The point is that this is an implicit formal parameter (containing the address of the object whose method you are calling). It is not a local variable.
Look at the generated code of your program. I compiled (on Linux/Debian/Sid/x86-64 with GCC 4.9.1) your example arman.cc with
gcc -O1 -fverbose-asm -S arman.cc
and got the function main below
.globl main
.type main, @function
main:
.LFB512:
.cfi_startproc
.cfi_personality 0x3,__gxx_personality_v0
.cfi_lsda 0x3,.LLSDA512
pushq %rbx #
.cfi_def_cfa_offset 16
.cfi_offset 3, -16
subq $48, %rsp #,
.cfi_def_cfa_offset 64
movq $0, 16(%rsp) #, MEM[(struct _Vector_impl *)&vec]._M_start
movq $0, 24(%rsp) #, MEM[(struct _Vector_impl *)&vec]._M_finish
movq $0, 32(%rsp) #, MEM[(struct _Vector_impl *)&vec]._M_end_of_storage
movq $0, (%rsp) #, MEM[(struct A *)&__x]
movq %rsp, %rcx #,
movl $100, %edx #,
movl $0, %esi #,
leaq 16(%rsp), %rdi #, tmp92
.LEHB0:
call _ZNSt6vectorI1ASaIS0_EE14_M_fill_insertEN9__gnu_cxx17__normal_iteratorIPS0_S2_EEmRKS0_ #
.LEHE0:
movq 16(%rsp), %rdi # MEM[(struct _Vector_base *)&vec]._M_impl._M_start, D.10014
testq %rdi, %rdi # D.10014
je .L34 #,
call _ZdlPv #
jmp .L34 #
.L33:
movq %rax, %rbx #, tmp91
movq 16(%rsp), %rdi # MEM[(struct _Vector_base *)&vec]._M_impl._M_start, D.10014
testq %rdi, %rdi # D.10014
je .L32 #,
call _ZdlPv #
.L32:
movq %rbx, %rdi # tmp91,
.LEHB1:
call _Unwind_Resume #
.LEHE1:
.L34:
movl $0, %eax #,
addq $48, %rsp #,
.cfi_def_cfa_offset 16
popq %rbx #
.cfi_def_cfa_offset 8
ret
.cfi_endproc
You see that some space for your vec is allocated on the stack (e.g. with subq $48, %rsp etc...) and then the address of that zone on the stack is passed as the this formal argument (using the usual x86-64 ABI conventions, which dictates (p 20) that the first argument of function is passed thru register %rdi) so you could say that this is, at the beginning of some member function, in the register %rdi ...