istream object don't read any char - c++

Question

Why istream object after calling readsome() method don't give any chars in buffer? Is there any error in class construction?
StreamBuffer.h
#ifndef StreamBuffer_h
#define StreamBuffer_h
#include <string>
#include <fstream>
#include <iostream>
#include <iterator>
enum StreamBufferState
{
STREAMBUFFER_OK = 0,
STREAMBUFFER_EOF = 1
};
class StreamBuffer
{
std::fstream file;
std::istream istrm;
int maxBufferSize;
std::string buffer;
public:
StreamBuffer(int maxBuffSize, const std::string& filename);
~StreamBuffer();
void SetMaxBufferSize(unsigned int maxBuffSize);
StreamBufferState FullBufferWithData();
std::string GetDataBuffer();
};
#endif
StreamBuffer.cpp
#include "StreamBuffer.h"
using namespace std;
StreamBuffer::StreamBuffer(int maxBuffSize, const std::string& filename) : istrm( !filename.empty() ? file.rdbuf() : cin.rdbuf() )
{
SetMaxBufferSize(maxBuffSize);
if(!filename.empty())
{
file.open(filename.c_str(),ios::in | ios::binary);
}
else
{
std::cin>>noskipws;
}
}
StreamBuffer::~StreamBuffer()
{
file.close();
}
void StreamBuffer::SetMaxBufferSize(unsigned int maxBuffSize)
{
maxBufferSize = maxBuffSize;
}
StreamBufferState StreamBuffer::FullBufferWithData()
{
istrm.readsome((char*)&buffer[0],maxBufferSize);
if(istrm.eof())
return STREAMBUFFER_EOF;
return STREAMBUFFER_OK;
}
std::string StreamBuffer::GetDataBuffer()
{
string buf = buffer;
return buf;
}
File is opened, but readsome() don't read buffer.

Answer 1

You have undefined behavior in your code, as you try read into an empty string. You need to set the size of buffer.
An unrelated logical error: In the FullBufferWithData function you will return "OK" even if there is an error reading the file.