another template question

This is a discussion on another template question within the C++ Programming forums, part of the General Programming Boards category; Is there any way to be able to do this: Code: class record : public base_record< record::pair_type > { public: ...

  1. #1
    l2u
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    another template question

    Is there any way to be able to do this:

    Code:
    class record : public base_record<record::pair_type> {
    public:
    	typedef std::pair<std::string, int> pair_type;
    	record() { }
    };
    I know pair_type typedef should be defined before using it.. Just wondering if this is possible?

    Thanks a lot for help

  2. #2
    Frequently Quite Prolix dwks's Avatar
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    Use typename?
    Code:
    class record : public base_record<typename record::pair_type> {
    I think that would work.
    dwk

    Seek and ye shall find. quaere et invenies.

    "Simplicity does not precede complexity, but follows it." -- Alan Perlis
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  3. #3
    l2u
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    It doesnt.

  4. #4
    Frequently Quite Prolix dwks's Avatar
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    I fiddled around with Dinkumware a little, but couldn't find a way to do this. Any variations, like this,
    Code:
    #include <iostream>
    #include <string>
    
    template <typename t>
    class base_record {};
    
    class record;
    typedef std::pair<std::string, int> record::pair_type;
    
    class record : public base_record<typename record::pair_type> {
    public:
        //typedef std::pair<std::string, int> pair_type;
        record() { }
    };
    
    int main() {
        return 0;
    }
    cause this sort of error:
    Code:
    sourceFile.cpp(8) : error C2027: use of undefined type 'record'
            sourceFile.cpp(7) : see declaration of 'record'
    I would suspect that you can't do this; however, I'm not sure. Maybe someone more knowledgeable about C++ will be able to tell you.
    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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  5. #5
    Cat without Hat CornedBee's Avatar
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    Impossible, sorry. But if you want to avoid redundant typing, perhaps you can put the pair_type typedef in the base class and pass the full type as a template parameter?
    Code:
    template<class Pair>
    class record_base
    {
    public:
      typedef Pair pair_type;
    };
    
    class record : public record_base< std::pair<...> >
    {
    };
    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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