std::Move

Meets

[root@localhost mthread]# cat 2.cpp
#include <iostream>
#include <utility>
#include <vector>
#include <string>

int main()
{
    std::string str = "Hello";
    std::vector<std::string> v;

    // uses the push_back(const T&) overload, which means
    // we'll incur the cost of copying str
    v.push_back(str);
    std::cout << "After copy, str is \"" << str << "\"\n";

    // uses the rvalue reference push_back(T&&) overload,
    // which means no strings will copied; instead, the contents
    // of str will be moved into the vector.  This is less
    // expensive, but also means str might now be empty.
    v.push_back(std::move(str));
    std::cout << "After move, str is \"" << str << "\"\n";

    std::cout << "The contents of the vector are \"" << v[0]
                                         << "\", \"" << v[1] << "\"\n";
}
[root@localhost mthread]# gcc --version
gcc (GCC) 4.4.6 20120305 (Red Hat 4.4.6-4)
Copyright (C) 2010 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

[root@localhost mthread]# g++ -std=c++0x 2.cpp -o 2  
[root@localhost mthread]# ./2
After copy, str is "Hello"
After move, str is "Hello"
The contents of the vector are "Hello", "Hello"

为什么move 之后还是  “hello"?

socay2

std::move 是C++11的？
我看它的原型是这样的：
Defined in header <algorithm>
               
template< class InputIt, class OutputIt >
OutputIt move( InputIt first, InputIt last, OutputIt d_first );                 (since C++11)

不是有三个参数么，LZ怎么直接给了个 string

变异老鼠

g++ 4.7.2 测试无误。

楼主最好看看 g++ 4.4 的 vector 有没有重载 push_back(T&&)。

Meets

回复 3# 变异老鼠


    这里和push_back 的关系应该不大， 你的是什么操作系统？ 4.7 现在是不是还不稳定？

变异老鼠

回复 4# Meets

没有 push_back(T&&) 就只能调用 push_back(T const&)，实际做的还是复制而不是移动。

std::move 只是把一个 lvalue 变成 xvalue，并不是说调用了 std::move 就一定发生移动操作。

gcc 4.7 系列都发布大半年了……