Re: zero memory

From:
Gianni Mariani <gi3nospam@mariani.ws>
Newsgroups:
comp.lang.c++
Date:
Sat, 07 Apr 2007 06:40:01 -0700
Message-ID:
<46179f31$0$17246$5a62ac22@per-qv1-newsreader-01.iinet.net.au>
Christopher Pisz wrote:

What is the C++ way to zero out memory after calling operator new on a
struct?

A constructor is not possible in this case nor is a class, because the
people using my code c-stlye cast a pointer to the first member of the
struct to a pointer to the entrire struct later on in the code.


Note that there are a number of win32 structs that must be initialized
not only with zeros but with a member set to the size of the struct.
e.g. IMAGEHLP_SYMBOL64

The code below (checked on gcc) should automatically initialize an
entire struct to zeros and insert the right value in the "sizeofstruct"
member if it exists.

I did have a problem with visual studio on this technique a while back,
let's hope things have improved.

// This is used to initialize Win32 structs that contain a
// "sizeofstruct" structure.

struct NoMemb { char a[1]; };
struct Memb_SizeOfStruct { char a[2]; };
struct Memb_sizeofstruct { char a[3]; };

template <int w_val>
struct InitSizeOf;

template <>
struct InitSizeOf< sizeof( NoMemb ) >
{
     template <typename U>
     inline static void ObjInitSel( U & obj )
     {
     }
};

template <>
struct InitSizeOf< sizeof( Memb_SizeOfStruct ) >
{
     template <typename U>
     inline static void ObjInitSel( U & obj )
     {
         obj.SizeOfStruct = sizeof( obj );
     }
};

template <>
struct InitSizeOf< sizeof( Memb_sizeofstruct ) >
{
     template <typename U>
     inline static void ObjInitSel( U & obj )
     {
         obj.sizeofstruct = sizeof( obj );
     }
};

template <typename T>
struct InitObject
{
     private:

     struct xA {};
     struct xB : xA {};

     template <int w_size>
     struct Detect
     {
     };

     public:

     template <typename U>
     inline static Memb_sizeofstruct ObjInitSel(
         U & obj, xB * b, Detect< sizeof(&U::sizeofstruct) > * = 0
     );

     template <typename U>
     inline static Memb_SizeOfStruct ObjInitSel(
         U & obj, xB * b, Detect< sizeof(&U::SizeOfStruct) > * = 0
     );

     template <typename U>
     inline static NoMemb ObjInitSel( U & obj, xA * a );

     inline static void ObjInit( T & obj )
     {
         typedef xB * bp;

         InitSizeOf<sizeof( ObjInitSel(obj, bp()))>::ObjInitSel( obj );
     }
};

struct InitStruct
{

     template <typename T>
     inline operator T ()
     {
         T obj = T();
         InitObject<T>().ObjInit( obj );

         return obj;
     }

     template <typename T>
     inline operator T * ()
     {
         T * obj = new T();

         InitObject<T>().ObjInit( * obj );

         return obj;
     }

};

/////////// test code

struct A
{
     int a;
     char x[10];
};

struct B
{
     int SizeOfStruct;
     char x[15];
};

struct C
{
     int sizeofstruct;
     char x[22];
};

B Bfoo()
{
     return InitStruct();
}

C Cfoo()
{
     return InitStruct();
}

A Afoo()
{
     return InitStruct();
}

int main()
{

     // make dynamically allocated version
     C * c = InitStruct();

     delete c;

     Bfoo();
     Cfoo();
     Afoo();
}

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