Creating Function Templates

This is a discussion on Creating Function Templates within the C++ Programming forums, part of the General Programming Boards category; What is a directive? A using directive is basically a statement of the form: using namespace namespace_name ; It makes ...

  1. #16
    C++ Witch laserlight's Avatar
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    What is a directive?
    A using directive is basically a statement of the form:
    using namespace namespace_name;

    It makes all names in namespace_name (e.g., std) accessible.
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  2. #17
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    I am working on a template to accomplish the same thing as the overloaded function - namely accept and display two parameters. This time I will write this as a non-member function.

    I have this in my header file (I haven't removed the directive yet, until I know how to fully qualify the template)
    Code:
    template<class DTYPE1 text, class DTYPE2 someVal>
    for definition
    Code:
    DTYPE1, DTYPE2 showStuff(DTYPE1 text, DTYPE2 someVal)
    {
        cout << text << " " << someVal << endl;
    }
    to call:
    Code:
    showStuff(string "Text ", int 7);
    showStuff(string "More text ", double 36.65);
    The C++ book I am working from has next to nothing to say about more than one data type. I am sure that there needs to be a template argument list but how?
    Last edited by marQade; 12-03-2007 at 12:08 PM. Reason: oops.

  3. #18
    C++まいる!Cをこわせ! Elysia's Avatar
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    That is incorrect.
    A function can only return one type.
    A template functions look like this:
    Code:
    template<typename NameOfTypeHere1, typename NameOfTypeHere2> ReturnTypeHere foo(NameOfTypeHere1 myfirstarg, NameOfTypeHere2 myarg2);

  4. #19
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    Ha, so there are STILL no shortcuts in the header file? A template only shortens the definitions?

  5. #20
    C++まいる!Cをこわせ! Elysia's Avatar
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    Huh? What's that supposed to mean?
    A template is used to allow the compiler to substitute the type and generate code instead of you making lots of overloaded functions.

  6. #21
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    What I was thinking there for a minute was that my header file would contain the template prefix, followed by, well basically by more declarations that look like overloaded functions, then the definition would only contain a template definition. I'll try to work this out again and come back to you.

  7. #22
    C++まいる!Cをこわせ! Elysia's Avatar
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    Every function, declaration or definition must have the exact same structure, and thus the "template" part must also appear before every declaration as well as definition.
    Here's a sample:
    Code:
    // Declaration:
    template<typename T> class CMemoryManagerBase abstract
    {
    	// ...
    	void SetNewPointer(T* pNew) throw(...);
    	// ...
    }
    
    // Definition:
    #define MMB(return_type) template<typename T> return_type CMemoryManagerBase<T>
    MMB(void)::SetNewPointer(T* pNew) throw(...)
    {
    	// ...
    }
    This is a sample from my own code - a template class. Adding the template to the class basically adds the "template" part before all the functions.
    See the definition. I also put the template part before there and the function looks exactly like the declaration (although I use a MACRO to shorten the name).
    That's how you use templates.
    You can have more than one template, as I showed. The principle is the same.
    Last edited by Elysia; 12-03-2007 at 12:27 PM.

  8. #23
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    Thank you for your patience with me...

    so far I have this much done:

    Code:
    template<class T1, class T2>
    void showStuff(T1 text, T2 someVal)
    in my header file, and
    Code:
    template<class T1, class T2>
    void showStuff(T1 text, T2 someVal
    {
       cout << text << " " << someVal << endl;
    }
    Then call these like so:

    Code:
    	showStuff("Text here ", 36.52);
    	showStuff("More text ", 7);
    and receive an external error message.

  9. #24
    C++まいる!Cをこわせ! Elysia's Avatar
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    Don't forget ; on your declarations.
    Templates should also reside in headers. There is an excellent tutorial on why, but unfortunately, I don't have a link to it.

  10. #25
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    Okay, I'm heading out to class again...but I will come back to you in about 2 hours. I placed the ';' after the 'void showStuff(T1 text, T2 someVal);' but still get an external error. Do you mean that my header file should contain something like
    Code:
    template<class T1, class T2>
    void showAbs(T1 text, T2 someVal)
    {
    	cout << text << " " << someVal << endl;
    }
    I will look for a tutorial this afternoon.

  11. #26
    C++まいる!Cをこわせ! Elysia's Avatar
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    Place both declaration and definition in the header, or just skip the declaration and only do your definition (works fine for templates).

  12. #27
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    I understand now! Thank you so much for all your time and patience.

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