CODEWITHSUNDEEP
c++
John Humphreys
John Humphreys

Reputation: 39294

When is the vtable used?

Is the vtable only used for virtual function lookups, or does it get used for normal member function lookups as well?

Upvotes: 2

Views: 863

Answers (7)

j_kubik
j_kubik

Reputation: 6181

This is always implementation dependent, but most of the compilers will use vtable when:

  • virtual function is being called;
  • when using dynamic_cast addresses of vtables are used. Actual vtable is not used, but it must exist to have an address.
  • virtual-y inherited class is being accesed.

virtual-y inherited class access includes both virtual and non-virtual method calls, but also field access, and even pointer casting

class foo{
public:
    virtual void bar(){}
};

class foo2: public virtual foo{
public:
    virtual void bar2(){}
};

int main(int argc, char *argv[])
{
    foo2* f2= new foo2();
    f2->bar();
    foo* f1 = f2;
}

Line foo* f1 = f2; will also read vtable (although firstly check will be made for f2==NULL, in such case f1 will be also NULL).

Upvotes: 0

Thomas
Thomas

Reputation: 19

And method pointer lookup. There is at least a check to see if the pointer points on virtual member function.

Upvotes: -1

Matthew
Matthew

Reputation: 25763

As far as I know, the vtable is only created (and thus used) when that method being called is specified as Virtual. If it's virtual, then it will use the vtable, if it is not virtual, then it will be statically binded.

This determination is done at compile time.

Upvotes: 0

john
john

Reputation: 87959

dynamic_cast would also use the vtable, I believe.

Upvotes: 1

Oliver Charlesworth
Oliver Charlesworth

Reputation: 272517

Technically, this is an implementation detail. The C++ standard says nothing about vtables or vptrs.

But typically, a compiler will only choose to use the vtable/vptr mechanism when a member function is being called in a polymorphic sense (i.e. via a pointer/referene to base class). If it knows at compile-time what to do, then there's no need for the indirection.

Upvotes: 9

0x5f3759df
0x5f3759df

Reputation: 2359

The standard doesn't dictate how inheritance is implemented, so a vtable doesn't necessarily exist. But as far as I know all the current major compilers only use vtables to call virtual functions.

Upvotes: 1

Jerry Coffin
Jerry Coffin

Reputation: 490158

Only for virtual function lookups. A non-virtual member function doesn't require much special -- it's just a normal function that receives this as a hidden parameter.

Upvotes: 3

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