============================
 #include <sys/stat.h>
    ...
    struct stat results;
    if (stat("input.bin", &results) == 0)
    // The size of the file in bytes is in
    // results.st_size
    else
    // An error occurred
============================

fsize = myfile.tellg();
// The file pointer is currently at the beginning
myfile.seekg(0, ios::end);
// Place the file pointer at the end of file
fsize = myfile.tellg() - fsize;
myfile.close();
cout << "size is: " << fsize << " bytes.\n";

=============================