バイナリ ファイルが部分的にしかコピーされないのはなぜですか?どうすれば修正できますか?

int length;
char * buffer;

ifstream is;
is.open ("C:\Final.gif", ios::binary );
// get length of file:
is.seekg (0, ios::end);
length = is.tellg();
is.seekg (0, ios::beg);
// allocate memory:
buffer = new char [length];
// read data as a block:
is.read (buffer,length);
is.close();

FILE *pFile;
pFile = fopen ("C:\myfile.gif", "w");
fwrite (buffer , 1 , sizeof(buffer) , pFile );

#include <fstream>
#include <iterator>
#include <algorithm>

int main()
{
    std::ifstream input( "C:\Final.gif", std::ios::binary );
    std::ofstream output( "C:\myfile.gif", std::ios::binary );

    std::copy( 
        std::istreambuf_iterator<char>(input), 
        std::istreambuf_iterator<char>( ),
        std::ostreambuf_iterator<char>(output));
}

#include <fstream>
#include <iterator>
#include <vector>

int main()
{
    std::ifstream input( "C:\Final.gif", std::ios::binary );

    // copies all data into buffer
    std::vector<unsigned char> buffer(std::istreambuf_iterator<char>(input), {});

}