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"

為什么move 之后還是  “hello"?

socay2

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

不是有三個(gè)參數(shù)么，LZ怎么直接給了個(gè) string

變異老鼠

g++ 4.7.2 測(cè)試無誤。

樓主最好看看 g++ 4.4 的 vector 有沒有重載 push_back(T&&)。

Meets

回復(fù) 3# 變異老鼠


    這里和push_back 的關(guān)系應(yīng)該不大， 你的是什么操作系統(tǒng)？ 4.7 現(xiàn)在是不是還不穩(wěn)定？

變異老鼠

回復(fù) 4# Meets

沒有 push_back(T&&) 就只能調(diào)用 push_back(T const&)，實(shí)際做的還是復(fù)制而不是移動(dòng)。

std::move 只是把一個(gè) lvalue 變成 xvalue，并不是說調(diào)用了 std::move 就一定發(fā)生移動(dòng)操作。

gcc 4.7 系列都發(fā)布大半年了……