which the template and its explicit instantiation were in a different
    (than 'main') translation unit. Can you try your code on a different
    compiler? I am going to try in GCC on Linux. Will report with results
    in a few minutes...
    Tried it with GCC 4.2.3 on Ubuntu. Here's the output:

    Ubuntu-bazarov:~$ g++ test_template.c pp
    /tmp/ccCNJWBg.o: In function `main':
    test_template.c pp:(.text+0x12) : undefined reference to `Function()'
    collect2: ld returned 1 exit status
    Ubuntu-bazarov:~$ g++ -c test_template.c pp -o test_template.o
    Ubuntu-bazarov:~$ nm test_template.o
    U _Z8Functionv
    U __gxx_personali ty_v0
    00000000 T main
    Ubuntu-bazarov:~$

    As we can see, the function 'Function' does not get defined in the
    module although the template Test is instantiated. That looks like an
    oversight on GCC part :*)

    V
    --
    Please remove capital 'A's when replying by e-mail
    I do not respond to top-posted replies, please don't ask

      Comment

      • H9XLrv5oXVNvHiUI@spambox.us
        #17
        Re: Template friend function injection

        On Jul 11, 8:33 pm, Victor Bazarov <v.Abaza...@com Acast.netwrote:
        Victor Bazarov wrote:
        H9XLrv5oXVNvH.. .@spambox.us wrote:
        On Jul 11, 7:53 pm, Victor Bazarov <v.Abaza...@com Acast.netwrote:
        >H9XLrv5oXVNvH. ..@spambox.us wrote:
        >>[..]
        >>From what I know a function gets inlined only when declaration and
        >>definition occur in the same place, which is not true for my code.
        >Is that code different from what you posted?
        >
        I don't understand the question. I'm talking about the code I posted,
        which doesn't compile as well as my real code I'm working on.
        >
        I wasn't sure whether you meant the code you posted or some other code
        in which the template and its explicit instantiation were in a different
        (than 'main') translation unit.  Can you try your code on a different
        compiler?  I am going to try in GCC on Linux.  Will report with results
        in a few minutes...
        >
        Tried it with GCC 4.2.3 on Ubuntu.  Here's the output:
        >
             Ubuntu-bazarov:~$ g++ test_template.c pp
             /tmp/ccCNJWBg.o: In function `main':
             test_template.c pp:(.text+0x12) : undefined reference to `Function()'
             collect2: ld returned 1 exit status
             Ubuntu-bazarov:~$ g++ -c test_template.c pp -o test_template.o
             Ubuntu-bazarov:~$ nm test_template.o
                      U _Z8Functionv
                      U __gxx_personali ty_v0
             00000000 T main
             Ubuntu-bazarov:~$
        >
        As we can see, the function 'Function' does not get defined in the
        module although the template Test is instantiated.  That looks like an
        oversight on GCC part :*)
        >
        V
        --
        Please remove capital 'A's when replying by e-mail
        I do not respond to top-posted replies, please don't ask
        Ok, so the function gets probably inlined? Could you try to copy/paste
        the printf() call many times so that the compiler doesn't inline it?
        Or is there somethin for not making GCC inline a function? I know on
        MSVC there's the __declspec(noin line) but I don't know if GCC has some
        equivalent (I guess it does though).

          Comment

          • H9XLrv5oXVNvHiUI@spambox.us
            #18
            Re: Template friend function injection

            Also, I tried this on Open Watcom and it compiles and link it without
            any problem and the executable works of course.

              Comment

              • H9XLrv5oXVNvHiUI@spambox.us
                #19
                Re: Template friend function injection

                On Jul 11, 8:44 pm, H9XLrv5oXVNvH.. .@spambox.us wrote:
                On Jul 11, 8:33 pm, Victor Bazarov <v.Abaza...@com Acast.netwrote:
                >
                >
                >
                Victor Bazarov wrote:
                H9XLrv5oXVNvH.. .@spambox.us wrote:
                >On Jul 11, 7:53 pm, Victor Bazarov <v.Abaza...@com Acast.netwrote:
                >>H9XLrv5oXVNvH ...@spambox.us wrote:
                >>>[..]
                >>>From what I know a function gets inlined only when declaration and
                >>>definition occur in the same place, which is not true for my code.
                >>Is that code different from what you posted?
                >
                >I don't understand the question. I'm talking about the code I posted,
                >which doesn't compile as well as my real code I'm working on.
                >
                I wasn't sure whether you meant the code you posted or some other code
                in which the template and its explicit instantiation were in a different
                (than 'main') translation unit.  Can you try your code on a different
                compiler?  I am going to try in GCC on Linux.  Will report with results
                in a few minutes...
                >
                Tried it with GCC 4.2.3 on Ubuntu.  Here's the output:
                >
                     Ubuntu-bazarov:~$ g++ test_template.c pp
                     /tmp/ccCNJWBg.o: In function `main':
                     test_template.c pp:(.text+0x12) : undefined reference to `Function()'
                     collect2: ld returned 1 exit status
                     Ubuntu-bazarov:~$ g++ -c test_template.c pp -o test_template.o
                     Ubuntu-bazarov:~$ nm test_template.o
                              U _Z8Functionv
                              U __gxx_personali ty_v0
                     00000000 T main
                     Ubuntu-bazarov:~$
                >
                As we can see, the function 'Function' does not get defined in the
                module although the template Test is instantiated.  That looks like an
                oversight on GCC part :*)
                >
                V
                --
                Please remove capital 'A's when replying by e-mail
                I do not respond to top-posted replies, please don't ask
                >
                Ok, so the function gets probably inlined? Could you try to copy/paste
                the printf() call many times so that the compiler doesn't inline it?
                Or is there somethin for not making GCC inline a function? I know on
                MSVC there's the __declspec(noin line) but I don't know if GCC has some
                equivalent (I guess it does though).
                Nop, even with the attribute __attribute__ ((noinline)) it still gives
                that error about Function() not being defined.

                  Comment

                  • =?ISO-8859-1?Q?Marcel_M=FCller?=
                    #20
                    Re: Template friend function injection

                    H9XLrv5oXVNvHiU I@spambox.us wrote:
                    On Jul 11, 6:53 pm, Marcel Müller <news.5.ma...@s pamgourmet.org>
                    wrote:
                    >
                    >I expected something like that. That still gives you the chance, to use
                    >the functions defined inside Test not before the definition of test.
                    >>
                    >Furthermore you can define a template version of Function outside Test
                    >for this purpose.
                    >>
                    >Marcel
                    >
                    Can you explain this point better? Especially the last sentence. If I
                    define a template version of Function() then I would not be able to
                    call it from main, am I wrong? Can you provide an example? Thank you.
                    template <typename T>
                    void Function(const T& arg);

                    template <typename T>
                    class Test
                    {
                    public:
                    friend void Function<T>(con st T&);
                    };

                    template <typename T>
                    void Function(const T& arg)
                    { printf("Functio n()"); getchar(); }

                    int main(int argc, char* argv[])
                    {
                    Function(Test<i nt>());
                    }


                    But from your question I guess that you want to do something entirely
                    different. You want to control by the statement
                    template class Test<int>;
                    which version of Function is implemented, isn't it? The argument list of
                    Function does not depend on T at all, but the body does. This is evil.
                    Think what is happening if you write
                    template class Test<int>;
                    template class Test<char>;

                    You will get an error in this case, but only if both of them are defined
                    in the same module. Otherwise you violate the ODR and get undefined
                    behavior.

                    In this case a simple
                    typedef int myFunctionType;
                    would do a better job.


                    Marcel

                      Comment

                      • H9XLrv5oXVNvHiUI@spambox.us
                        #21
                        Re: Template friend function injection

                        On Jul 11, 9:27 pm, Marcel Müller <news.5.ma...@s pamgourmet.org>
                        wrote:
                        H9XLrv5oXVNvH.. .@spambox.us wrote:
                        On Jul 11, 6:53 pm, Marcel Müller <news.5.ma...@s pamgourmet.org>
                        wrote:
                        >
                        I expected something like that. That still gives you the chance, to use
                        the functions defined inside Test not before the definition of test.
                        >
                        Furthermore you can define a template version of Function outside Test
                        for this purpose.
                        >
                        Marcel
                        >
                        Can you explain this point better? Especially the last sentence. If I
                        define a template version of Function() then I would not be able to
                        call it from main, am I wrong? Can you provide an example? Thank you.
                        >
                        template <typename T>
                        void Function(const T& arg);
                        >
                        template <typename T>
                        class Test
                        {
                        public:
                                friend void Function<T>(con st T&);
                        >
                        };
                        >
                        template <typename T>
                        void Function(const T& arg)
                        { printf("Functio n()"); getchar(); }
                        >
                        int main(int argc, char* argv[])
                        {
                                Function(Test<i nt>());
                        >
                        }
                        >
                        But from your question I guess that you want to do something entirely
                        different. You want to control by the statement
                           template class Test<int>;
                        which version of Function is implemented, isn't it? The argument list of
                        Function does not depend on T at all, but the body does. This is evil.
                        Think what is happening if you write
                           template class Test<int>;
                           template class Test<char>;
                        >
                        You will get an error in this case, but only if both of them are defined
                        in the same module. Otherwise you violate the ODR and get undefined
                        behavior.
                        >
                        In this case a simple
                           typedef int myFunctionType;
                        would do a better job.
                        >
                        Marcel
                        Yes I know most of the times this would be seen as a bad design choice
                        but in this case is instead a perfect solution to a problem because I
                        know as a design principle that the Test class template will never be
                        instantiated more than one time so there won't be any duplicate
                        definition of Function(). Also if you follow the conversation with
                        Victor Bazarov he informed me that my code is completely standard-
                        compliant thus making this a possible GCC bug. I've also double-
                        checked that this code compiles and links fine on the Open Watcom
                        compiler.

                          Comment

                          • H9XLrv5oXVNvHiUI@spambox.us
                            #22
                            Re: Template friend function injection

                            On Jul 11, 8:51 pm, H9XLrv5oXVNvH.. .@spambox.us wrote:
                            Also, I tried this on Open Watcom and it compiles and link it without
                            any problem and the executable works of course.
                            Furthermore I was wrong about having a way to compile this code in
                            visualc++. In fact that code was slightly different and visualc++
                            seems not only unable to compile any kind of code like this, but it
                            also crashes when you try to make it to.
                            So basically so far the only compiler able to compile this kind of
                            code seems to be this Open Watcom.

                              Comment

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