template specialization question
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Thread: template specialization question

  1. #1
    Join Date
    Aug 2009
    Posts
    14

    template specialization question

    Suppose I've got a template, and I want to only instantiate to a specific list of types, and nothing else, like this:

    template<typename T> T addone(T x) {
    return x+1;
    }

    The problem I have is that I only want functions of actual numbers to be created. I don't want it going crazy and trying to do pointer arithmetic and every other **** thing that has a + operator. I only want:

    int addone(int x)
    unsigned int addone(unsigned int x)
    long addone(long x)
    unsigned long addone(unsigned long x)
    // etc, for short, char, ...

    I don't want:
    T* addone(T* stupidpointer)

    Is there a way to do specialization such that nothing except specialized templates can be instantiated implicitly? This is causing me headaches in combination with overload resolution because it keeps trying to instantiate my templates to stupid things to resolve the overload.

  2. #2
    Join Date
    Apr 1999
    Posts
    27,427

    Re: template specialization question

    Quote Originally Posted by drwowe View Post
    Suppose I've got a template, and I want to only instantiate to a specific list of types, and nothing else, like this:

    template<typename T> T addone(T x) {
    return x+1;
    }

    The problem I have is that I only want functions of actual numbers to be created. I don't want it going crazy and trying to do pointer arithmetic and every other **** thing that has a + operator.
    Boost has an is_arithmetic<T> template.

    http://www.boost.org/doc/libs/1_39_0...rithmetic.html

    Regards,

    Paul McKenzie

  3. #3
    Join Date
    Aug 2009
    Posts
    14

    Re: template specialization question

    Thanks! It seems like it's on the right track. All the examples in the documentation provide generic template functions, and the use the traits to create optimized functions for certain kinds of types.

    Is there a way to adapt this technique so the generic functions are never created in the first place?

  4. #4
    Join Date
    Aug 2004
    Location
    Bucuresti
    Posts
    38

    Re: template specialization question

    You can try this:

    Code:
     
    template<typename T> 
    T addone(T x) 
    {
         return x+1;
    }
     
    template <typename T>
    T* addone(T* p)
    {
       // error at compile time, only if the method is called 
       return 1+1;
       // or, at runtime
       // assert(0);
     
       return NULL;
    }
    The drawback with this code is that the error message is not very appropriate. I suggest you to consult this book Andrei Alexandrescu - "Modern C++ Design: Generic Programming and Design Patterns Applied" - it may help you to find a better solution.
    Regards,
    Cosmin

  5. #5
    Join Date
    Jul 2002
    Location
    Portsmouth. United Kingdom
    Posts
    2,712

    Re: template specialization question

    If your compiler support the TR1 extensions to the language then the <type_traits> header is available.

    The compile time error can be generated by using a 'static assert'

    static_assert is not supported by current compilers (as far as I know), but it can be emulated. See http://www.csn.ul.ie/~caolan/Fragments/C++StaticAssert.html or the Boost version can be used.
    "It doesn't matter how beautiful your theory is, it doesn't matter how smart you are. If it doesn't agree with experiment, it's wrong."
    Richard P. Feynman

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