this: the implicit parameter in OOP

I was recently reminded that the variable this is an implicit parameter passed to all methods in OOP such as C++. We can observe this by comparing a regular function vs a method belonging to some class:

#include <iostream>

void greet() {
    std::cout << "Hello World\n";
}

class Human {
public:
    void greet() {
        std::cout << "Hello World\n";
    }
};

int main() {
    greet();
    Human human = Human();
    human.greet();
}

Output:

$ g++ test.C
$ ./a.out 
Hello World
Hello World

Both may output the same thing but under the hood is where the differences shines. Note: I’ll be only showing the code of interest.

For greet:

Dump of assembler code for function _Z5greetv:
   0x0000000000401126 <+0>:	push   %rbp
   0x0000000000401127 <+1>:	mov    %rsp,%rbp
   0x000000000040112a <+4>:	mov    $0x402280,%esi

For Human::greet:

Dump of assembler code for function _ZN5Human5greetEv:
   0x000000000040115c <+0>:     push   %rbp
   0x000000000040115d <+1>:     mov    %rsp,%rbp
   0x0000000000401160 <+4>:     sub    $0x10,%rsp
   0x0000000000401164 <+8>:     mov    %rdi,-0x8(%rbp)
   0x0000000000401168 <+12>:	mov    $0x402280,%esi

For greet (_Z5greetv), we can immediately tell that the function takes in no arguments both by the mangled name (i.e. mangled name ends with v to indicate its parameter is void) and from the disassembled code. However, the same cannot be said for Human::greet. The mangled name _ZN5Human5greetEv does suggest that the method does not take in any parameter and hence the suffix v for void. But observe the following line in <+8>:

mov    %rdi,-0x8(%rbp)

This is our implicit argument, this, which contains the address of the object itself. We can observe this via gdb:

(gdb) p &human
$2 = (Human *) 0x7fffffffdc4f
...
(gdb) i r rdi
rdi            0x7fffffffdc4f      140737488346191

Notice our rdi register contains the same address as our object human: 0x7fffffffdc4f.