void* or char* for generic buffer representation?

I'm designing a Buffer class whose purpose is to represent a chunk of memory.

My underlying buffer is a char* (well, a boost::shared_array<char> actually, but it doesn't really matter).

I'm stuck at deciding what prototype to choose for my constructor:

Should I go with:

Buffer(const void* buf, size_t buflen);

Or with:

Buffer(const char* buf, size_t buflen);

Or something else ?

What is usually done, and why ?