void* in a struct

[cma]

Have you anytime heard about VARIANT type?? variant data type is used to extensively in COM programming. This may help you to hold different data type value. but It needs some considerable time to spend on this datatype to use your requirements, that I leave it to you. You can get more details in MSDN and in Codeguru also. Search for Variant.

Variant

There's also Boost Variant and Boost Any with included usage examples.