Performance and footprint of virtual function

[George2]

Hello everyone,


I want to know why virtual function's footprint and performance is compared with normal function call.

Here is my analysis, please review whether I am correct. Thanks.

1. foorprint

class Foo
{
protected:
        char * pName;
        int nSize;
        virtual void copyName(char* pN) {return;}
        void virFunc() {return;}
};
 

Class Foo instance will consume 12 bytes other than 8 bytes to hold an additonal __vfptr pointing to the entry address of virtual function table. If we remove virtual keyword, the footprint of class Foo instance will consume only 8 bytes.

2. performance

2.1 Virtual function call's performance is worse than normal function call is because it needs to use __vfptr to find the virtual function table, then invokes the function. Here using __vfptr brings one more level of indirection compared with normal function call, which is the most important reason making the performance worse.

2.2 Are there anything else matters performance?
2.3 How to find normal function call address can be determined during compile time and virtual function call address can only be determined during runtime?

Please comment whether my understanding of 1 and 2 are correct or not?


thanks in advance,
George

[weaknessforcats]

1. is correct.

2.1 is silly. The virtual function is called by:

obj->vtblptr->func

Using a function pointer is;

fptr->funct

Using a funciton pointer argument to call a function:

ptr ->copy as arg->call func

So you can say that virtual functions are worse than a normal function call becuse of the added layer of indirection. Please note this applies to ALL function arguments that are pointers since they have to be copied and dereferenced later.

The most you can say is using a pointer is slower than not using a pointer.

The icing on this cake is that ALL functions are called by address (i.e. a pointer).

2.3 is incorrect. All function addresses are known at compile time. Face it, if they are unknown then you can't link.

[George2]

Thanks for your great reply, weaknessforcats !

1.

Using a function pointer is;

fptr->funct

Using a funciton pointer argument to call a function:

ptr ->copy as arg->call func

I do not quite understand what is the type of fptr and funct? What do you mean "ptr ->copy as arg->call func"? Could you show some simple pseudo code please?

2.

2.3 is incorrect. All function addresses are known at compile time. Face it, if they are unknown then you can't link.

Do not agree. The type can not be determined during compile time, so compile could only pass pointer to the base address of virtual function table. Compiler can not find the exact function to be invoked during compile time (since it is virtual).


regards,
George

[weaknessforcats]

Georege2, take a deep breath and relax.

I do not quite understand what is the type of fptr and funct? What do you mean "ptr ->copy as arg->call func"? Could you show some simple pseudo code please?

What I meant to convey was that all functions are called by address. You always use a function pointer to call a funcion. That is, the name of a function is the address of the function. To call, you just jump to that address.

If you use a function pointer variable, you just jump th the address in the variable.

It's a single jump.

Overhead is 0.

Do not agree. The type can not be determined during compile time, so compile could only pass pointer to the base address of virtual function table. Compiler can not find the exact function to be invoked during compile time (since it is virtual).

You don't know what you are talking about. The virtual function table contains the addresses of the class virtual functions, which are known at compile time by the compiler. Otherwise, the virtual table couldn't be built.

When you call a virtual function, the compiler has inserted code to access the virtual funciton table to a particular offset for that function and just jumps to the function at that address.

It's a simple function pointer operation.

BTW: No types can be discovered at run time. All you have are the compiled bits.

[George2]

Thanks weaknessforcats ,


I have understood your points now. My question is answered.

Georege2, take a deep breath and relax.


What I meant to convey was that all functions are called by address. You always use a function pointer to call a funcion. That is, the name of a function is the address of the function. To call, you just jump to that address.

If you use a function pointer variable, you just jump th the address in the variable.

It's a single jump.

Overhead is 0.




You don't know what you are talking about. The virtual function table contains the addresses of the class virtual functions, which are known at compile time by the compiler. Otherwise, the virtual table couldn't be built.

When you call a virtual function, the compiler has inserted code to access the virtual funciton table to a particular offset for that function and just jumps to the function at that address.

It's a simple function pointer operation.

BTW: No types can be discovered at run time. All you have are the compiled bits.

regards,
George