It doesn't have to use vtables at all though most do. Generally, you don't need to know those details. You just need to know that when you call a virtual function, it do what you expect regardless of how it does it.
That said, I'll clarify a couple points about a typical implementation. A class with virtual functions has a hidden pointer we'll call vptr that points to the vtable for that class. Assume we have an employee class:. When we derive from this class, it will also have a vptr, and it must be in the same spot in this case, at the beginning. When exactly does the compiler create a virtual function table? OR 2 when the immediate base class contains at least one virtual function.
OR 3 when any parent class at any level of the hierarchy contains at least one virtual function. Since B does not declare f as virtual, does class C get dynamic polymorphism? Improve this question. The language does not even have a concept of vTables. Unless you have some inside information on the next architectural breakthrough in modern microprocessor design I would stop with all this propaganda about vtables not being part of the language.
Even when you think you know how they work you still really don't. Di-0xide: Even if you were manually manipulating v-tables which you are not, don't be foolish enough to claim you are. You could not get any information from a discussion like this. There is nowhere near enough space for the details required.
Show 3 more comments. Active Oldest Votes. If a class declares non-inherited virtual methods such as when it doesn't have a base class , then it must have its own vtable. If a class has a different set of overriding methods than its first base class, then it must have its own vtable, and cannot reuse the base's. Destructors commonly require this. If a class has multiple base classes, with the second or later base having virtual methods: If no earlier bases have virtual methods and the Empty Base Optimization was applied to all earlier bases, then treat this base as the first base class.
Otherwise, the class must have its own vtable. If a class has any virtual base classes, it must have its own vtable. Improve this answer. Roger Pate Roger Pate. Add a comment. Implementation-wise, the compiler is allowed to do whatever it wants to accomplish that result. Oh, drat, I turned you into a banana. Answer a vtable is created when a class declaration contains a virtual function. Read on for discussion and tests -- explanation -- When you specify a member function as virtual, there is a chance that you may try to use sub-classes via a base-class polymorphically at run-time.
You do not incur the cost of the vtable if you avoid the virtual keyword. I think you mean when a class contains a virtual function. Could be: When does an object get an implicit pointer to V-Table inserted into it? AnT AnT k 39 39 gold badges silver badges bronze badges. Khaled Alshaya Khaled Alshaya The behavior is defined in chapter Hans Passant Hans Passant k gold badges silver badges bronze badges.
Sign up or log in Sign up using Google. The entries in the virtual table point to the most-derived version of the function objects of that class are allowed to call.
Note that because dPtr is a base pointer, it only points to the Base portion of d1. First, the program recognizes that function1 is a virtual function. This has been set to Dfunction1. Would it still call Dfunction1? The answer is no. Second, we have to index the virtual table to find the correct function to call.
Only then can we call the function. As a result, we have to do 3 operations to find the function to call, as opposed to 2 operations for a normal indirect function call, or one operation for a direct function call. However, with modern computers, this added time is usually fairly insignificant.
Virtual functions are powerful, but they do have a performance cost. Previous Post
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