探讨一下C++11的move函数

在C++11中，标准库在<utility>中提供了一个有用的函数std::move，std::move并不能移动任何东西，
它唯一的功能是将一个左值强制转化为右值引用，继而可以通过右值引用使用该值，以用于移动语义。
从实现上讲，std::move基本等同于一个类型转换：static_cast<T&&>(lvalue);

std::move函数可以以非常简单的方式将左值引用转换为右值引用。

通过std::move，可以避免不必要的拷贝操作。

std::move是为性能而生。

std::move是将对象的状态或者所有权从一个对象转移到另一个对象，只是转移，没有内存的搬迁或者内存拷贝。

#include "move.hpp"
#include <iostream>
#include <utility>
#include <vector>
#include <string>

//////////////////////////////////////////////////////
// reference: http://en.cppreference.com/w/cpp/utility/move
int test_move1()
{
    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 be 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";

    return 0;
}

void ProcessValue(int& i)
{
    std::cout << "LValue processed: " << i << std::endl;
}

void ProcessValue(int&& i)
{
    std::cout << "RValue processed: " << i << std::endl;
}

int test_move2()
{
    int a = 0;
    ProcessValue(a);
    // std::move函数可以以非常简单的方式将左值引用转换为右值引用
    ProcessValue(std::move(a));

    return 0;
}
int test_move3()
{
    std::string foo = "foo-string";
    std::string bar = "bar-string";
    std::vector<std::string> myvector;

    // The first call to myvector.push_back copies the value of foo into
    // the vector (foo keeps the value it had before the call).
    // The second call moves the value of bar into the vector.
    // This transfers its content into the vector(while bar loses its value,
    // and now is in a valid but unspecified state)
    myvector.push_back(foo);                    // copies
    myvector.push_back(std::move(bar));         // moves

    std::cout << "myvector contains:";
    for (std::string& x : myvector)
        std::cout << ' ' << x;
    std::cout << '\n';

    return 0;
}