使用std::string和std::string&& 区别

概述

C++11 右值引用 一般搭配std::move使用
如下为正确的使用方法

#include <string>
void test(std::string&& src) {
    
    }

int main() {
    
    
  std::string s1("hello");
  test(std::move(s1));
}

分析

#include <string>
void test(std::string&& src) {
    
    }

int main() {
    
    
  std::string s1("hello");
  test(std::move(s1));
}

对应的汇编代码为：

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"

而若不加两个&&

#include <string>
void test(std::string src) {
    
    }

int main() {
    
    
  std::string s1("hello");
  test(std::move(s1));
}

对应的汇编为

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"

本质上是多了一次构造和析构，因此std::move通常需要和&&搭配使用