user defined template question

This is a discussion on user defined template question within the C++ Programming forums, part of the General Programming Boards category; I wrot e a template which successfully works for ints, floats, doubles etc. It includes a method which does a ...

  1. #1
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    user defined template question

    I wrot e a template which successfully works for ints, floats, doubles etc. It includes a method which does a swap between two members of the class.
    For example:
    the class decl.
    Code:
    template< class T1, class T2 > 
    class Pair {
              public:
                     Pair();
                     Pair(const T1 &x, const T2 &y) : first(x), second(y) {}
                     ~Pair();
                     void setPairX(const T1 &x);
                     void setPairY(const T2 &y);
                     T1 getPairX() const { return first; }  //inline def
                     T2 getPairY() const { return second; } // inline def
                     void swap();
              private:
                      T1 first;
                      T2 second;
    };
    the void swap definition looks like this:\
    Code:
    template< class T1, class T2 >
    void Pair< T1, T2 >::swap()
    {
         union { T1 tmpX; T2 tmpY; };
     
         if (typeid(first) == typeid(T1)) {
            tmpX = first;
            first = static_cast<T1>(second);
            second = static_cast<T2>(tmpX);
         }
         else {
             tmpY = first;
             first = static_cast<T2>(second);
             second = static_cast<T1>(tmpY);
         }
    }
    My question is: why this doesn't work for "strings"? I get compiler errors, complaining about the anonymous union I used in swap method. Why is that? These two do not work together?
    I'm kinda new to C++, trying hard to memorize all the rules...

  2. #2
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    C++ classes (with constructors, destructors, member functions, etc) cannot be a member of a union. The C++ string type is a C++ class.

  3. #3
    Registered User whiteflags's Avatar
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    Uh, you didn't give the union a name. it should be something like
    Code:
     union { T1 tmpX; T2 tempY; } TmpTypes;
    Also, a union only sets aside enough memory to hold it's largest data member. I need an example:
    Code:
     union {
       double dollar;
       int yen;
    } currency;
    I can only use dollars or yen, there isn't enough space to keep track of both of them. You might want to use a class instead.
    Last edited by whiteflags; 04-25-2006 at 01:51 AM.

  4. #4
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    Thanks, Grumpy, I did not know that.

    @citizen, this is an "anonymous union", it's working fine for built-in types. And as far as I know, it is legal in C++.

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