Templates and Typedefs

Discussion in 'C++' started by dwrayment, Aug 12, 2003.

  1. dwrayment

    dwrayment Guest

    here is an example of what im trying to do in the most basic form,

    template<class T, class I> class A : public T {

    typedef struct {I i; T t;} e;
    // functions not that important, not the issue
    }

    struct {... } typea;
    struct {... } typeb;
    A<type,int> _a;
    A<typea,int> _b;



    Problem: when instantiating _b, entry.T is of the form typea
    and not typeb. so when i try doing something like
    entry.T = (T &)(*this) it produces cannot convert
    from typea to typeb and vice versa.


    I have also tried just declaring struct e without typedef and the same thing
    happens. I think the solution involves the keyword
    typename but Im having trouble figureing it out.

    If anyone can help id appreciate it. Thanks
    dwrayment, Aug 12, 2003
    #1
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  2. "dwrayment" <> wrote...
    > here is an example of what im trying to do in the most basic form,
    >
    > template<class T, class I> class A : public T {
    >
    > typedef struct {I i; T t;} e;
    > // functions not that important, not the issue
    > }
    >
    > struct {... } typea;
    > struct {... } typeb;


    Did you mean to write

    struct typea {};
    struct typeb {};

    ???

    > A<type,int> _a;


    What's "type"? Did you mean "typeb"?

    > A<typea,int> _b;
    >
    >
    >
    > Problem: when instantiating _b, entry.T is of the form typea


    What's "entry"? Did you mean "e"? And there is no e.T. There is
    only e.t. Did you mean "e.t" instead of "entry.T"?

    > and not typeb. so when i try doing something like
    > entry.T = (T &)(*this) it produces cannot convert
    > from typea to typeb and vice versa.


    Why would you do that?

    > I have also tried just declaring struct e without typedef and the same

    thing
    > happens. I think the solution involves the keyword
    > typename but Im having trouble figureing it out.
    >
    > If anyone can help id appreciate it. Thanks


    You'll have to excuse me, I am trying to get all the ducks in a row
    here. What are you trying to do, actually?

    This (irrelevant things removed):

    template<class T, class I> struct A : public T
    {
    struct
    {
    I i;
    T t;
    } entry;

    A() { entry.t = *this; }
    };

    struct typea {};

    int main()
    {
    A<typea, int> b;
    }


    compiles OK (and is valid code).

    Victor
    Victor Bazarov, Aug 12, 2003
    #2
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  3. dwrayment

    dwrayment Guest

    fist off im using pseudocode so if its not "technically" right is not
    supposed to be it only supposed to show the meaning. aka e.T

    However, i did make a small typeo

    A<type,int> _a; should be A<typea,int> _a;
    and
    A<typea,int> _b; should be A<typeb,int> _b;

    and yes its compiles fine in your example because theres only one
    instatiation of the template class. it works fine for me also with one
    case.

    derive another template class using a different type as T.
    (hence the meaning of typea and typeb, which are preferabley complex and
    not just double or char etc)

    Aka

    > int main()
    > {
    > A<typea, int> _a;

    A<typeb, int> _b;

    //and make a member function that attempts to copy
    // ie class member funct copy(T &)
    // { e _entry;
    // read(_entry) <-psuedocode meaning get
    // some value for _entry
    // (T &)*this) = _entry.T; <- psuedocode}

    // Finally, make sure to call the function in main.

    _a.copy(.typea.); <-psuedocode
    _b.copy(.typeb.);

    > }
    >


    lastly why isnt iimportant.



    "Victor Bazarov" <> wrote in message
    news:...
    > "dwrayment" <> wrote...
    > > here is an example of what im trying to do in the most basic form,
    > >
    > > template<class T, class I> class A : public T {
    > >
    > > typedef struct {I i; T t;} e;
    > > // functions not that important, not the issue
    > > }
    > >
    > > struct {... } typea;
    > > struct {... } typeb;

    >
    > Did you mean to write
    >
    > struct typea {};
    > struct typeb {};
    >
    > ???
    >
    > > A<type,int> _a;

    >
    > What's "type"? Did you mean "typeb"?
    >
    > > A<typea,int> _b;
    > >
    > >
    > >
    > > Problem: when instantiating _b, entry.T is of the form typea

    >
    > What's "entry"? Did you mean "e"? And there is no e.T. There is
    > only e.t. Did you mean "e.t" instead of "entry.T"?
    >
    > > and not typeb. so when i try doing something like
    > > entry.T = (T &)(*this) it produces cannot convert
    > > from typea to typeb and vice versa.

    >
    > Why would you do that?
    >
    > > I have also tried just declaring struct e without typedef and the same

    > thing
    > > happens. I think the solution involves the keyword
    > > typename but Im having trouble figureing it out.
    > >
    > > If anyone can help id appreciate it. Thanks

    >
    > You'll have to excuse me, I am trying to get all the ducks in a row
    > here. What are you trying to do, actually?
    >
    > This (irrelevant things removed):
    >
    > template<class T, class I> struct A : public T
    > {
    > struct
    > {
    > I i;
    > T t;
    > } entry;
    >
    > A() { entry.t = *this; }
    > };
    >
    > struct typea {};
    >
    > int main()
    > {
    > A<typea, int> b;
    > }
    >
    >
    > compiles OK (and is valid code).
    >
    > Victor
    >
    >
    dwrayment, Aug 12, 2003
    #3
  4. "dwrayment" <> wrote...
    >
    > fist off im using pseudocode so if its not "technically" right is not
    > supposed to be it only supposed to show the meaning. aka e.T [...]


    Please read FAQ 5.8. You can find FAQ list here:
    http://www.parashift.com/c -faq-lite/

    Victor
    Victor Bazarov, Aug 13, 2003
    #4
  5. dwrayment

    dwrayment Guest

    i appreciate your trying to help but nothing at that site is useful to what
    i want to do. in the meantime i have come up with a "it works" solution
    that i am "content with" so i am not rushed to find an answer. but
    curiostity kills that cat, so i will work on it evenetuallly. i hate not
    being able to do something.


    "Victor Bazarov" <> wrote in message
    news:sdi_a.90819$...
    > "dwrayment" <> wrote...
    > >
    > > fist off im using pseudocode so if its not "technically" right is not
    > > supposed to be it only supposed to show the meaning. aka e.T [...]

    >
    > Please read FAQ 5.8. You can find FAQ list here:
    > http://www.parashift.com/c -faq-lite/
    >
    > Victor
    >
    >
    dwrayment, Aug 13, 2003
    #5
  6. dwrayment

    dwrayment Guest

    lmao. you choose not to use psuedocode because it says so in faq. tff






    "Sam Holden" <> wrote in message
    news:...
    > On Wed, 13 Aug 2003 05:34:42 GMT, dwrayment <> wrote:
    > > i appreciate your trying to help but nothing at that site is useful to

    what
    > > i want to do. in the meantime i have come up with a "it works"

    solution
    > > that i am "content with" so i am not rushed to find an answer. but
    > > curiostity kills that cat, so i will work on it evenetuallly. i hate

    not
    > > being able to do something.

    >
    > Other than the specific FAQ you were directed to that tells you not
    > to post "pseudocode" to "show the meaning".
    >
    > And if you had read FAQ 5.4 you (assuming you aren't antisocial) wouldn't
    > have done the bit below:
    >
    > > "Victor Bazarov" <> wrote in message
    > > news:sdi_a.90819$...
    > >> "dwrayment" <> wrote...
    > >> >
    > >> > fist off im using pseudocode so if its not "technically" right is not
    > >> > supposed to be it only supposed to show the meaning. aka e.T [...]
    > >>
    > >> Please read FAQ 5.8. You can find FAQ list here:
    > >> http://www.parashift.com/c -faq-lite/
    > >>
    > >> Victor
    > >>
    > >>

    >
    > Of course you are free to ignore the etiquette common in places
    > you visit, but it generally means people are less helpful.
    >
    > --
    > Sam Holden
    >
    dwrayment, Aug 14, 2003
    #6
  7. dwrayment

    Sam Holden Guest

    On Thu, 14 Aug 2003 04:18:36 GMT, dwrayment <> wrote:
    > lmao. you choose not to use psuedocode because it says so in faq. tff


    If I wanted to maximise my chances of getting useful help from
    others I would, yes.

    If I just wanted to get myself killfiled by those who might be able
    to help, then no I wouldn't.

    [snip full quote]

    --
    Sam Holden
    Sam Holden, Aug 14, 2003
    #7
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