overview
C++11 rvalue references are generally used with std::move.
The correct usage is as follows
#include <string>
void test(std::string&& src) {
}
int main() {
std::string s1("hello");
test(std::move(s1));
}
analyze
#include <string>
void test(std::string&& src) {
}
int main() {
std::string s1("hello");
test(std::move(s1));
}
The corresponding assembly code is:
test(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&&):
push rbp
mov rbp, rsp
mov QWORD PTR [rbp-8], rdi
nop
pop rbp
ret
.LC0:
.string "hello"
main:
push rbp
mov rbp, rsp
push rbx
sub rsp, 56
lea rax, [rbp-17]
mov rdi, rax
call std::allocator<char>::allocator() [complete object constructor]
lea rdx, [rbp-17]
lea rax, [rbp-64]
mov esi, OFFSET FLAT:.LC0
mov rdi, rax
call std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::basic_string<std::allocator<char> >(char const*, std::allocator<char> const&)
lea rax, [rbp-17]
mov rdi, rax
call std::allocator<char>::~allocator() [complete object destructor]
lea rax, [rbp-64]
mov rdi, rax
call std::remove_reference<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&>::type&& std::move<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&>(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&)
mov rdi, rax
call test(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&&)
lea rax, [rbp-64]
mov rdi, rax
call std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::~basic_string() [complete object destructor]
mov eax, 0
jmp .L11
mov rbx, rax
lea rax, [rbp-17]
mov rdi, rax
call std::allocator<char>::~allocator() [complete object destructor]
mov rax, rbx
mov rdi, rax
call _Unwind_Resume
.L11:
mov rbx, QWORD PTR [rbp-8]
leave
ret
.LC1:
.string "basic_string: construction from null is not valid"
And if you don't add two &&
#include <string>
void test(std::string src) {
}
int main() {
std::string s1("hello");
test(std::move(s1));
}
The corresponding compilation is
test(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >):
push rbp
mov rbp, rsp
mov QWORD PTR [rbp-8], rdi
nop
pop rbp
ret
.LC0:
.string "hello"
main:
push rbp
mov rbp, rsp
push rbx
sub rsp, 88
lea rax, [rbp-49]
mov rdi, rax
call std::allocator<char>::allocator() [complete object constructor]
lea rdx, [rbp-49]
lea rax, [rbp-96]
mov esi, OFFSET FLAT:.LC0
mov rdi, rax
call std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::basic_string<std::allocator<char> >(char const*, std::allocator<char> const&)
lea rax, [rbp-49]
mov rdi, rax
call std::allocator<char>::~allocator() [complete object destructor]
lea rax, [rbp-96]
mov rdi, rax
call std::remove_reference<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&>::type&& std::move<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&>(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&)
mov rdx, rax
lea rax, [rbp-48]
mov rsi, rdx
mov rdi, rax
call std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::basic_string(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&&) [complete object constructor]
lea rax, [rbp-48]
mov rdi, rax
call test(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >)
lea rax, [rbp-48]
mov rdi, rax
call std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::~basic_string() [complete object destructor]
lea rax, [rbp-96]
mov rdi, rax
call std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::~basic_string() [complete object destructor]
mov eax, 0
jmp .L11
mov rbx, rax
lea rax, [rbp-49]
mov rdi, rax
call std::allocator<char>::~allocator() [complete object destructor]
mov rax, rbx
mov rdi, rax
call _Unwind_Resume
.L11:
mov rbx, QWORD PTR [rbp-8]
leave
ret
.LC1:
.string "basic_string: construction from null is not valid"
Essentially, there is one more construction and destruction, so std::move usually needs to be used with &&