从文件缓冲区读取6个字节

Question

Guys, i need to read 6 bytes from filebuffer and store them as unsigned number.

ifstream ifs("dummy.txt", ios::binary);
unsigned __int64 result = 0;
ifs.read((char*)&result, 6);

This is correct?

Answer 1

First, the standard type of a 64 bit unsigned integer is either 'unsigned long long' or 'uint64_t'. And second, you have to know the format of the data in the file you're reading. I've never seen a format which uses six bytes, so it's difficult to guess, but supposing it's binary, you should use either:

uint64_t readSix( std::istream& src )
{
    uint64_t result = checkedGet( src ) ;
    result |= checkedGet( src ) <<  8;
    result |= checkedGet( src ) << 16;
    result |= checkedGet( src ) << 24;
    result |= checkedGet( src ) << 32;
    result |= checkedGet( src ) << 48;
    return result;
}

or

uint64_t readSix( std::istream& src )
{
    uint64_t result = checkedGet( src ) << 48;
    result |= checkedGet( src ) << 32;
    result |= checkedGet( src ) << 24;
    result |= checkedGet( src ) << 16;
    result |= checkedGet( src ) <<  8;
    result |= checkedGet( src );
    return result;
}

depending on the format, with:

unsigned char checkedGet( std::istream& src )
{
    int result = src.get();
    if ( result == EOF )
        throw UnexpectedEof();
    return result;
}