Since nobody complained, checked in cow_wrapper.hxx in a new module, under the tools project (which seemed to me the right place for a tools package): o3tl/inc/o3tl/cow_wrapper.hxx contains the stuff, the remainder of the module is currently a mere unit test for said template.
The scope for o3tl is admittedly kind of ambitious, as it should contain “…very
basic (template) functionality, comparable to what’s provided by boost
or stl, but specific to OOo (what comes to mind are e.g. stl adapters
for our own datatypes and UNO, and stuff that could in principle be
upstreamed to boost, but isn’t as of now).”
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