Assembly Language for Beginners

3.18. C++

“assign” method, the s1 and s2 strings are constructed. And with the help ofoperator+, the s3 string is
constructed.

Please note that there is no call to the c_str() method, because its code is tiny enough so the compiler

inlined it right there: if the string is shorter than 16 characters, a pointer to buffer is left inEAX, otherwise

the address of the string buffer located in theheapis fetched.

Next, we see calls to the 3 destructors, they are called if the string is longer than 16 characters: then the

buffers in theheaphave to be freed. Otherwise, since all three std::string objects are stored in the stack,

they are freed automatically, when the function ends.

As a consequence, processing short strings is faster, because of lessheapaccesses.

GCC code is even simpler (because the GCC way, as we saw above, is to not store shorter strings right in

the structure):

Listing 3.104: GCC 4.8.1

.LC0:

.string "Hello, "

.LC1:

.string "world!\n"

main:

push ebp

mov ebp, esp

push edi

push esi

push ebx

and esp, -16

sub esp, 32

lea ebx, [esp+28]

lea edi, [esp+20]

mov DWORD PTR [esp+8], ebx

lea esi, [esp+24]

mov DWORD PTR [esp+4], OFFSET FLAT:.LC0

mov DWORD PTR [esp], edi

call _ZNSsC1EPKcRKSaIcE

mov DWORD PTR [esp+8], ebx

mov DWORD PTR [esp+4], OFFSET FLAT:.LC1

mov DWORD PTR [esp], esi

call _ZNSsC1EPKcRKSaIcE

mov DWORD PTR [esp+4], edi

mov DWORD PTR [esp], ebx

call _ZNSsC1ERKSs

mov DWORD PTR [esp+4], esi

mov DWORD PTR [esp], ebx

call _ZNSs6appendERKSs

; inlined c_str():

mov eax, DWORD PTR [esp+28]

mov DWORD PTR [esp], eax

call puts

mov eax, DWORD PTR [esp+28]

lea ebx, [esp+19]

mov DWORD PTR [esp+4], ebx

sub eax, 12

mov DWORD PTR [esp], eax

call _ZNSs4_Rep10_M_disposeERKSaIcE

mov eax, DWORD PTR [esp+24]

mov DWORD PTR [esp+4], ebx

sub eax, 12

mov DWORD PTR [esp], eax

call _ZNSs4_Rep10_M_disposeERKSaIcE