Questions on typedef

This is a discussion on Questions on typedef within the C++ Programming forums, part of the General Programming Boards category; I notice you more experienced programmers use typedef a lot. I have some questions: 1. When should you use typedef? ...

  1. #1
    Use this: dudeomanodude's Avatar
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    Questions on typedef

    I notice you more experienced programmers use typedef a lot. I have some questions:

    1. When should you use typedef? When writing:
    Code:
    std::list<myType>::iterator
    100 times can be easier expressed as:
    Code:
    typedef std::list<myType>::iterator list_itr;
    list_itr it = a_list.begin();
    2. I notice often typedefs are written at global scope. Do typedefs follow normal scoping rules? Should they typically be written at global scope?

    EDIT:

    i guess that's actually:
    Code:
    std::list<myType>::std::iterator
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  2. #2
    Cat without Hat CornedBee's Avatar
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    1) You just answered your question. When the type becomes unwieldy, unreadable, or simply overly long for the number of times you'll use it, give it an alias.

    2) They follow scoping rules. They should be written in the scope where they're useful.
    All the buzzt!
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    Quote Originally Posted by dudeomanodude View Post
    Code:
    std::list<myType>::std::iterator
    I think that's extra.

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    How do you typedef the following:

    Code:
    template <typename T> return_type my_template_class
    ???

    Do you include up to return_type or up to my_template_class?

    EDIT: I don't have a compiler right now, so I can't play around with it myself.
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    C++ Witch laserlight's Avatar
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    Are you talking about template typedefs? That language feature will only be available in C++0x.
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    Quote Originally Posted by laserlight View Post
    Are you talking about template typedefs? That language feature will only be available in C++0x.
    I'm not sure...

    I mean if I have an implementation like this:
    Code:
    template <typename T> void template_container::do_something(){
    
        std::cout << "Hello there!\n";
    }
    can I shorten the whole template <typename T> void template_container line?
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  7. #7
    The larch
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    This doesn't look entirely valid, but you could surely shorten it by defining do_something within template_container definition
    I might be wrong.

    Thank you, anon. You sure know how to recognize different types of trees from quite a long way away.
    Quoted more than 1000 times (I hope).

  8. #8
    Cat without Hat CornedBee's Avatar
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    You can't shorten an out-of-line definition, really.
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  9. #9
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    Hmmm.

    I want to do something like this:

    Code:
    typedef template <typename T> void template_container void_t;
    
    void_t::do_something(){
    
        // ...
    
    }
    that's what I'm trying to get at.
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  10. #10
    C++まいる!Cをこわせ!
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    I don't think it's possible with typedefs. However, defines can still do the trick.
    (This is, of course, assuming template_container is a template, in which case in should look more like
    typedef template <typename T> void template_container<T> void_t;
    anyway.)
    Last edited by Elysia; 03-05-2008 at 12:45 PM.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

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    Code:
    #include <iostream>
    
    template <typename T> 
    class my_temp{
    
    	public:
    
    		void do_something();
    
    };
    
    typedef template <typename T> void my_temp void_t;
    void_t::do_something(){
    
        std::cout << "Hello there!\n";
    }
    
    int main(){
    
    	my_temp<int> example;
    	
    	example.do_something();
    	
    	return 0;
    }
    yea, it didn't like the typedef line.
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  12. #12
    C++まいる!Cをこわせ!
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    Unspecialized templates aren't allowed in C++. Unspecialized templates are where you specify the name of something that is a template, but do not specify a type. Example:
    Code:
    typedef my_temp mytypedef; // Illegal
    typedef my_temp<int> mytypedef; // Legal
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  13. #13
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    Unspecialized templates aren't allowed in C++. Unspecialized templates are where you specify the name of something that is a template, but do not specify a type.
    So basically when I write a template I can't abbreviate my template stuff (with typedefs), but when I use a template, it's fair game?
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  14. #14
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    Quote Originally Posted by dudeomanodude View Post
    So basically when I write a template I can't abbreviate my template stuff (with typedefs), but when I use a template, it's fair game?
    No, you sure can do it, but you must specify the type. You can't use a specialized type such as T and you can't omit it:

    Code:
    typedef template<typename T> my_temp<T> mytd; // Illegal
    typedef my_temp mytd; // Illegal
    typedef my_temp<int> mytd; // Legal
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  15. #15
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    Okay, so the real litmus test is that the template argument itself must be a defined type?

    I want to get that right for my benefit and anyone else's.
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