Can plastic pipe go DIRECTLY to a water heater?

I was told there should be at least one foot of copper or steel pipe between the heater and PEX or CPVC pipes. Looking on the web, all I see are opinions, no facts. Does the code have anything to say about it? What about the manufacturers of these plastic pipes?
I do see where the heat from the heater could cause failure of these pipes, so I'd rather just use the metal pipes, but I'm looking to find out what is supposed to be done.
I'm looking for FACTS and REQUIREMENTS, not opinions!
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On 1/21/2016 10:23 AM, snipped-for-privacy@invalid.no wrote:

http://www.acehardware.com/info/index.jsp?categoryId 80891
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On Thu, 21 Jan 2016 09:23:02 -0600, snipped-for-privacy@invalid.no wrote:

My water heater has been plumbed with plastic for 30 years (white PVC in, CPVC out). There is a short metal riser right at the stubs but it came with the heater. I am not sure if it is a thermal trap or if it holds the anodes.
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