C + + class intrinsic function call virtual function

The process of doing the project, encountered a problem.

The problem can be abstracted to the following question:

Ordinary people eat chopsticks, children eat spoon.

classpeople { Public:    voideat () {get_util_to_eat (); }    Virtual voidget_util_to_eat () {std::cout<<"people get chopsticks"<<Std::endl; }};classChildren: Publicpeople { Public:    voidget_util_to_eat () {std::cout<<"Children get Scoop"<<Std::endl; }};intMain () {people* people =NewChildren (); People-eat (); return 0;}

Output Result:

Get Scoop

This, of course, is in line with our expectations.

Because people is not a virtual function, the above program calls the Eat method in people, which involves a concept that I have been vague about, how to invoke a virtual method in a class method.

This involves a question that has to be said before:

class a {public:    void  print () {        "I am A" << Std::endl;    }}; int Main () {    a* a = NULL;    A-print ();     return 0 ;}

The above code will output what, according to the intuitive sense of NULL how the method can be called, the core bar.

But in fact, the output is:

I am A

When invoking a class function, the C + + compiler does not control whether the class is empty, but instead passes the address of the class as the this pointer to the function.

A->print (), it is equivalent to print (&A) in the compiler

There are cases where member variables are not used in print, so it is natural to run correctly.

Then look at the assembly code to get a better understanding of it. The following is the assembly code within the People class.

 21  void   eat () { 0x0000000000400bd2  <+0 ;: Push%rbp  0x 0000000000400bd3  <+1 ;: mov%rsp,% RBP  0x0000000000400bd6  <+4  > : Sub $0x10 ,%rsp  0x0000000000400bda  <+8 ;: mov%rdi,-0x8  (%RBP)//RSP represents the first parameter, which is a pointer to the class 

 get_util_to_eat ();     0x0000000000400be9  <+23 ;: mov-0x8  (%RBP),%rax//Put the class pointer in the RAX register  0x0000000000400bed  <+27 ;: mov (%rax),%< Span style= "COLOR: #000000" >rax//Take the first address value, which is the virtual table address  0x0000000000400bf0  <+ 30 ;: mov-0x8  (%RBP),%rdi//Put in RDI, the next time the function call takes the argument with  0x0000000000400bf4  <+34 ;: mov (%rax),%rax//Remove the address of the function in the virtual table  0x0000000000400bf7  <+37 ;: callq *%rax//Call change function 
   

The summary is that when entering a non-static member function of a class, the pointer to the class is carried by default (this), and then the member variable is used within the function, and the member method is equivalent to adding a this-> in front of it.

So back to the original question, the Chilren pointer was passed in the people::eat, so the Children::get_util_to_eat method was fetched from the virtual table and called when Get_util_to_eat was called.

C + + class intrinsic function call virtual function