Template arguement probelm.

This is a discussion on Template arguement probelm. within the C++ Programming forums, part of the General Programming Boards category; This is probably something stupid but I can't figure out what's wrong with the following code: Code: template <typename STATE, ...

  1. #1
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    Template arguement probelm.

    This is probably something stupid but I can't figure out what's wrong with the following code:
    Code:
    template <typename STATE, typename UNAIRY_FUNCTION>
    std::vector<STATE> solvePuzzle(const STATE &start, const UNAIRY_FUNCTION goalfunc ){
        //compile error on next line. Says invalid template arguments.
        std::queue<std::set<STATE>::iterator> tryNext;
        ...
    }
    It is too clear and so it is hard to see.
    A dunce once searched for fire with a lighted lantern.
    Had he known what fire was,
    He could have cooked his rice much sooner.

  2. #2
    Registered User whiteflags's Avatar
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    I don't recall seeing predicate functions as typename anywhere in the STL.

    I'm also pretty sure that vectors and other containers need instantiable objects - use class.

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    Quote Originally Posted by citizen View Post
    I don't recall seeing predicate functions as typename anywhere in the STL.

    I'm also pretty sure that vectors and other containers need instantiable objects - use class.
    I'm not sure what you mean but as far as I know the following are equal:
    template <typename STATE, typename UNAIRY_FUNCTION>
    template <class STATE, class UNAIRY_FUNCTION>
    It is too clear and so it is hard to see.
    A dunce once searched for fire with a lighted lantern.
    Had he known what fire was,
    He could have cooked his rice much sooner.

  4. #4
    Frequently Quite Prolix dwks's Avatar
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    Quote Originally Posted by King Mir View Post
    I'm not sure what you mean but as far as I know the following are equal:
    template <typename STATE, typename UNAIRY_FUNCTION>
    template <class STATE, class UNAIRY_FUNCTION>
    As far as I know, they are equivalent in most modern compilers. However, the class keyword is the older style. If you're using a really old compiler, you might need to use class.

    On the other hand, I think that usage of the class keyword is deprecated and support for it might be removed in the future. So if your compiler supports it you should use typename.
    dwk

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    Captain Crash brewbuck's Avatar
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    Quote Originally Posted by King Mir View Post
    This is probably something stupid but I can't figure out what's wrong with the following code:
    Code:
    template <typename STATE, typename UNAIRY_FUNCTION>
    std::vector<STATE> solvePuzzle(const STATE &start, const UNAIRY_FUNCTION goalfunc ){
        //compile error on next line. Says invalid template arguments.
        std::queue<std::set<STATE>::iterator> tryNext;
        ...
    }
    You're missing a "typename":

    Code:
    std::queue<typename std::set<STATE>::iterator> tryNext;
    Whenever you use a scoping operator on a template-dependent typename, you need to say "typename" in front of it.

  6. #6
    Captain Crash brewbuck's Avatar
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    Quote Originally Posted by citizen View Post
    I'm also pretty sure that vectors and other containers need instantiable objects - use class.
    There is absolutely no difference between the two in this context. There is only a difference when you use a template-template-parameter, as in:

    Code:
    template <template <typename> class C, typename X> // VALID
    template <template <typename> typename C, typename X> // INVALID

  7. #7
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    Quote Originally Posted by brewbuck View Post
    You're missing a "typename":

    Code:
    std::queue<typename std::set<STATE>::iterator> tryNext;
    Whenever you use a scoping operator on a template-dependent typename, you need to say "typename" in front of it.
    Thank you. Problem solved.
    It is too clear and so it is hard to see.
    A dunce once searched for fire with a lighted lantern.
    Had he known what fire was,
    He could have cooked his rice much sooner.

  8. #8
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    > On the other hand, I think that usage of the class keyword is deprecated and support for it might be removed in
    > the future.

    Are you sure? I've never heard this, and currently I use "class" if the corresponding argument is always a class, and "typename" otherwise.

  9. #9
    Frequently Quite Prolix dwks's Avatar
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    No, you're right. This usage of class is not actually deprecated. Templated classes...
    dwk

    Seek and ye shall find. quaere et invenies.

    "Simplicity does not precede complexity, but follows it." -- Alan Perlis
    "Testing can only prove the presence of bugs, not their absence." -- Edsger Dijkstra
    "The only real mistake is the one from which we learn nothing." -- John Powell


    Other boards: DaniWeb, TPS
    Unofficial Wiki FAQ: cpwiki.sf.net

    My website: http://dwks.theprogrammingsite.com/
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