128 bit data type

[pccX]

How can I use a data type that is bigger than 64 bits?

[firedraco]

std::bitset, or you can manually allocate stuff by manually allocating data with malloc or other stuff.

There is also another non-standard C only word that is called an "octaword"
http://en.wikipedia.org/wiki/Integer_(computer_science)

[helios]

GMP comes to mind.

[pccX]

What is GMP?

[naeem]

Look into this, it may be of some help to you...

http://msdn.microsoft.com/en-us/library/s3f49ktz(VS.71).aspx

[smilodon]

my computer supports a 96-bit long double (12 bytes), though it might be the compiler g++.

What are you looking for this data type to support?

[pccX]

I am looking for a data type of index of an array. 64 bit indexing is small for what I am doing.

[smilodon]

what do you need more than 64-bits of indexing for? 64bits should take care of any array size up to a couple of EB of memory

[PanGalactic]

smilodon is right. I don't think you've thought through the other limits that you might encounter using such an index.