C++ FAQ / Section 20 / FAQ 20.4

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Revised Jul 4, 2012

[20.4] What happens in the hardware when I call a virtual function? How many layers of indirection are there? How much overhead is there?

This is a drill-down of the previous FAQ. The answer is entirely compiler-dependent, so your mileage may vary, but most C++ compilers use a scheme similar to the one presented here.

Let's work an example. Suppose class Base has 5 virtual functions: virt0() through virt4().

// Your original C++ source code

class Base {
public:
  virtual arbitrary_return_type virt0(...arbitrary params...);
  virtual arbitrary_return_type virt1(...arbitrary params...);
  virtual arbitrary_return_type virt2(...arbitrary params...);
  virtual arbitrary_return_type virt3(...arbitrary params...);
  virtual arbitrary_return_type virt4(...arbitrary params...);
  ...
};

Step #1: the compiler builds a static table containing 5 function-pointers, burying that table into static memory somewhere. Many (not all) compilers define this table while compiling the .cpp that defines Base's first non-inline virtual function. We call that table the v-table; let's pretend its technical name is Base::__vtable. If a function pointer fits into one machine word on the target hardware platform, Base::__vtable will end up consuming 5 hidden words of memory. Not 5 per instance, not 5 per function; just 5. It might look something like the following pseudo-code:

// Pseudo-code (not C++, not C) for a static table defined within file Base.cpp

// Pretend FunctionPtr is a generic pointer to a generic member function
// (Remember: this is pseudo-code, not C++ code)
FunctionPtr Base::__vtable[5] = {
  &Base::virt0, &Base::virt1, &Base::virt2, &Base::virt3, &Base::virt4
};

Step #2: the compiler adds a hidden pointer (typically also a machine-word) to each object of class Base. This is called the v-pointer. Think of this hidden pointer as a hidden data member, as if the compiler rewrites your class to something like this:

// Your original C++ source code

class Base {
public:
  ...
  FunctionPtr* __vptr;  ← supplied by the compiler, hidden from the programmer
  ...
};

Step #3: the compiler initializes this->__vptr within each constructor. The idea is to cause each object's v-pointer to point at its class's v-table, as if it adds the following instruction in each constructor's init-list:

Base::Base(...arbitrary params...)
  : __vptr(&Base::__vtable[0])  ← supplied by the compiler, hidden from the programmer
  ...
{
  ...
}

Now let's work out a derived class. Suppose your C++ code defines class Der that inherits from class Base. The compiler repeats steps #1 and #3 (but not #2). In step #1, the compiler creates a hidden v-table, keeping the same function-pointers as in Base::__vtable but replacing those slots that correspond to overrides. For instance, if Der overrides virt0() through virt2() and inherits the others as-is, Der's v-table might look something like this (pretend Der doesn't add any new virtuals):

// Pseudo-code (not C++, not C) for a static table defined within file Der.cpp

// Pretend FunctionPtr is a generic pointer to a generic member function
// (Remember: this is pseudo-code, not C++ code)
FunctionPtr Der::__vtable[5] = {
  &Der::virt0, &Der::virt1, &Der::virt2, &Base::virt3, &Base::virt4
};                                        ^^^^----------^^^^---inherited as-is

In step #3, the compiler adds a similar pointer-assignment at the beginning of each of Der's constructors. The idea is to change each Der object's v-pointer so it points at its class's v-table. (This is not a second v-pointer; it's the same v-pointer that was defined in the base class, Base; remember, the compiler does not repeat step #2 in class Der.)

Finally, let's see how the compiler implements a call to a virtual function. Your code might look like this:

// Your original C++ code

void mycode(Base* p)
{
  p->virt3();
}

The compiler has no idea whether this is going to call Base::virt3() or Der::virt3() or perhaps the virt3() method of another derived class that doesn't even exist yet. It only knows for sure that you are calling virt3() which happens to be the function in slot #3 of the v-table. It rewrites that call into something like this:

// Pseudo-code that the compiler generates from your C++

void mycode(Base* p)
{
  p->__vptr[3](p);
}

On typical hardware, the machine-code is two 'load's plus a call:

1. The first load gets the v-pointer, storing it into a register, say r1.
2. The second load gets the word at r1 + 3*4 (pretend function-pointers are 4-bytes long, so r1+12 is the pointer to the right class's virt3() function). Pretend it puts that word into register r2 (or r1 for that matter).
3. The third instruction calls the code at location r2.

Conclusions:

• Objects of classes with virtual functions have only a small space-overhead compared to those that don't have virtual functions.
• Calling a virtual function is fast — almost as fast as calling a non-virtual function.
• You don't get any additional per-call overhead no matter how deep the inheritance gets. You could have 10 levels of inheritance, but there is no "chaining" — it's always the same — fetch, fetch, call.

Caveat: I've intentionally ignored multiple inheritance, virtual inheritance and RTTI. Depending on the compiler, these can make things a little more complicated. If you want to know about these things, DO NOT EMAIL ME, but instead ask comp.lang.c++.

Caveat: Everything in this FAQ is compiler-dependent. Your mileage may vary.