Why is this error? C++ stringstream instance?

I am creating this wrapper class:
When I compile this it keep on telling me that there is an error in the mDataStream object definition in the .h file. I do not understand why. I have seen several examples on th enet and I do not see a problem. Can someobe tell me why?

//BinaryStream.h

#ifndef BinaryStream_EXISTS
#define BinaryStream_EXISTS

#include <string>
#include <sstream>

class BinaryStream;

namespace Wrsp
{
    class BinaryStream
    {
        public:
          void read(char *, int);
          void write(const char*, int);
 
          void operator<<(const char* );
          void operator>>(char* );  
        private:
           std::stringstream mDataStream(std::stringstream::in | std::stringstream::out | std::stringstream::binary);
    };
}


//BinaryStream.cpp
#include "BinaryStream.h"

namespace Wrsp
{
    void BinaryStream::read(char* buffer, int buffersize)
    {
        mDataStream.read(buffer, buffersize);
    }


    void BinaryStream::write(const char* buffer, int buffersize)
    {
        mDataStream.write(buffer, buffersize);
    }


    void BinaryStream::operator<<(const char* inChar )
    {
        write(inChar, 1);
    }
     

    void BinaryStream::operator>>(char* inChar)
    {  
        read(inChar, 1);
    }  
}