We all know what virtual functions are in C, but how are they implemented at a deep level?Can the vtable be modified or even directly accessed at runtime?Does the vtable exist for all classes, or only those that have at least one virtual function?Do abstract classes simply have a NULL for the function pointer of at least one entry?Does having a single virtual function slow down the whole class? Or only the call to the function that is virtual? And does the speed get affected if the virtual function is actually overwritten or not, or does this have no effect so long as it is virtual.

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How are virtual functions implemented at a deep level?From:Whenever a program has a virtual function declared, a v - table is constructed for the class. The v-table consists of addresses to the virtual functions for classes that contain one or more virtual functions. The object of the class containing the virtual function contains a virtual pointer that points to the base address of the virtual table in memory. Whenever there is a virtual function call, the v-table is used to resolve to the function address. An object of the class that contains one or more virtual functions contains a virtual pointer called the vptr at the very beginning of the object in the memory.

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Hence the size of the object in this case increases by the size of the pointer. This vptr contains the base address of the virtual table in memory. Note that virtual tables are class specific, i.e., there is only one virtual table for a class irrespective of the number of virtual functions it contains. This virtual table in turn contains the base addresses of one or more virtual functions of the class. At the time when a virtual function is called on an object, the vptr of that object provides the base address of the virtual table for that class in memory.

This table is used to resolve the function call as it contains the addresses of all the virtual functions of that class. This is how dynamic binding is resolved during a virtual function call. Can the vtable be modified or even directly accessed at runtime?Universally, I believe the answer is 'no'.

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You could do some memory mangling to find the vtable but you still wouldn't know what the function signature looks like to call it. Anything that you would want to achieve with this ability (that the language supports) should be possible without access to the vtable directly or modifying it at runtime. Also note, the C language spec does not specify that vtables are required - however that is how most compilers implement virtual functions. Does the vtable exist for all objects, or only those that have at least one virtual function?I believe the answer here is 'it depends on the implementation' since the spec doesn't require vtables in the first place. However, in practice, I believe all modern compilers only create a vtable if a class has at least 1 virtual function. There is a space overhead associated with the vtable and a time overhead associated with calling a virtual function vs a non-virtual function. Do abstract classes simply have a NULL for the function pointer of at least one entry?The answer is it is unspecified by the language spec so it depends on the implementation.

Calling the pure virtual function results in undefined behavior if it is not defined (which it usually isn't) (ISO/IEC 10.4-2). In practice it does allocate a slot in the vtable for the function but does not assign an address to it. This leaves the vtable incomplete which requires the derived classes to implement the function and complete the vtable. Some implementations do simply place a NULL pointer in the vtable entry; other implementations place a pointer to a dummy method that does something similar to an assertion.Note that an abstract class can define an implementation for a pure virtual function, but that function can only be called with a qualified-id syntax (ie., fully specifying the class in the method name, similar to calling a base class method from a derived class). This is done to provide an easy to use default implementation, while still requiring that a derived class provide an override. Does having a single virtual function slow down the whole class or only the call to the function that is virtual?This is getting to the edge of my knowledge, so someone please help me out here if I'm wrong!I believe that only the functions that are virtual in the class experience the time performance hit related to calling a virtual function vs.

A non-virtual function. The space overhead for the class is there either way. Note that if there is a vtable, there is only 1 per class, not one per object.

Does the speed get affected if the virtual function is actually overridden or not, or does this have no effect so long as it is virtual?I don't believe the execution time of a virtual function that is overridden decreases compared to calling the base virtual function. However, there is an additional space overhead for the class associated with defining another vtable for the derived class vs the base class. Additional Resources:(via way back machine). This answer has been incorporated into the. Do abstract classes simply have a NULL for the function pointer of at least one entry?The answer for that is that it is unspecified - calling the pure virtual function results in undefined behavior if it is not defined (which it usually isn't) (ISO/IEC 10.4-2). Some implementations do simply place a NULL pointer in the vtable entry; other implementations place a pointer to a dummy method that does something similar to an assertion.Note that an abstract class can define an implementation for a pure virtual function, but that function can only be called with a qualified-id syntax (ie., fully specifying the class in the method name, similar to calling a base class method from a derived class).

This is done to provide an easy to use default implementation, while still requiring that a derived class provide an override.

To implement virtual functions, C uses a special form of late binding known as the virtual table. The virtual table is a lookup table of functions used to resolve function calls in a dynamic/late binding manner. The virtual table sometimes goes by other names, such as “vtable”, “virtual function table”, “virtual method table”, or “dispatch table”.Because knowing how the virtual table works is not necessary to use virtual functions, this section can be considered optional reading.The virtual table is actually quite simple, though it’s a little complex to describe in words. First, every class that uses virtual functions (or is derived from a class that uses virtual functions) is given its own virtual table. This table is simply a static array that the compiler sets up at compile time. A virtual table contains one entry for each virtual function that can be called by objects of the class. Each entry in this table is simply a function pointer that points to the most-derived function accessible by that class.Second, the compiler also adds a hidden pointer to the base class, which we will call.vptr.vptr is set (automatically) when a class instance is created so that it points to the virtual table for that class.

Unlike the.this pointer, which is actually a function parameter used by the compiler to resolve self-references,.vptr is a real pointer. Consequently, it makes each class object allocated bigger by the size of one pointer. It also means that.vptr is inherited by derived classes, which is important.By now, you’re probably confused as to how these things all fit together, so let’s take a look at a simple example. ;When a class object is created,.vptr is set to point to the virtual table for that class. For example, when a object of type Base is created,.vptr is set to point to the virtual table for Base. When objects of type D1 or D2 are constructed,.vptr is set to point to the virtual table for D1 or D2 respectively.Now, let’s talk about how these virtual tables are filled out.

Because there are only two virtual functions here, each virtual table will have two entries (one for function1, and one for function2). Remember that when these virtual tables are filled out, each entry is filled out with the most-derived function an object of that class type can call.The virtual table for Base objects is simple. An object of type Base can only access the members of Base. Base has no access to D1 or D2 functions. Consequently, the entry for function1 points to Base::function1, and the entry for function2 points to Base::function2.The virtual table for D1 is slightly more complex. An object of type D1 can access members of both D1 and Base. However, D1 has overridden function1, making D1::function1 more derived than Base::function1.

Consequently, the entry for function1 points to D1::function1. D1 hasn’t overridden function2, so the entry for function2 will point to Base::function2.The virtual table for D2 is similar to D1, except the entry for function1 points to Base::function1, and the entry for function2 points to D2::function2.Here’s a picture of this graphically:Although this diagram is kind of crazy looking, it’s really quite simple: the.vptr in each class points to the virtual table for that class. The entries in the virtual table point to the most-derived version of the function objects of that class are allowed to call.So consider what happens when we create an object of type D1. In this case, when b is created, vptr points to Base’s virtual table, not D1’s virtual table.

Consequently, bPtr-vptr will also be pointing to Base’s virtual table. Base’s virtual table entry for function1 points to Base::function1. Thus, bPtr-function1 resolves to Base::function1, which is the most-derived version of function1 that a Base object should be able to call.By using these tables, the compiler and program are able to ensure function calls resolve to the appropriate virtual function, even if you’re only using a pointer or reference to a base class!Calling a virtual function is slower than calling a non-virtual function for a couple of reasons: First, we have to use the.vptr to get to the appropriate virtual table. 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.Also as a reminder, any class that uses virtual functions has a vptr, and thus each object of that class will be bigger by one pointer. Virtual functions are powerful, but they do have a performance cost. Hi Alex,Can you please let me know when are the vtable loaded with the function addresses at compile time or at run time. If it is at runtime then is it when the instance of a class created else vtables are loaded same as static variables before the main function starts?Are the vtable member variables, I think they are not. So are this stored in global scope.?Are the vtables static tables?Again Thanks is a small word for such a detailed tutorial it always helps me making my concepts strongerRegards,Mehul.

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The virtual table for each class points to the most-derived versions of functions available to objects of that class. The virtual table pointer in a class object points at the virtual table for the Derived class.Therefore, for a Base pointer pointing to a Derived class, baseptr-vptr will be pointing to Derived's virtual table (because the object is actually a Derived object, and therefore vptr will be set to point to Derived's virtual table, per the above), and thus any function call made through that table will route to the most derived version of the function that a Derived object can call.Does that help?. 0:000 dps poi(bas) L4 (dump VFT of bas)0022aba0 00221154 tests!ILT+335(?f0baseUAEXXZ) f0 at index 00022aba4 0022138e tests!ILT+905(?f1baseUAEXXZ) f1 at index 10022aba8 00221230 tests!ILT+555(?f2baseUAEXXZ) f2 at index 20022abac 002213c5 tests!ILT+960(?f3baseUAEXXZ) f3 at index 30:000 dps poi(der1) L4 (dump VFT of der1, funcs was shuffled, do you remember?