big double vectors in C++

I have vector a(8000000000);
So 8000000000*8/1024/1024/1024=7.45Gb , so I need 7.45Gb RAM in order my program with this huge size to work, and I have that RAM in my computer but it doesn't work, WHY?

It compiles, but when I run, it gives error terminate called after throwing an instance of 'std::bad_alloc' what(): std::bad_alloc Aborted (core dumped)

Thanks in advance