Re: How to get rid of the new-initializer in a new-expression

From:
Jeff Schwab <jeff@schwabcenter.com>
Newsgroups:
comp.lang.c++.moderated
Date:
Fri, 22 Feb 2008 23:41:48 CST
Message-ID:
<8oidnVDu-ZCNhiLanZ2dnUVZ_u2mnZ2d@comcast.com>
Matthias Hofmann wrote:

"Jeff Schwab" <jeff@schwabcenter.com> schrieb im Newsbeitrag
news:zq6dnZqTEfaeYyDanZ2dnUVZ_gednZ2d@comcast.com...

Would you consider something like this?

    template<typename T>
    T* New() { return new T; }

    template<typename T, typename U>
    T* New(U const& u) { return new T(u); }

    template<typename T, typename U, typename V>
    T* New(U const& u, V const& v) { return new T(u, v); }

    // ... Up to some large number of arguments.

    struct X {
         X() { }
         X(int) { }
         X(int, double) { }
    };

    int main() {

         X* p1 = New<X>();
         X* p2 = New<X>(2);
         X* p3 = New<X>(2, 3.0);

         // ...

         return 0;
    }


Not a bad idea. Unfortunately, I had to give up on my memory tracker
because
I ran into a problem with the array form of the new operator. It has to do
with the requirement of a constant expression for array bounds:

#include <cstddef>

// Extracts the type of an object.
template <class T>
struct extract { typedef T type; };

// Extracts the type of an array.
template <class T, std::size_t N>
struct extract<T[N]> { typedef T type; };

template <class T>
struct TrackNewImpl
{
      static T* TrackNew( T* ptr )
      {
          // Do memory tracking for objects.

          return ptr;
      }
};

template <class T, std::size_t N>
struct TrackNewImpl<T[N]>
{
      static T* TrackNew( T* ptr )
      {
          // Do memory tracking for arrays.

          return ptr;
      }
};

template <class T> typename extract<T>::type*
     TrackNew( typename extract<T>::type* ptr )
{
      return TrackNewImpl<T>::TrackNew( ptr );
}

#define NEW( T ) TrackNew<T>( new T )

int* f( std::size_t size )
{
     // Error: Constant expression
     // required for array bounds.
     return NEW( int[size] );
}

int main()
{
     int* p = f( 32 );

     return 0;
}

Guess there's no solution for this?


There are plenty of alternatives, so it depends what you consider a
solution.

There's no fundamental problem; it's just that int[size] isn't a static
type, so you can't parameterize TrackNew with it. It makes sense, since
the whole point of using dynamic allocation here is that the array size
isn't known until run-time. I think I know how to do this in a way
you'll like; I'll try to post it this weekend, when I have an hour or so
to spend on it.

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