example

// esi contains the struct ptr
// vtable = *(DWORD*)ptr
mov eax,dword ptr [esi]

// function = *(DWORD*)vtable
mov edx,dword ptr [eax]

// put the class pointer in ecx
// this = ptr
mov ecx,esi

// call the function!
// function()
call edx

example 2.asm

// Someone memset our call destroying out vtable!
mov dword ptr [ecx], 0
mov eax, dword ptr [ecx]
mov edx, dword ptr [eax]
call edx