problem instantiating template with pointers

This is a discussion on problem instantiating template with pointers within the C++ Programming forums, part of the General Programming Boards category; I can't figure out why this won't compile: Code: template <class T, T init = T()> class test { }; ...

  1. #1
    Guest Sebastiani's Avatar
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    Question problem instantiating template with pointers

    I can't figure out why this won't compile:

    Code:
    template <class T, T init = T()> class test { };
    
    int
    main(void)
    {
    	test<int, 1024> a; /* ok */
    	test<int> b; /* ok */
    	test<int*, 0> c; /* no */
    	test<int*> d; /* ok / no (depends on the compiler) */
    }
    any ideas?
    Code:
    #include <cmath>
    #include <complex>
    bool euler_flip(bool value)
    {
        return std::pow
        (
            std::complex<float>(std::exp(1.0)), 
            std::complex<float>(0, 1) 
            * std::complex<float>(std::atan(1.0)
            *(1 << (value + 2)))
        ).real() < 0;
    }

  2. #2
    vae victus! skorman00's Avatar
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    MSVC 2003 doesn't seem to mind. Perhaps it doesn't like converting 0 to a pointer? Try NULL, or (int *)0.

  3. #3
    Guest Sebastiani's Avatar
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    I tried that too, but with mixed results...
    Code:
    #include <cmath>
    #include <complex>
    bool euler_flip(bool value)
    {
        return std::pow
        (
            std::complex<float>(std::exp(1.0)), 
            std::complex<float>(0, 1) 
            * std::complex<float>(std::atan(1.0)
            *(1 << (value + 2)))
        ).real() < 0;
    }

  4. #4
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    What's the error message? With Comeau online it is:
    Code:
    "ComeauTest.c", line 7: error: argument of type "int" is incompatible with template
              parameter of type "int *"
      	test<int*, 0> c; /* no */
    which would seem to indicate skorman00 is correct. The (int *)0 fixes it there.

  5. #5
    Guest Sebastiani's Avatar
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    o.k., thanks. but mingw still won't compile what you guys suggested nor the last two examples I posted - the complaint is: 'a casts to a type other than an integral or enumeration type cannot appear in a constant-expression'. not sure, but it may be something in the standard (which mingw sticks pretty close to). I wonder if there's a workaround for that?
    Code:
    #include <cmath>
    #include <complex>
    bool euler_flip(bool value)
    {
        return std::pow
        (
            std::complex<float>(std::exp(1.0)), 
            std::complex<float>(0, 1) 
            * std::complex<float>(std::atan(1.0)
            *(1 << (value + 2)))
        ).real() < 0;
    }

  6. #6
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    I would imagine it would be a bug in the compiler. VC++ 2003 and Comeau are fairly standards compliant themselves, and since 0 is a valid pointer you should be able to use it there.

    So it doesn't compile d above? What is the error when you only try d?

  7. #7
    Guest Sebastiani's Avatar
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    it says: ' "0u" is not a valid template argument - it must be the address of an object with external linkage'.

    >> I would imagine it would be a bug in the compiler.

    it could very well be. but mingw is one of the strictest compilers out there - in fact, I've noticed that if something compiles with mingw, it will probably compile with any other compiler. not that it doesn't have it's share of bugs - it does (there have been a few internal compiler errors using templates I've come across), but all in all it seems to follow the standard to a 'T', making it a great tool for writing portable code. another thing I noticed with this particular project is mingw won't let you use operator delete on a void*. It seems to me that in both of these cases the compiler should have issued a warning instead of an error...odd.
    Code:
    #include <cmath>
    #include <complex>
    bool euler_flip(bool value)
    {
        return std::pow
        (
            std::complex<float>(std::exp(1.0)), 
            std::complex<float>(0, 1) 
            * std::complex<float>(std::atan(1.0)
            *(1 << (value + 2)))
        ).real() < 0;
    }

  8. #8
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    I would search over in comp.lang.c++.moderated if you haven't already, and consider asking about this there if you don't find anything. Compiler writers themselves often answer there, so you are more likely to get a good explanation. I'm not sure I even want to try to figure out where to look in the standard for such an issue.

  9. #9
    Guest Sebastiani's Avatar
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    well said.
    Code:
    #include <cmath>
    #include <complex>
    bool euler_flip(bool value)
    {
        return std::pow
        (
            std::complex<float>(std::exp(1.0)), 
            std::complex<float>(0, 1) 
            * std::complex<float>(std::atan(1.0)
            *(1 << (value + 2)))
        ).real() < 0;
    }

  10. #10
    Just Lurking Dave_Sinkula's Avatar
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    I don't know why I chose to wander into the standard for this other than just curiousity. A bit I happened upon (14.3.2p5):
    for a non-type template-parameter of type pointer to object, qualification conversions (4.4) and the array-to-pointer conversion (4.2) are applied. [Note: In particular, neither the null pointer conversion (4.10) nor the derived-to-base conversion (4.10) are applied. Although 0 is a valid template-argument for a non-type template-parameter of integral type, it is not a valid template-argument for a non-type template-parameter of pointer type. ]
    FWIW
    7. It is easier to write an incorrect program than understand a correct one.
    40. There are two ways to write error-free programs; only the third one works.*

  11. #11
    Guest Sebastiani's Avatar
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    ah, so basically, even though it compiles on some compilers, it shouldn't. but I'm confused about the default template parameter now...calling a constructor isn't exactly a constant expression - I wonder if that's invalid, too? anyway, thanks for looking that up, Dave.
    Code:
    #include <cmath>
    #include <complex>
    bool euler_flip(bool value)
    {
        return std::pow
        (
            std::complex<float>(std::exp(1.0)), 
            std::complex<float>(0, 1) 
            * std::complex<float>(std::atan(1.0)
            *(1 << (value + 2)))
        ).real() < 0;
    }

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