Template stream operator overloading

I want to be able to put the template class below in a cin and cout and thereby be able to input/output the value contained within. Has anyone done this before and know how it could be done?

Code:

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template<class T> SomeClass
{
public:
    SomeClass(T inval) { value = inval; }
protected:
    T value;
}

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Example of what I want to be able to do:

Code:

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SomeClass someObject<int>(5);
cout << someObject << endl;

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When run, I want this code to output '5'. Thanks

Quote:

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Originally Posted by Etherous

Has anyone done this before and know how it could be done?

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Quite trivial to implement:

Code:

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template<class T> class SomeClass
{
public:
    SomeClass(T inval) : value(inval) {} //This is real initialization.

    friend std::ostream& operator<<(std::ostream& outputStream, const SomeClass& instance)
    {
        return outputStream << instance.value;
    }

protected:
    T value;
};

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EDIT: Forgot about the second part of your question; it's the same idea with operator>>, but you'll be dealing with std::istream (in the case of std::cin).

Well, not surprisingly, it works. I feel stupid because it's such a simple thing to do. I just haven't needed to do this for quite some time. Thanks.

Is there any approach that works with any class including different variable ?

This is tedious because you need to write this every time for every class.

Quote:

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Originally Posted by Peter_APIIT

Is there any approach that works with any class including different variable ?

This is tedious because you need to write this every time for every class.

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The compiler can't read your mind (yet). :sick:

I mean does generic programming able to handle such a tedious task for every class.

I have a template class called basic_posint, in which I have declared a friend function

Code:

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template<unsigned BASE>
class base_posint : public integer {
    ...
public:
    friend std::ostream& operator <<(std::ostream&, const basic_posint);
}

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but the compiler gives me the following error:

Code:

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integer.hpp:85: error: ISO C++ forbids declaration of `basic_posint' with no type

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The function is declared as public. What does it mean, doesn't it recognize basic_posint as a typename?

Edit: I also tried your example template class, and it compiled without any warnings. On the other hand, if I replaced the function body by a semi colon (hence defining the function somewhere else), I got the following warning:

Code:

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integer.hpp:48: warning: friend declaration `std::ostream& operator<<(std::ostream&, const SomeClass<T>&)' declares a non-template function
integer.hpp:48: warning: (if this is not what you intended, make sure the function template has already been declared and add <> after the function name here) -Wno-non-template-friend disables this warning

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Is it supposed to be like this? Do you have to define the function within the class to prevent this warning? (I'm compiling with g++)

Quote:

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Originally Posted by TriKri

I have a template class called basic_posint

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Your class template is named base_posint. You need to decide if you want to use base_posint or basic_posint.

Quote:

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Originally Posted by TriKri

Do you have to define the function within the class to prevent this warning? (I'm compiling with g++)

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No. Read this FAQ on Why do I get linker errors when I use template friends? for a solution.

Quote:

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Originally Posted by laserlight

Your class template is named base_posint. You need to decide if you want to use base_posint or basic_posint.


No. Read this FAQ on Why do I get linker errors when I use template friends? for a solution.

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Doh! XD Sometimes I feel really stupid. I replaced basic_posint with base_posint, and the problem got solved. Strange, huh? :P

Concerning the warnings I got from the compiler, I had to do both of the things that were mentioned in your link in order to get it to compile. If I remove the <> template indication I get a warning and then undefined reference to `operator<<(std::ostream&, base_posint<10u> const&)' and it won't compile, and if I remove the pre-declaration I get an error saying my friend function does not match any template declaration. But now it works just fine. Thanks!