What is _S_empty_rep_storage used for in this code?

Question

While reversing a C++ program compiled with g++, I've seen a _ZNSs4_Rep20_S_empty_rep_storageE being used. Running it through c++filt shows that before mangling it's a:

std::basic_string<char, std::char_traits<char>, std::allocator<char> >::_Rep::_S_empty_rep_storage

But what is this _S_empty_rep_storage used for? I included an assembly snippet below where it's used:

mov     rax, cs:_ZNSs4_Rep20_S_empty_rep_storageE_ptr
...
add     rax, 18h
...
mov     [rsp+328h+var_308], rax
mov     [rsp+328h+var_2F8], rax
mov     [rsp+328h+var_2E8], rax
...
lea     r14, [rsp+328h+var_308]
lea     rsi, [rsp+328h+var_2D8] ; std::string *
mov     rdi, r14        ; this
call    __ZNSs4swapERSs ; std::string::swap(std::string &)
lea     rdi, [rsp+328h+var_2D8] ; this
lea     r13, [rsp+328h+var_2F8]
lea     r12, [rsp+328h+var_2E8]
call    __ZNSsD1Ev      ; std::string::~string()

So my question is: What's the purpose of _S_empty_rep_storage here? Also why are var_308, var_2f8 and var_2e8 lea'd into r12-14? These registers are not used later on.

Answer 1

Check the comments at the beginning of libstdc++'s basic_string.h to see how GCC's std::string works.Basically, _S_empty_rep_storage is a pre-initialized (in fact, zeroed out) representation of an empty string, used to initialize the string in a default constructor. So var_308, var_2F8 and var_2E8 are three std::string objects, initialized to an empty string.

As for r12-r14, they seem to be used as temporary variables. We can at leas see that r14 is used to initialize rdi - the this pointer for the std::string::swap() call, so presumably r12 and r13 are also used later.