Assembly Language for Beginners

3.18. C++

lea ecx, DWORD PTR _c$[ebp]
call ?at@?$vector@HV?$allocator@H@std@@@std@@QAEAAHI@Z ; std::vector<int,std::allocator<int⤦
Ç> >::at
mov edx, DWORD PTR [eax]
push edx
push OFFSET $SG52650 ; '%d'
call DWORD PTR impprintf
add esp, 8
mov eax, 8
shl eax, 2
mov ecx, DWORD PTR _c$[ebp]
mov edx, DWORD PTR [ecx+eax]
push edx
push OFFSET $SG52651 ; '%d'
call DWORD PTR impprintf
add esp, 8
lea ecx, DWORD PTR _c$[ebp]
call ?_Tidy@?$vector@HV?$allocator@H@std@@@std@@IAEXXZ ; std::vector<int,std::allocator<int⤦
Ç> >::_Tidy
xor eax, eax
mov esp, ebp
pop ebp
ret 0
_main ENDP

We see how the.at()method checks the bounds and throws an exception in case of error. The number
that the lastprintf()call prints is just taken from the memory, without any checks.

One may ask, why not use the variables like “size” and “capacity”, like it was done instd::string.
Supposedly, this was done for faster bounds checking.

The code GCC generates is in general almost the same, but the.at()method is inlined:

Listing 3.114: GCC 4.8.1 -fno-inline-small-functions -O1

main proc near
push ebp
mov ebp, esp
push edi
push esi
push ebx
and esp, 0FFFFFFF0h
sub esp, 20h
mov dword ptr [esp+14h], 0
mov dword ptr [esp+18h], 0
mov dword ptr [esp+1Ch], 0
lea eax, [esp+14h]
mov [esp], eax
call _Z4dumpP14vector_of_ints ; dump(vector_of_ints )
mov dword ptr [esp+10h], 1
lea eax, [esp+10h]
mov [esp+4], eax
lea eax, [esp+14h]
mov [esp], eax
call _ZNSt6vectorIiSaIiEE9push_backERKi ; std::vector<int,std::allocator>::push_back(⤦
Çint const&)
lea eax, [esp+14h]
mov [esp], eax
call _Z4dumpP14vector_of_ints ; dump(vector_of_ints )
mov dword ptr [esp+10h], 2
lea eax, [esp+10h]
mov [esp+4], eax
lea eax, [esp+14h]
mov [esp], eax
call _ZNSt6vectorIiSaIiEE9push_backERKi ; std::vector<int,std::allocator>::push_back(⤦
Çint const&)
lea eax, [esp+14h]
mov [esp], eax
call _Z4dumpP14vector_of_ints ; dump(vector_of_ints *)
mov dword ptr [esp+10h], 3
lea eax, [esp+10h]