Where is the vptr (virtual pointer) initialized in a class having only parameterized constructors?
To be strictly technical this is completely implementation defined.
However, almost known compilers implement dynamic dispatch through vptr and v-table mechanism. All of these compilers will initialize the vptr to point to its own v-table in each constructor's member initialization list.
Something like:
Base::Base(...arbitrary params...)
: __vptr(&Base::__vtable[0]) ← supplied by the compiler, hidden from the programmer
{
}
This C++ FAQ explains a gist of what exactly happens.