C where is the vtable

This is a pretty low level, but I will do all the hard work for you. I do not think that most of the future posts will describe the details of such a low level. Disclaimer : everything written here depends on the implementation, may change in any future version, so you should not rely on it.

We consider this for educational purposes only. However, this is not important now. The size Virtualclass is 8 bytes on a bit machine. Because inside there is a hidden pointer pointing to a vtable. This article talks about their content and how they are used. To get a deeper understanding of what vtables look like, let's look at the following code with gdb to find out how memory is allocated:.

Here is what we learned from the above: — Despite the fact that classes do not have data members, there is a hidden pointer to vtable; — vtable for p1 and p2 is the same. We will also discuss this later. Let's continue our gdb session to see the contents of vtables. I will use the x command, which displays the memory on the screen. We are going to output bytes in hexadecimal format, starting with 0xb Why exactly this address?

There also may be multiple vtable pointers, if your class multiply-inherits from other classes with vtables. The vtables themselves are statically allocated somewhere in your address space. You got some of these right, but whoever wrote the questions tricked you on at least one question:. Your function is not virtual, it is thus statically called : the compiler inserts a jump to the code segment corresponding to your function. No additional memory is used per instance.

Were your function virtual, your instance would carry a vpointer, which would be dereferenced to find its class' vtable, which would then be indexed to find the function pointer to be called, and finally jump there. The additional cost is thus one vtable per class probably the size of one function pointer, times the number of virtuals functions of your class , and one pointer per instance. For the example, assume the actual ToString method index is 0 in the vtable:. Then the actual method definition is pasted into an implementation function and the vtable itself is created, with pointers to the implementation function.

This vtable will be assigned to objects of the Base type when they are created:. We're a friendly, industry-focused community of developers, IT pros, digital marketers, and technology enthusiasts learning and sharing knowledge.

Hi , I want to know, how to implement vtable in C. Answered by Tom Gunn 1, in a post from 12 Years Ago. That is my … Jump to Post. Note: This example is not completely portable, but neither is a compiler. Implement them as function pointers. This Vtable is helpfull in 2 ways a. C doesn't define casting from-to arbitrary type. That book is really complicated. I suggest you start with something simpler. Add a comment.

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