Why basic_ifstream show me wrong results?

Question

I have a binary file. There are 2288*2288 longitude float values stored in top half section, and the same number of latitude float values occupied the bottom half. I used the following code to load them into a float vector. It can run like a charm, but gave me incorrect results. With regard to my binary file, the float vector should be filled with a total of 2288*2288*2=10469888 elements, but only 159005, all their values are the same 200.0000. Would you please explain what's wrong with my code?

Thank you in advance!

bool LoadData(const char* pszDataFile)
{   
    typedef char_traits<float> traits_type;
    typedef std::codecvt<float, char, mbstate_t> cvt;

    std::basic_ifstream<float, traits_type> input( pszDataFile, std::ios::binary );
    std::locale loc(std::locale(), new cvt());
    input.imbue(loc);

    std::vector<float> fvBuffer;    

    // Copies all data into buffer 
    std::copy(std::istreambuf_iterator<float>(input),          
              std::istreambuf_iterator<float>( ),         
              std::back_inserter(fvBuffer)); 

    long nSzie = fvBuffer.size();  // Wrong vector size (159005)

    return true;
}

Answer 1

To make std::basic_ifstream works, you have to define your trait_type so that it provides everything expected by input stream, and I'm not sure that it will be possible. That will be far more than just a codecvt<float, char, mbstate_t> (which you seem to think is already present while the standard demand only the wchar_t, char and char, char specializations).

If you want a binary input iterator, you'll have to write one yourself to work with basic_ifstream, something like this (run and give the expected result, but not debugged further):

#include <fstream>
#include <algorithm>
#include <vector>
#include <iterator>
#include <iostream>

template <typename T>
class BinaryInputIterator
    : public std::iterator<std::input_iterator_tag, T, std::ptrdiff_t,
                           const T*, const T&>
{
public:
    BinaryInputIterator();
    BinaryInputIterator(std::istream&);
    // Compiler generated version OK:
    // BinaryInputIterator(BinaryInputIterator const& other);
    // BinaryInputIterator& operator=(BinaryInputIterator const& other);
    ~BinaryInputIterator();

    T const& operator*() const;
    T const* operator->() const;
    BinaryInputIterator& operator++();
    BinaryInputIterator operator++(int);

private:
    std::istream* myStream;
    T myValue;

    friend bool operator==
       (BinaryInputIterator const& l, BinaryInputIterator const& r)
    {
        return ((l.myStream == NULL && (r.myStream == NULL || !*r.myStream))
                || (r.myStream == NULL && (l.myStream == NULL || !*l.myStream)));
    }
    friend bool operator!=
       (BinaryInputIterator const& l, BinaryInputIterator const& r)
    {
        return !(l == r);
    }
};

template <typename T>
BinaryInputIterator<T>::BinaryInputIterator()
    : myStream(0)
{}

template <typename T>
BinaryInputIterator<T>::BinaryInputIterator(std::istream& is)
    : myStream(&is)
{
    myStream->read(reinterpret_cast<char*>(&myValue), sizeof myValue);
}

template <typename T>
BinaryInputIterator<T>::~BinaryInputIterator()
{}

template <typename T>
T const& BinaryInputIterator<T>::operator*() const
{
    return myValue;
}

template <typename T>
T const* BinaryInputIterator<T>::operator->() const
{
    return &myValue;
}

template <typename T>
BinaryInputIterator<T>& BinaryInputIterator<T>::operator++()
{
    myStream->read(reinterpret_cast<char*>(&myValue), sizeof myValue);
    return *this;
}

template <typename T>
BinaryInputIterator<T> BinaryInputIterator<T>::operator++(int)
{
    BinaryInputIterator result(this);
    ++*this;
    return result;
}

int main()
{
 {
    std::ofstream os("foo.dta");
    std::vector<float> vect1;
    vect1.push_back(4.2);
    vect1.push_back(3.14);
    os.write(reinterpret_cast<char*>(&vect1[0]), sizeof(float)*vect1.size());
 }
 {  
    std::ifstream is("foo.dta");
    std::vector<float> vect2;
    std::copy(BinaryInputIterator<float>(is),
              BinaryInputIterator<float>(),
              std::back_inserter(vect2));
    std::copy(vect2.begin(), vect2.end(), std::ostream_iterator<float>(std::cout, "\n"));
 }
}