Why are double and long double completely the same on my 64 bit machine?

Question

This question may sound like for beginners, however when I found that out I thought I'm either a beginner or my comp is missing something:

int main()
{
    cout << sizeof(double) << endl;
    cout << sizeof(long double) << endl;

    cout << DBL_DIG << endl;
    cout << LDBL_DIG << endl;

    return 0;
}

PROGRAM OUTPUT:

8

8

15

15

I thought long double is 10 bytes and has 18 decimal digits while double is 8 bytes and has 15 digits but it seems I was wrong.

Why is that so?

Using MSVC 2010 on 64bit machine.

Answer 1

In MSVC++, long double is a synonym for double as you've found out. Apparently this is to take advantage of SSE/SSE2/SSE3 instruction sets which are limited to 64-bit operations.

See also here for more information.