Reverse Engineering for Beginners

CHAPTER 51. C++ CHAPTER 51. C++

As it can be seen, there is no allocated buffer whenmain()starts. After the firstpush_back()call, a buffer is allocated.
And then, after eachpush_back() call, both array size and buffer size (capacity) are increased. But the buffer address
changes as well, becausepush_back()reallocates the buffer in theheapeach time. It is costly operation, that’s why it is
very important to predict the size of the array in the future and reserve enough space for it with the.reserve()method.
The last number is garbage: there are no array elements at this point, so a random number is printed. This illustrates the
fact thatoperator[]ofstd::vectordoes not check of the index is in the array’s bounds. The slower.at()method,
however, does this checking and throws anstd::out_of_rangeexception in case of error.

Let’s see the code:

Listing 51.33: MSVC 2012 /GS- /Ob1

$SG52650 DB '%d', 0aH, 00H
$SG52651 DB '%d', 0aH, 00H

_this$ = -4 ; size = 4
Pos$ = 8 ; size = 4
?at@?$vector@HV?$allocator@H@std@@@std@@QAEAAHI@Z PROC ; std::vector<int,std::allocator⤦
Ç>::at, COMDAT
; _this$ = ecx
push ebp
mov ebp, esp
push ecx
mov DWORD PTR _this$[ebp], ecx
mov eax, DWORD PTR _this$[ebp]
mov ecx, DWORD PTR _this$[ebp]
mov edx, DWORD PTR [eax+4]
sub edx, DWORD PTR [ecx]
sar edx, 2
cmp edx, DWORD PTR Pos$[ebp]
ja SHORT $LN1@at
push OFFSET ??_C@_0BM@NMJKDPPO@invalid?5vector?$DMT?$DO?5subscript?$AA@
call DWORD PTR _imp?_Xout_of_range@std@@YAXPBD@Z
$LN1@at:
mov eax, DWORD PTR _this$[ebp]
mov ecx, DWORD PTR [eax]
mov edx, DWORD PTR __Pos$[ebp]
lea eax, DWORD PTR [ecx+edx*4]
$LN3@at:
mov esp, ebp
pop ebp
ret 4
?at@?$vector@HV?$allocator@H@std@@@std@@QAEAAHI@Z ENDP ; std::vector<int,std::allocator⤦
Ç>::at

_c$ = -36 ; size = 12
$T1 = -24 ; size = 4
$T2 = -20 ; size = 4
$T3 = -16 ; size = 4
$T4 = -12 ; size = 4
$T5 = -8 ; size = 4
$T6 = -4 ; size = 4
_main PROC
push ebp
mov ebp, esp
sub esp, 36
mov DWORD PTR _c$[ebp], 0 ; Myfirst
mov DWORD PTR _c$[ebp+4], 0 ; Mylast
mov DWORD PTR _c$[ebp+8], 0 ; Myend
lea eax, DWORD PTR _c$[ebp]
push eax
call ?dump@@YAXPAUvector_of_ints@@@Z ; dump
add esp, 4
mov DWORD PTR $T6[ebp], 1
lea ecx, DWORD PTR $T6[ebp]
push ecx
lea ecx, DWORD PTR _c$[ebp]
call ?push_back@?$vector@HV?$allocator@H@std@@@std@@QAEX$$QAH@Z ; std::vector<int,std::⤦
Çallocator >::push_back
lea edx, DWORD PTR _c$[ebp]
push edx