Implicit template argument deduction for classes

This is a discussion on Implicit template argument deduction for classes within the C++ Programming forums, part of the General Programming Boards category; Hi all, I just discovered that class template arguements are never implicitly deduced from their constructor arguements. Am I correct ...

  1. #1
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    Implicit template argument deduction for classes

    Hi all,

    I just discovered that class template arguements are never implicitly deduced from their constructor arguements. Am I correct in thinking that the following code won't work (as I don't have access to a compiler)?

    Code:
    template<typename T> class basic_broken { /* ... */ };
    
    template<typename T>
    class broken { // bridge class
      basic_broken<T>* broken_impl;
    public:
      broken( T* t )
        : broken_impl(t)
      {}
    };
    
    void f()
    {
      int s;
      broken(&s); // Error: can't deduce template type of class broken?
    }

  2. #2
    Registered User Noir's Avatar
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    Yeah, that looks broken to me.

  3. #3
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    I feared so. Thanks Noir.

  4. #4
    Registered User Noir's Avatar
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    Why fear? It's not that big of an issue, and if you need that kind of thing, you can wrap the construction of an object in a factory function that's also a template and deduces its arguments:
    Code:
    template <class T>
    broken<T> make_more_broken( T arg ) {
      return broken<T>( arg );
    }

  5. #5
    Cat without Hat CornedBee's Avatar
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    Well, there's the forwarding issue. You have to write a function for each constructor. It's not insurmountable, but it sure is a hassle.
    All the buzzt!
    CornedBee

    "There is not now, nor has there ever been, nor will there ever be, any programming language in which it is the least bit difficult to write bad code."
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  6. #6
    Registered User Noir's Avatar
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    If you use default arguments smartly and don't bog down the class with constructors, it's not even a hassle.

  7. #7
    The larch
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    Quote Originally Posted by Noir
    Why fear? It's not that big of an issue, and if you need that kind of thing, you can wrap the construction of an object in a factory function that's also a template and deduces its arguments:
    Code:
    template <class T>
    broken<T> make_more_broken( T arg ) {
      return broken<T>( arg );
    }
    Er, how would I call this function? The only ways that work seem to be

    Code:
    broken<int> code = make_more_broken(42);
    //or
    broken<int> code(make_more_broken(42));

  8. #8
    semi-colon generator ChaosEngine's Avatar
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    Quote Originally Posted by anon
    Er, how would I call this function? The only ways that work seem to be

    Code:
    broken<int> code = make_more_broken(42);
    //or
    broken<int> code(make_more_broken(42));
    mostly these kinds of functions are used with other templates.

    e.g.
    Code:
    template <typename T>
    void print_thing(T t);
    
    print_thing(make_more_broken(42));
    in the next version of C++ the auto keyword will do what you want.
    Code:
    auto someVar = make_more_broken(42);
    "I saw a sign that said 'Drink Canada Dry', so I started"
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  9. #9
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    Quote Originally Posted by anon View Post
    Er, how would I call this function? The only ways that work seem to be

    Code:
    broken<int> code = make_more_broken(42);
    //or
    broken<int> code(make_more_broken(42));
    Isn't the point of the factory functions so you can do something like:
    Code:
    broken code = make_more_broken(42); // template arg of code automatically deduced
    Will that not work?

  10. #10
    Cat without Hat CornedBee's Avatar
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    But you can't do that either. The point is that you can pass the result of the make_* function directly to a function template, e.g.

    Code:
    std::copy(in.begin(), in.end(), make_back_inserter(foo));
    Until the auto keyword in C++09, if you want a variable, you'll have to explicitly declare its full type.
    All the buzzt!
    CornedBee

    "There is not now, nor has there ever been, nor will there ever be, any programming language in which it is the least bit difficult to write bad code."
    - Flon's Law

  11. #11
    Frequently Quite Prolix dwks's Avatar
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    Until the auto keyword in C++09, if you want a variable, you'll have to explicitly declare its full type.
    The auto keyword already exists; are they redefining it?
    dwk

    Seek and ye shall find. quaere et invenies.

    "Simplicity does not precede complexity, but follows it." -- Alan Perlis
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  12. #12
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    I believe they are just adding to its uses.

    http://www.open-std.org/jtc1/sc22/wg...2004/n1607.pdf
    Last edited by Daved; 03-30-2007 at 06:24 PM.

  13. #13
    Frequently Quite Prolix dwks's Avatar
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    Okay, thanks for the link.
    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


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    Unofficial Wiki FAQ: cpwiki.sf.net

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    Projects: codeform, xuni, atlantis, nort, etc.

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